# postconf(5) - man - phpMan

[POSTCONF(5)](https://www.chedong.com/phpMan.php/man/POSTCONF/5/markdown)                              File Formats Manual                             [POSTCONF(5)](https://www.chedong.com/phpMan.php/man/POSTCONF/5/markdown)



## NAME
       postconf - Postfix configuration parameters

## SYNOPSIS
       **postconf** _parameter_ ...

       **postconf** **-e** "_parameter=value_" ...

## DESCRIPTION
       The Postfix main.cf configuration file specifies parameters that control the operation of the
       Postfix mail system. Typically the file contains only a small subset of all  parameters;  pa‐
       rameters not specified are left at their default values.

       The general format of the main.cf file is as follows:

       •      Each  logical line has the form "parameter = value".  Whitespace around the "=" is ig‐
              nored, as is whitespace at the end of a logical line.

       •      Empty lines and whitespace-only lines are ignored, as are lines whose first non-white‐
              space character is a `#'.

       •      A  logical  line  starts  with non-whitespace text. A line that starts with whitespace
              continues a logical line.

       •      A parameter value may refer to other parameters.

              •      The expressions "$name" and "${name}" are recursively replaced with  the  value
                     of  the  named  parameter. The parameter name must contain only characters from
                     the set [a-zA-Z0-9_]. An undefined parameter value is replaced with  the  empty
                     value.

              •      The expressions "${name?value}" and "${name?{value}}" are replaced with "value"
                     when "$name" is non-empty. The parameter name must contain only characters from
                     the  set  [a-zA-Z0-9_].  These forms are supported with Postfix versions >= 2.2
                     and >= 3.0, respectively.

              •      The expressions "${name:value}" and "${name:{value}}" are replaced with "value"
                     when "$name" is empty. The parameter name must contain only characters from the
                     set [a-zA-Z0-9_]. These forms are supported with Postfix versions >= 2.2 and >=
                     3.0, respectively.

              •      The  expression  "${name?{value1}:{value2}}"  is  replaced  with  "value1" when
                     "$name" is non-empty, and with "value2" when "$name" is empty.  The "{}" is re‐
                     quired  for  "value1",  optional  for "value2". The parameter name must contain
                     only characters from the set [a-zA-Z0-9_].  This form is supported with Postfix
                     versions >= 3.0.

              •      The  first  item  inside  "${...}"  may be a relational expression of the form:
                     "{value3} == {value4}". Besides the "==" (equality) operator  Postfix  supports
                     "!="  (inequality),  "<", "<=", ">=", and ">". The comparison is numerical when
                     both operands are all digits,  otherwise  the  comparison  is  lexicographical.
                     These forms are supported with Postfix versions >= 3.0.

              •      Each  "value" is subject to recursive named parameter and relational expression
                     evaluation, except where noted.

              •      Whitespace before or after each "{value}" is ignored.

              •      Specify "$$" to produce a single "$" character.

              •      The legacy form "$(...)" is equivalent to the preferred form "${...}".

       •      When the same parameter is defined multiple times, only the last  instance  is  remem‐
              bered.

       •      Otherwise, the order of main.cf parameter definitions does not matter.

       The  remainder of this document is a description of all Postfix configuration parameters. De‐
       fault values are shown after the parameter name in parentheses, and can be looked up with the
       "**postconf** **-d**" command.

       Note: this is not an invitation to make changes to Postfix configuration parameters. Unneces‐
       sary changes can impair the operation of the mail system.

**2bounce**___**notice**___**recipient** **(default:** **postmaster)**
       The recipient of undeliverable mail that cannot be returned to the sender.  This  feature  is
       enabled with the notify_classes parameter.

**access**___**map**___**defer**___**code** **(default:** **450)**
       The  numerical Postfix SMTP server response code for an [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) map "defer" action, includ‐
       ing "defer_if_permit" or "defer_if_reject". Prior to Postfix 2.6, the response is  hard-coded
       as "450".

       Do not change this unless you have a complete understanding of RFC 5321.

       This feature is available in Postfix 2.6 and later.

**access**___**map**___**reject**___**code** **(default:** **554)**
       The numerical Postfix SMTP server response code for an [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) map "reject" action.

       Do not change this unless you have a complete understanding of RFC 5321.

**address**___**verify**___**cache**___**cleanup**___**interval** **(default:** **12h)**
       The amount of time between [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) address verification database cleanup runs. This feature
       requires that the database supports the "delete" and "sequence" operators.   Specify  a  zero
       interval to disable database cleanup.

       After  each  database  cleanup run, the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) daemon logs the number of entries that were
       retained and dropped. A cleanup run is logged as "partial" when the daemon  terminates  early
       after "**postfix** **reload**", "**postfix** **stop**", or no requests for $max_idle seconds.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.7.

**address**___**verify**___**default**___**transport** **(default:** **$default**___**transport)**
       Overrides the default_transport parameter setting for address verification probes.

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**local**___**transport** **(default:** **$local**___**transport)**
       Overrides the local_transport parameter setting for address verification probes.

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**map** **(default:** **see** **postconf** **-d** **output)**
       Lookup  table for persistent address verification status storage.  The table is maintained by
       the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) service, and is opened before the process releases privileges.

       The lookup table is persistent by default (Postfix 2.7 and later).  Specify  an  empty  table
       name  to  keep  the  information  in  volatile memory which is lost after "**postfix** **reload**" or
       "**postfix** **stop**". This is the default with Postfix version 2.6 and earlier.

       Specify a location in a file system that will not fill up. If the database becomes corrupted,
       the  world  comes  to  an  end.  To  recover  delete (NOT: truncate) the file and do "**postfix**
       **reload**".

       Postfix daemon processes do not use root privileges when opening this file (Postfix  2.5  and
       later).   The  file  must  therefore  be  stored  under a Postfix-owned directory such as the
       data_directory.  As a migration aid, an attempt to open the file under a  non-Postfix  direc‐
       tory is redirected to the Postfix-owned data_directory, and a warning is logged.

       Examples:

       address_verify_map = hash:/var/lib/postfix/verify
       address_verify_map = btree:/var/lib/postfix/verify

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**negative**___**cache** **(default:** **yes)**
       Enable  caching  of failed address verification probe results.  When this feature is enabled,
       the cache may pollute quickly with garbage.  When this feature is disabled, Postfix will gen‐
       erate an address probe for every lookup.

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**negative**___**expire**___**time** **(default:** **3d)**
       The time after which a failed probe expires from the address verification cache.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**negative**___**refresh**___**time** **(default:** **3h)**
       The time after which a failed address verification probe needs to be refreshed.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**pending**___**request**___**limit** **(default:** **see** **postconf** **-d** **output)**
       A  safety  limit  that  prevents  address verification requests from overwhelming the Postfix
       queue. By default, the number of pending requests is limited to 1/4 of the active queue maxi‐
       mum size (qmgr_message_active_limit). The queue manager enforces the limit by tempfailing re‐
       quests that exceed the limit. This affects only unknown addresses and inactive addresses that
       have  expired,  because the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) daemon automatically refreshes an active address before
       it expires.

       This feature is available in Postfix 3.1 and later.

**address**___**verify**___**poll**___**count** **(default:** **normal:** **3,** **overload:** **1)**
       How many times to query the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) service for the completion of an  address  verification
       request in progress.

       By  default,  the  Postfix  SMTP  server  polls the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) service up to three times under
       non-overload conditions, and only once when under overload.  With  Postfix  version  2.5  and
       earlier, the SMTP server always polls the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) service up to three times by default.

       Specify  1 to implement a crude form of greylisting, that is, always defer the first delivery
       request for a new address.

       Examples:

       # Postfix <= 2.6 default
       address_verify_poll_count = 3
       # Poor man's greylisting
       address_verify_poll_count = 1

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**poll**___**delay** **(default:** **3s)**
       The delay between queries for the completion of an address verification request in progress.

       The default polling delay is 3 seconds.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**positive**___**expire**___**time** **(default:** **31d)**
       The time after which a successful probe expires from the address verification cache.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**positive**___**refresh**___**time** **(default:** **7d)**
       The time after which a successful address verification probe needs to be refreshed.  The  ad‐
       dress verification status is not updated when the probe fails (optimistic caching).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**relay**___**transport** **(default:** **$relay**___**transport)**
       Overrides the relay_transport parameter setting for address verification probes.

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**relayhost** **(default:** **$relayhost)**
       Overrides  the  relayhost parameter setting for address verification probes. This information
       can be overruled with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**sender** **(default:** **$double**___**bounce**___**sender)**
       The sender address to use in address verification probes; prior to Postfix  2.5  the  default
       was  "postmaster". To avoid problems with address probes that are sent in response to address
       probes, the Postfix SMTP server excludes the probe  sender  address  from  all  SMTPD  access
       blocks.

       Specify an empty value (address_verify_sender =) or <> if you want to use the null sender ad‐
       dress. Beware, some sites reject mail from <>, even though RFCs require that  such  addresses
       be accepted.

       Examples:

       address_verify_sender = <>
       address_verify_sender = <postmaster@my.domain>

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**sender**___**dependent**___**default**___**transport**___**maps**   **(default:**  **$sender**___**dependent**___**default**___**trans**‐‐
       **port**___**maps)**
       Overrides the sender_dependent_default_transport_maps parameter setting for address verifica‐
       tion probes.

       This feature is available in Postfix 2.7 and later.

**address**___**verify**___**sender**___**dependent**___**relayhost**___**maps** **(default:** **$sender**___**dependent**___**relayhost**___**maps)**
       Overrides  the  sender_dependent_relayhost_maps  parameter  setting  for address verification
       probes.

       This feature is available in Postfix 2.3 and later.

**address**___**verify**___**sender**___**ttl** **(default:** **0s)**
       The time between changes in the time-dependent portion of address verification  probe  sender
       addresses.  The  time-dependent portion is appended to the localpart of the address specified
       with the address_verify_sender parameter. This feature is ignored when the probe  sender  ad‐
       dresses is the null sender, i.e. the address_verify_sender value is empty or <>.

       Historically, the probe sender address was fixed. This has caused such addresses to end up on
       spammer mailing lists, and has resulted in wasted network and processing resources.

       To enable time-dependent probe sender addresses, specify a non-zero time value  (an  integral
       value  plus  an optional one-letter suffix that specifies the time unit).  Specify a value of
       at least several hours, to avoid problems with senders that use greylisting.  Avoid nice  TTL
       values,  to  make  the  result less predictable.  Time units are: s (seconds), m (minutes), h
       (hours), d (days), w (weeks).

       This feature is available in Postfix 2.9 and later.

**address**___**verify**___**service**___**name** **(default:** **verify)**
       The name of the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) address verification service. This service maintains the status  of
       sender and/or recipient address verification probes, and generates probes on request by other
       Postfix processes.

**address**___**verify**___**transport**___**maps** **(default:** **$transport**___**maps)**
       Overrides the transport_maps parameter setting for address verification probes.

       This feature is available in Postfix 2.1 and later.

**address**___**verify**___**virtual**___**transport** **(default:** **$virtual**___**transport)**
       Overrides the virtual_transport parameter setting for address verification probes.

       This feature is available in Postfix 2.1 and later.

**alias**___**database** **(default:** **see** **postconf** **-d** **output)**
       The alias databases for [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery that are updated with "**newaliases**" or  with  "**send**‐‐
       **mail** **-bi**".

       This  is  a  separate  configuration  parameter  because  not  all  the tables specified with
       $alias_maps have to be local files.

       Examples:

       alias_database = hash:/etc/aliases
       alias_database = hash:/etc/mail/aliases

**alias**___**maps** **(default:** **see** **postconf** **-d** **output)**
       The alias databases that are used for [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery. See [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) for  syntax  details.
       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.  Note: these  lookups  are  recur‐
       sive.

       The  default list is system dependent.  On systems with NIS, the default is to search the lo‐
       cal alias database, then the NIS alias database.

       If you change the alias database, run  "**postalias**  **/etc/aliases**"  (or  wherever  your  system
       stores  the  mail  alias  file),  or simply run "**newaliases**" to build the necessary DBM or DB
       file.

       The [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery  agent  disallows  regular  expression  substitution  of  $1  etc.  in
       alias_maps, because that would open a security hole.

       The  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery  agent  will  silently  ignore requests to use the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) server
       within alias_maps. Instead it will open the table directly. Before Postfix version  2.2,  the
       [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent will terminate with a fatal error.

       Examples:

       alias_maps = hash:/etc/aliases, nis:mail.aliases
       alias_maps = hash:/etc/aliases

**allow**___**mail**___**to**___**commands** **(default:** **alias,** **forward)**
       Restrict [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) mail delivery to external commands.  The default is to disallow delivery to
       "|command" in :include:  files (see [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) for the text that defines this terminology).

       Specify zero or more of: **alias**, **forward** or **include**, in order to allow commands in [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown),
       .forward files or in :include:  files, respectively.

       Example:

       allow_mail_to_commands = alias,forward,include

**allow**___**mail**___**to**___**files** **(default:** **alias,** **forward)**
       Restrict  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  mail  delivery to external files. The default is to disallow "/file/name"
       destinations in :include:  files (see [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) for the text that defines this terminology).

       Specify zero or more of: **alias**, **forward** or **include**, in order to allow  "/file/name"  destina‐
       tions in [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown), .forward files and in :include:  files, respectively.

       Example:

       allow_mail_to_files = alias,forward,include

**allow**___**min**___**user** **(default:** **no)**
       Allow  a sender or recipient address to have `-' as the first character.  By default, this is
       not allowed, to avoid accidents with software that passes email  addresses  via  the  command
       line.  Such  software  would  not be able to distinguish a malicious address from a bona fide
       command-line option. Although this can be prevented by inserting  a  "--"  option  terminator
       into the command line, this is difficult to enforce consistently and globally.

       As  of  Postfix version 2.5, this feature is implemented by trivial-[**rewrite**(8)](https://www.chedong.com/phpMan.php/man/rewrite/8/markdown).  With earlier
       versions this feature was implemented by [**qmgr**(8)](https://www.chedong.com/phpMan.php/man/qmgr/8/markdown) and was limited to recipient addresses only.

**allow**___**percent**___**hack** **(default:** **yes)**
       Enable the rewriting of the form "user%domain" to "user@domain".  This is enabled by default.

       Note: as of Postfix version 2.2, message header address rewriting happens only  when  one  of
       the following conditions is true:

       •      The message is received with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command,

       •      The  message  is  received  from  a  network  client  that  matches  $local_header_re‐
              write_clients,

       •      The message is received from the network, and the remote_header_rewrite_domain parame‐
              ter specifies a non-empty value.

       To  get  the  behavior  before  Postfix  version 2.2, specify "local_header_rewrite_clients =
       static:all".

       Example:

       allow_percent_hack = no

**allow**___**untrusted**___**routing** **(default:** **no)**
       Forward mail with sender-specified routing (user[@%!]remote[@%!]site) from untrusted  clients
       to destinations matching $relay_domains.

       By  default,  this  feature  is  turned off.  This closes a nasty open relay loophole where a
       backup MX host can be tricked into forwarding junk mail to a primary MX host which then spams
       it out to the world.

       This  parameter  also controls if non-local addresses with sender-specified routing can match
       Postfix access tables. By default, such addresses cannot match Postfix access tables, because
       the address is ambiguous.

**alternate**___**config**___**directories** **(default:** **empty)**
       A  list  of non-default Postfix configuration directories that may be specified with "-c con‐
       fig_directory" on the command line (in the case of [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown), with the "-C" option), or via
       the MAIL_CONFIG environment parameter.

       This  list must be specified in the default Postfix main.cf file, and will be used by set-gid
       Postfix commands such as [**postqueue**(1)](https://www.chedong.com/phpMan.php/man/postqueue/1/markdown) and [**postdrop**(1)](https://www.chedong.com/phpMan.php/man/postdrop/1/markdown).

       Specify absolute pathnames, separated by comma or space. Note: $name expansion  is  not  sup‐
       ported.

**always**___**add**___**missing**___**headers** **(default:** **no)**
       Always  add (Resent-) From:, To:, Date: or Message-ID: headers when not present.  Postfix 2.6
       and later add these headers only when clients match the local_header_rewrite_clients  parame‐
       ter  setting.   Earlier Postfix versions always add these headers; this may break DKIM signa‐
       tures that cover non-existent headers.  The undisclosed_recipients_header  parameter  setting
       determines whether a To: header will be added.

**always**___**bcc** **(default:** **empty)**
       Optional  address that receives a "blind carbon copy" of each message that is received by the
       Postfix mail system.

       Note: with Postfix 2.3 and later the BCC address is added as if it  was  specified  with  NO‐
       TIFY=NONE.  The sender will not be notified when the BCC address is undeliverable, as long as
       all down-stream software implements RFC 3461.

       Note: with Postfix 2.2 and earlier the sender will be notified when the BCC address is  unde‐
       liverable.

       Note:  automatic BCC recipients are produced only for new mail.  To avoid mailer loops, auto‐
       matic BCC recipients are not generated after Postfix forwards mail internally, or after Post‐
       fix generates mail itself.

**anvil**___**rate**___**time**___**unit** **(default:** **60s)**
       The time unit over which client connection rates and other rates are calculated.

       This feature is implemented by the [**anvil**(8)](https://www.chedong.com/phpMan.php/man/anvil/8/markdown) service which is available in Postfix version 2.2
       and later.

       The default interval is relatively short. Because of  the  high  frequency  of  updates,  the
       [**anvil**(8)](https://www.chedong.com/phpMan.php/man/anvil/8/markdown)  server  uses  volatile  memory only. Thus, information is lost whenever the process
       terminates.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**anvil**___**status**___**update**___**time** **(default:** **600s)**
       How frequently the [**anvil**(8)](https://www.chedong.com/phpMan.php/man/anvil/8/markdown) connection and rate limiting server logs peak usage information.

       This feature is available in Postfix 2.2 and later.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**append**___**at**___**myorigin** **(default:** **yes)**
       With locally submitted mail, append the string "@$myorigin" to mail addresses without  domain
       information. With remotely submitted mail, append the string "@$remote_header_rewrite_domain"
       instead.

       Note 1: this feature is enabled by default and must not be turned off.  Postfix does not sup‐
       port domain-less addresses.

       Note  2:  with Postfix version 2.2, message header address rewriting happens only when one of
       the following conditions is true:

       •      The message is received with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command,

       •      The  message  is  received  from  a  network  client  that  matches  $local_header_re‐
              write_clients,

       •      The message is received from the network, and the remote_header_rewrite_domain parame‐
              ter specifies a non-empty value.

       To get the behavior before  Postfix  version  2.2,  specify  "local_header_rewrite_clients  =
       static:all".

**append**___**dot**___**mydomain** **(default:** **Postfix** **>=** **3.0:** **no,** **Postfix** **<** **3.0:** **yes)**
       With  locally  submitted mail, append the string ".$mydomain" to addresses that have no ".do‐
       main" information. With remotely  submitted  mail,  append  the  string  ".$remote_header_re‐
       write_domain" instead.

       Note  1: this feature is enabled by default. If disabled, users will not be able to send mail
       to "user@partialdomainname" but will have to specify full domain names instead.

       Note 2: with Postfix version 2.2, message header address rewriting happens only when  one  of
       the following conditions is true:

       •      The message is received with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command,

       •      The  message  is  received  from  a  network  client  that  matches  $local_header_re‐
              write_clients,

       •      The message is received from the network, and the remote_header_rewrite_domain parame‐
              ter specifies a non-empty value.

       To  get  the  behavior  before  Postfix  version 2.2, specify "local_header_rewrite_clients =
       static:all".

**application**___**event**___**drain**___**time** **(default:** **100s)**
       How long the [**postkick**(1)](https://www.chedong.com/phpMan.php/man/postkick/1/markdown) command waits for a request to enter the Postfix daemon process  in‐
       put buffer before giving up.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

       This feature is available in Postfix 2.1 and later.

**authorized**___**flush**___**users** **(default:** **static:anyone)**
       List of users who are authorized to flush the queue.

       By default, all users are allowed to flush the queue.  Access is always granted if the invok‐
       ing  user  is the super-user or the $mail_owner user.  Otherwise, the real UID of the process
       is looked up in the system password file, and access is granted only if the corresponding lo‐
       gin  name is on the access list.  The username "unknown" is used for processes whose real UID
       is not found in the password file.

       Specify a list of user names, "/file/name" or  "type:table"  patterns,  separated  by  commas
       and/or  whitespace.  The  list  is  matched  left to right, and the search stops on the first
       match. A "/file/name" pattern is replaced by its contents; a  "type:table"  lookup  table  is
       matched when a name matches a lookup key (the lookup result is ignored).  Continue long lines
       by starting the next line with whitespace. Specify "!pattern" to  exclude  a  name  from  the
       list. The form "!/file/name" is supported only in Postfix version 2.4 and later.

       This feature is available in Postfix 2.2 and later.

**authorized**___**mailq**___**users** **(default:** **static:anyone)**
       List of users who are authorized to view the queue.

       By  default, all users are allowed to view the queue.  Access is always granted if the invok‐
       ing user is the super-user or the $mail_owner user.  Otherwise, the real UID of  the  process
       is looked up in the system password file, and access is granted only if the corresponding lo‐
       gin name is on the access list.  The username "unknown" is used for processes whose real  UID
       is not found in the password file.

       Specify  a  list  of  user  names, "/file/name" or "type:table" patterns, separated by commas
       and/or whitespace. The list is matched left to right, and  the  search  stops  on  the  first
       match.  A  "/file/name"  pattern  is replaced by its contents; a "type:table" lookup table is
       matched when a name matches a lookup key (the lookup result is ignored).  Continue long lines
       by starting the next line with whitespace. Specify "!pattern" to exclude a user name from the
       list. The form "!/file/name" is supported only in Postfix version 2.4 and later.

       This feature is available in Postfix 2.2 and later.

**authorized**___**submit**___**users** **(default:** **static:anyone)**
       List of users who are authorized to submit mail with the [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command  (and  with  the
       privileged [**postdrop**(1)](https://www.chedong.com/phpMan.php/man/postdrop/1/markdown) helper command).

       By  default, all users are allowed to submit mail.  Otherwise, the real UID of the process is
       looked up in the system password file, and access is granted only if the corresponding  login
       name  is  on the access list.  The username "unknown" is used for processes whose real UID is
       not found in the password file. To deny mail submission access to all users specify an  empty
       list.

       Specify  a  list  of  user  names, "/file/name" or "type:table" patterns, separated by commas
       and/or whitespace. The list is matched left to right, and  the  search  stops  on  the  first
       match.  A  "/file/name"  pattern  is replaced by its contents; a "type:table" lookup table is
       matched when a name matches a lookup key (the lookup result is ignored).  Continue long lines
       by starting the next line with whitespace. Specify "!pattern" to exclude a user name from the
       list. The form "!/file/name" is supported only in Postfix version 2.4 and later.

       Example:

       authorized_submit_users = !www, static:all

       This feature is available in Postfix 2.2 and later.

**authorized**___**verp**___**clients** **(default:** **$mynetworks)**
       What remote SMTP clients are allowed to specify the XVERP  command.   This  command  requests
       that mail be delivered one recipient at a time with a per recipient return address.

       By default, only trusted clients are allowed to specify XVERP.

       This parameter was introduced with Postfix version 1.1.  Postfix version 2.1 renamed this pa‐
       rameter to smtpd_authorized_verp_clients and changed the default to none.

       Specify a list of network/netmask patterns, separated by commas and/or whitespace.  The  mask
       specifies  the  number  of  bits  in the network part of a host address. You can also specify
       hostnames or .domain names (the initial dot causes the domain to match any  name  below  it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is replaced by its contents; a
       "type:table" lookup table is matched when a table entry matches a lookup string  (the  lookup
       result  is  ignored).  Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude an address or network block from the list. The  form  "!/file/name"  is
       supported only in Postfix version 2.4 and later.

       Note:  IP  version  6  address  information  must  be  specified  inside  []  in  the  autho‐
       rized_verp_clients value, and in files specified with "/file/name".  IP version  6  addresses
       contain the ":" character, and would otherwise be confused with a "type:table" pattern.

**backwards**___**bounce**___**logfile**___**compatibility** **(default:** **yes)**
       Produce additional [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown) logfile records that can be read by Postfix versions before 2.0.
       The current and more extensible "name = value" format is needed in order  to  implement  more
       sophisticated functionality.

       This feature is available in Postfix 2.1 and later.

**berkeley**___**db**___**create**___**buffer**___**size** **(default:** **16777216)**
       The  per-table  I/O  buffer  size  for programs that create Berkeley DB hash or btree tables.
       Specify a byte count.

       This feature is available in Postfix 2.0 and later.

**berkeley**___**db**___**read**___**buffer**___**size** **(default:** **131072)**
       The per-table I/O buffer size for programs that read Berkeley DB hash or btree tables.  Spec‐
       ify a byte count.

       This feature is available in Postfix 2.0 and later.

**best**___**mx**___**transport** **(default:** **empty)**
       Where  the  Postfix SMTP client should deliver mail when it detects a "mail loops back to my‐
       self" error condition. This happens when the local MTA is the best SMTP mail exchanger for  a
       destination   not   listed  in  $mydestination,  $inet_interfaces,  $proxy_interfaces,  $vir‐
       tual_alias_domains, or $virtual_mailbox_domains.  By default, the Postfix SMTP client returns
       such mail as undeliverable.

       Specify,  for  example,  "best_mx_transport  =  local" to pass the mail from the Postfix SMTP
       client to the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent. You can specify any message  delivery  "transport"  or
       "transport:nexthop"  that  is defined in the master.cf file. See the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) manual page
       for the syntax and meaning of "transport" or "transport:nexthop".

       However, this feature is expensive because it ties up a Postfix SMTP client process while the
       [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery  agent  is  doing its work. It is more efficient (for Postfix) to list all
       hosted domains in a table or database.

### biff (default: yes)
       Whether or not to use the local biff service.  This service sends "new mail" notifications to
       users who have requested new mail notification with the UNIX command "biff y".

       For compatibility reasons this feature is on by default.  On systems with lots of interactive
       users, the biff service can be a performance drain.  Specify "biff = no" in main.cf  to  dis‐
       able.

**body**___**checks** **(default:** **empty)**
       Optional lookup tables for content inspection as specified in the **body**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) manual page.

       Note:  with  Postfix  versions  before 2.0, these rules inspect all content after the primary
       message headers.

**body**___**checks**___**size**___**limit** **(default:** **51200)**
       How much text in a message body segment (or attachment, if you prefer to use  that  term)  is
       subjected  to  body_checks  inspection.  The amount of text is limited to avoid scanning huge
       attachments.

       This feature is available in Postfix 2.0 and later.

**bounce**___**notice**___**recipient** **(default:** **postmaster)**
       The recipient of postmaster notifications with the message headers of mail that  Postfix  did
       not  deliver and of SMTP conversation transcripts of mail that Postfix did not receive.  This
       feature is enabled with the notify_classes parameter.

**bounce**___**queue**___**lifetime** **(default:** **5d)**
       Consider a bounce message as undeliverable, when delivery fails with a temporary  error,  and
       the time in the queue has reached the bounce_queue_lifetime limit.  By default, this limit is
       the same as for regular mail.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is d (days).

       Specify 0 when mail delivery should be tried only once.

       This feature is available in Postfix 2.1 and later.

**bounce**___**service**___**name** **(default:** **bounce)**
       The  name  of  the  [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown) service. This service maintains a record of failed delivery at‐
       tempts and generates non-delivery notifications.

       This feature is available in Postfix 2.0 and later.

**bounce**___**size**___**limit** **(default:** **50000)**
       The maximal amount of original message text that is  sent  in  a  non-delivery  notification.
       Specify a byte count.  A message is returned as either message/rfc822 (the complete original)
       or as text/rfc822-headers (the headers only).  With Postfix version 2.4 and earlier,  a  mes‐
       sage is always returned as message/rfc822 and is truncated when it exceeds the size limit.

       Notes:

       •      If you increase this limit, then you should increase the mime_nesting_limit value pro‐
              portionally.

       •      Be careful when making changes.  Excessively large values will result in the  loss  of
              non-delivery notifications, when a bounce message size exceeds a local or remote MTA's
              message size limit.

**bounce**___**template**___**file** **(default:** **empty)**
       Pathname of a configuration file with bounce message templates.  These override the  built-in
       templates  of delivery status notification (DSN) messages for undeliverable mail, for delayed
       mail, successful delivery, or delivery verification. The [**bounce**(5)](https://www.chedong.com/phpMan.php/man/bounce/5/markdown) manual page describes  how
       to edit and test template files.

       Template  message body text may contain $name references to Postfix configuration parameters.
       The result of $name expansion can be previewed with "**postconf** **-b** _file_name_" before  the  file
       is placed into the Postfix configuration directory.

       This feature is available in Postfix 2.3 and later.

**broken**___**sasl**___**auth**___**clients** **(default:** **no)**
       Enable  interoperability  with  remote SMTP clients that implement an obsolete version of the
       AUTH command (RFC 4954). Examples of such clients are MicroSoft Outlook Express version 4 and
       MicroSoft Exchange version 5.0.

       Specify  "broken_sasl_auth_clients  =  yes"  to  have  Postfix  advertise  AUTH  support in a
       non-standard way.

**canonical**___**classes** **(default:** **envelope**___**sender,** **envelope**___**recipient,** **header**___**sender,** **header**___**recipient)**
       What addresses are subject to canonical_maps address mapping.  By default, canonical_maps ad‐
       dress mapping is applied to envelope sender and recipient addresses, and to header sender and
       header recipient addresses.

       Specify one or more of: envelope_sender, envelope_recipient, header_sender, header_recipient

       This feature is available in Postfix 2.2 and later.

**canonical**___**maps** **(default:** **empty)**
       Optional address mapping lookup tables for message headers and envelopes. The mapping is  ap‐
       plied to both sender and recipient addresses, in both envelopes and in headers, as controlled
       with the canonical_classes parameter. This is typically used to clean up dirty addresses from
       legacy  mail  systems, or to replace login names by Firstname.Lastname.  The table format and
       lookups are documented in [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown). For an overview of Postfix address manipulations  see
       the ADDRESS_REWRITING_README document.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.  Note: these  lookups  are  recur‐
       sive.

       If  you  use this feature, run "**postmap** **/etc/postfix/canonical**" to build the necessary DBM or
       DB file after every change. The changes will become visible after a minute or so.  Use "**post**‐‐
       **fix** **reload**" to eliminate the delay.

       Note:  with  Postfix  version  2.2,  message header address mapping happens only when message
       header address rewriting is enabled:

       •      The message is received with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command,

       •      The  message  is  received  from  a  network  client  that  matches  $local_header_re‐
              write_clients,

       •      The message is received from the network, and the remote_header_rewrite_domain parame‐
              ter specifies a non-empty value.

       To get the behavior before  Postfix  version  2.2,  specify  "local_header_rewrite_clients  =
       static:all".

       Examples:

       canonical_maps = dbm:/etc/postfix/canonical
       canonical_maps = hash:/etc/postfix/canonical

**cleanup**___**replace**___**stray**___**cr**___**lf** **(default:** **yes)**
       Replace  each stray <CR> or <LF> character in message content with a space character, to pre‐
       vent outbound SMTP smuggling, and to make the evaluation of Postfix-added DKIM or other  sig‐
       natures independent from how a remote mail server handles such characters.

       SMTP  does not allow such characters unless they are part of a <CR><LF> sequence, and differ‐
       ent mail systems handle such stray characters in an  implementation-dependent  manner.  Stray
       <CR>  or  <LF> characters could be used for outbound SMTP smuggling, where an attacker uses a
       Postfix server to send message content with a non-standard End-of-DATA sequence that triggers
       inbound SMTP smuggling at a remote SMTP server.

       The  replacement  happens  before  all other content management, and before Postfix may add a
       DKIM etc. signature; if the signature were created first, the  replacement  could  invalidate
       the signature.

       In  addition  to  preventing  SMTP smuggling, replacing stray <CR> or <LF> characters ensures
       that the result of signature validation by later mail system will not depend on how that mail
       system handles those stray characters in an implementation-dependent manner.

       This feature is available in Postfix >= 3.9, 3.8.5, 3.7.10, 3.6.14, and 3.5.24.

**cleanup**___**service**___**name** **(default:** **cleanup)**
       The  name  of the [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) service. This service rewrites addresses into the standard form,
       and performs [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown) address mapping and [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown) aliasing.

       This feature is available in Postfix 2.0 and later.

**command**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The location of all postfix administrative commands.

**command**___**execution**___**directory** **(default:** **empty)**
       The [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent working directory for delivery to external command.   Failure  to
       change directory causes the delivery to be deferred.

       The command_execution_directory value is not subject to Postfix configuration parameter $name
       expansion. Instead, the following $name expansions are  done  on  command_execution_directory
       before  the directory is used. Expansion happens in the context of the delivery request.  The
       result of $name expansion is filtered with the character set that is specified with the  exe‐
       cution_directory_expansion_filter parameter.

       **$user**  The recipient's username.

       **$shell** The recipient's login shell pathname.

       **$home**  The recipient's home directory.

### $recipient
              The full recipient address.

### $extension
              The optional recipient address extension.

### $domain
              The recipient domain.

       **$local** The entire recipient localpart.

       **$recipient**___**delimiter**
              The  address extension delimiter that was found in the recipient address (Postfix 2.11
              and later), or the system-wide recipient address extension delimiter (Postfix 2.10 and
              earlier).

### ${name?value}
              Expands to _value_ when _$name_ is non-empty.

### ${name:value}
              Expands to _value_ when _$name_ is empty.

       Instead of $name you can also specify ${name} or $(name).

       This feature is available in Postfix 2.2 and later.

**command**___**expansion**___**filter** **(default:** **see** **postconf** **-d** **output)**
       Restrict the characters that the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent allows in $name expansions of $mail‐
       box_command and $command_execution_directory.  Characters outside the  allowed  set  are  re‐
       placed by underscores.

**command**___**time**___**limit** **(default:** **1000s)**
       Time  limit  for  delivery  to external commands. This limit is used by the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery
       agent, and is the default time limit for delivery by the [**pipe**(8)](https://www.chedong.com/phpMan.php/man/pipe/8/markdown) delivery agent.

       Note: if you set this time limit to a large value you must update the global ipc_timeout  pa‐
       rameter as well.

**compatibility**___**level** **(default:** **0)**
       A  safety  net that causes Postfix to run with backwards-compatible default settings after an
       upgrade to a newer Postfix version.

       With backwards compatibility turned on (the main.cf compatibility_level value  is  less  than
       the  Postfix  built-in value), Postfix looks for settings that are left at their implicit de‐
       fault value, and logs a message when a backwards-compatible default setting is required.

           using backwards-compatible default setting _name=value_
               to [accept a specific client request]

           using backwards-compatible default setting _name=value_
               to [enable specific Postfix behavior]

       See COMPATIBILITY_README for specific message details. If such a message  is  logged  in  the
       context  of a legitimate request, the system administrator should make the backwards-compati‐
       ble setting permanent in main.cf or master.cf, for example:

           # **postconf** _name=value_
           # **postfix** **reload**

       When no more backwards-compatible settings need  to  be  made  permanent,  the  administrator
       should  turn  off  backwards  compatibility  by  updating  the compatibility_level setting in
       main.cf:

           # **postconf** **compatibility**___**level=**_N_
           # **postfix** **reload**

       For _N_ specify the number that is logged in your [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) warning message:

           warning: To disable backwards compatibility use "postconf
               compatibility_level=_N_" and "postfix reload"

       Starting with Postfix version 3.6, the compatibility level in the above  warning  message  is
       the  Postfix  version that introduced the last incompatible change. The level is formatted as
       _major.minor.patch_, where _patch_ is usually omitted and defaults to zero. Earlier compatibility
       levels are 0, 1 and 2.

       NOTE:  this  also introduces support for the "<level", "<=level", and other operators to com‐
       pare compatibility levels.  With the standard operators "<", "<=", etc., compatibility  level
       "3.10" would be smaller than "3.9" which is undesirable.

       This feature is available in Postfix 3.0 and later.

**config**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The  default  location  of the Postfix main.cf and master.cf configuration files. This can be
       overruled via the following mechanisms:

       •      The MAIL_CONFIG environment variable (daemon processes and commands).

       •      The "-c" command-line option (commands only).

       With Postfix commands that run with set-gid privileges, a  config_directory  override  either
       requires root privileges, or it requires that the directory is listed with the alternate_con‐
       fig_directories parameter in the default main.cf file.

**confirm**___**delay**___**cleared** **(default:** **no)**
       After sending a "your message is delayed" notification, inform  the  sender  when  the  delay
       clears  up. This can result in a sudden burst of notifications at the end of a prolonged net‐
       work outage, and is therefore disabled by default.

       See also: delay_warning_time.

       This feature is available in Postfix 3.0 and later.

**connection**___**cache**___**protocol**___**timeout** **(default:** **5s)**
       Time limit for connection cache connect, send or receive operations.  The time limit  is  en‐
       forced in the client.

       This feature is available in Postfix 2.3 and later.

**connection**___**cache**___**service**___**name** **(default:** **scache)**
       The name of the [**scache**(8)](https://www.chedong.com/phpMan.php/man/scache/8/markdown) connection cache service.  This service maintains a limited pool of
       cached sessions.

       This feature is available in Postfix 2.2 and later.

**connection**___**cache**___**status**___**update**___**time** **(default:** **600s)**
       How frequently the [**scache**(8)](https://www.chedong.com/phpMan.php/man/scache/8/markdown) server logs usage statistics with connection cache hit and  miss
       rates for logical destinations and for physical endpoints.

**connection**___**cache**___**ttl**___**limit** **(default:** **2s)**
       The  maximal  time-to-live  value that the [**scache**(8)](https://www.chedong.com/phpMan.php/man/scache/8/markdown) connection cache server allows. Requests
       that specify a larger TTL will be stored with the maximum allowed TTL. The  purpose  of  this
       additional control is to protect the infrastructure against careless people. The cache TTL is
       already bounded by $max_idle.

**content**___**filter** **(default:** **empty)**
       After the message is queued, send the entire message to the specified  _transport:destination_.
       The  _transport_  name  specifies  the  first field of a mail delivery agent definition in mas‐
       ter.cf; the syntax of the next-hop _destination_ is described in the manual page of the  corre‐
       sponding  delivery  agent.  More information about external content filters is in the Postfix
       FILTER_README file.

       Notes:

       •      This setting has lower precedence than a FILTER action that is  specified  in  an  **ac**‐‐
              [**cess**(5)](https://www.chedong.com/phpMan.php/man/cess/5/markdown), **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) or **body**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) table.

       •      The meaning of an empty next-hop filter _destination_ is version dependent.  Postfix 2.7
              and later will use the recipient domain; earlier versions will use $myhostname.  Spec‐
              ify  "default_filter_nexthop = $myhostname" for compatibility with Postfix 2.6 or ear‐
              lier, or specify a content_filter value with an explicit next-hop _destination_.

**cyrus**___**sasl**___**config**___**path** **(default:** **empty)**
       Search path for Cyrus SASL application configuration files, currently used only to locate the
       $smtpd_sasl_path.conf file.  Specify zero or more directories separated by a colon character,
       or an empty value to use Cyrus SASL's built-in search path.

       This feature is available in Postfix 2.5 and later when compiled with Cyrus  SASL  2.1.22  or
       later.

**daemon**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The directory with Postfix support programs and daemon programs.  These should not be invoked
       directly by humans. The directory must be owned by root.

**daemon**___**table**___**open**___**error**___**is**___**fatal** **(default:** **no)**
       How a Postfix daemon process handles errors while opening lookup tables: gradual  degradation
       or immediate termination.

### no  (default)
              Gradual degradation: a daemon process logs a message of type "error" and continues ex‐
              ecution with reduced functionality. Features that do not depend on the unavailable ta‐
              ble  will work normally, while features that depend on the table will result in a type
              "warning" message.
              When the notify_classes parameter value contains the "data" class,  the  Postfix  SMTP
              server and client will report transcripts of sessions with an error because a table is
              unavailable.

        **yes**  (historical behavior)
              Immediate termination: a daemon process logs a type "fatal" message and terminates im‐
              mediately.  This option reduces the number of possible code paths through Postfix, and
              may therefore be slightly more secure than the default.

       For the sake of sanity, the number of type "error" messages is limited to 13 over  the  life‐
       time of a daemon process.

       This feature is available in Postfix 2.9 and later.

**daemon**___**timeout** **(default:** **18000s)**
       How  much  time a Postfix daemon process may take to handle a request before it is terminated
       by a built-in watchdog timer.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**data**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The  directory with Postfix-writable data files (for example: caches, pseudo-random numbers).
       This directory must be owned by the mail_owner account, and must not be shared with non-Post‐
       fix software.

       This feature is available in Postfix 2.5 and later.

**debug**___**peer**___**level** **(default:** **2)**
       The  increment  in  verbose logging level when a nexthop destination, remote client or server
       name or network address matches a pattern given with the debug_peer_list parameter.

       Per-nexthop debug logging is available in Postfix 3.6 and later.

**debug**___**peer**___**list** **(default:** **empty)**
       Optional list of nexthop destination, remote client or server name or  network  address  pat‐
       terns  that,  if matched, cause the verbose logging level to increase by the amount specified
       in $debug_peer_level.

       Per-nexthop debug logging is available in Postfix 3.6 and later.

       Specify domain names, network/netmask patterns, "/file/name" patterns or "type:table"  lookup
       tables. The right-hand side result from "type:table" lookups is ignored.

       Pattern  matching  of  domain  names  is  controlled  by  the  presence  or  absence  of "de‐
       bug_peer_list" in the parent_domain_matches_subdomains parameter value.

       Examples:

       debug_peer_list = 127.0.0.1
       debug_peer_list = example.com

**debugger**___**command** **(default:** **empty)**
       The external command to execute when a Postfix daemon program is invoked with the -D option.

       Use "command .. & sleep 5" so that the debugger can attach before the process marches on.  If
       you  use  an  X-based debugger, be sure to set up your XAUTHORITY environment variable before
       starting Postfix.

       Note: the command is subject to $name expansion, before it is passed to the  default  command
       interpreter. Specify "$$" to produce a single "$" character.

       Example:

       debugger_command =
           PATH=/usr/bin:/usr/X11R6/bin
           ddd $daemon_directory/$process_name $process_id & sleep 5

**default**___**database**___**type** **(default:** **see** **postconf** **-d** **output)**
       The  default database type for use in [**newaliases**(1)](https://www.chedong.com/phpMan.php/man/newaliases/1/markdown), [**postalias**(1)](https://www.chedong.com/phpMan.php/man/postalias/1/markdown) and [**postmap**(1)](https://www.chedong.com/phpMan.php/man/postmap/1/markdown) commands. On
       many UNIX systems the default type is either **dbm** or **hash**. The default setting is frozen  when
       the Postfix system is built.

       Examples:

       default_database_type = hash
       default_database_type = dbm

**default**___**delivery**___**slot**___**cost** **(default:** **5)**
       How often the Postfix queue manager's scheduler is allowed to preempt delivery of one message
       with another.

       Each transport maintains a so-called "available delivery slot counter" for each message.  One
       message can be preempted by another one when the other message can be delivered using no more
       delivery slots (i.e., invocations of delivery agents) than the current  message  counter  has
       accumulated (or will eventually accumulate - see about slot loans below). This parameter con‐
       trols how often is the counter incremented - it happens after each default_delivery_slot_cost
       recipients have been delivered.

       The cost of 0 is used to disable the preempting scheduling completely.  The minimum value the
       scheduling algorithm can use is 2 - use it if you want to  maximize  the  message  throughput
       rate. Although there is no maximum, it doesn't make much sense to use values above say 50.

       The  only  reason why the value of 2 is not the default is the way this parameter affects the
       delivery of mailing-list mail. In  the  worst  case,  delivery  can  take  somewhere  between
       (cost+1/cost) and (cost/cost-1) times more than if the preemptive scheduler was disabled. The
       default value of 5 turns out to provide reasonable message response times while  making  sure
       the  mailing-list  deliveries  are  not extended by more than 20-25 percent even in the worst
       case.

       Use _transport__delivery_slot_cost to specify a transport-specific override, where _transport_ is
       the master.cf name of the message delivery transport.

       Examples:

       default_delivery_slot_cost = 0
       default_delivery_slot_cost = 2

**default**___**delivery**___**slot**___**discount** **(default:** **50)**
       The default value for transport-specific _delivery_slot_discount settings.

       This  parameter speeds up the moment when a message preemption can happen. Instead of waiting
       until the full amount of delivery slots required is available, the preemption can happen when
       transport_delivery_slot_discount   percent  of  the  required  amount  plus  transport_deliv‐
       ery_slot_loan still remains to be accumulated.  Note that the full amount will still have  to
       be accumulated before another preemption can take place later.

       Use  _transport__delivery_slot_discount  to specify a transport-specific override, where _trans__‐
       _port_ is the master.cf name of the message delivery transport.

**default**___**delivery**___**slot**___**loan** **(default:** **3)**
       The default value for transport-specific _delivery_slot_loan settings.

       This parameter speeds up the moment when a message preemption can happen. Instead of  waiting
       until the full amount of delivery slots required is available, the preemption can happen when
       transport_delivery_slot_discount  percent  of  the  required  amount  plus   transport_deliv‐
       ery_slot_loan  still remains to be accumulated.  Note that the full amount will still have to
       be accumulated before another preemption can take place later.

       Use _transport__delivery_slot_loan to specify a transport-specific override, where _transport_ is
       the master.cf name of the message delivery transport.

**default**___**delivery**___**status**___**filter** **(default:** **empty)**
       Optional  filter to replace the delivery status code or explanatory text of successful or un‐
       successful deliveries.  This does not allow the  replacement  of  a  successful  status  code
       (2.X.X) with an unsuccessful status code (4.X.X or 5.X.X) or vice versa.

       Note:  the (smtp|lmtp)_delivery_status_filter is applied only once per recipient: when deliv‐
       ery is successful, when delivery is rejected with 5XX, or when there are no more alternate MX
       or  A  destinations.  Use smtp_reply_filter or lmtp_reply_filter to inspect responses for all
       delivery attempts.

       The following parameters can be used to implement a  filter  for  specific  delivery  agents:
       lmtp_delivery_status_filter,    local_delivery_status_filter,    pipe_delivery_status_filter,
       smtp_delivery_status_filter or virtual_delivery_status_filter. These parameters  support  the
       same filter syntax as described here.

       Specify  zero  or more "type:table" lookup table names, separated by comma or whitespace. For
       each successful or unsuccessful delivery to a recipient, the tables are queried in the speci‐
       fied order with one line of text that is structured as follows:

           enhanced-status-code SPACE explanatory-text

       The  first  table  match wins. The lookup result must have the same structure as the query, a
       successful status code (2.X.X) must be replaced with a successful status code, an  unsuccess‐
       ful  status  code (4.X.X or 5.X.X) must be replaced with an unsuccessful status code, and the
       explanatory text field must be non-empty. Other results will result in a warning.

       Example 1: convert specific soft TLS errors into hard errors, by overriding the first  number
       in the enhanced status code.

           /etc/postfix/main.cf:
               smtp_delivery_status_filter = pcre:/etc/postfix/smtp_dsn_filter

           /etc/postfix/smtp_dsn_filter:
               /^4(\.\d+\.\d+ TLS is required, but host \S+ refused to start TLS: .+)/
                   5$1
               /^4(\.\d+\.\d+ TLS is required, but was not offered by host .+)/
                   5$1
               # Do not change the following into hard bounces. They may
               # result from a local configuration problem.
               # 4.\d+.\d+ TLS is required, but our TLS engine is unavailable
               # 4.\d+.\d+ TLS is required, but unavailable
               # 4.\d+.\d+ Cannot start TLS: handshake failure

       Example  2: censor the per-recipient delivery status text so that it does not reveal the des‐
       tination command or filename when a remote sender requests confirmation of successful  deliv‐
       ery.

           /etc/postfix/main.cf:
               local_delivery_status_filter = pcre:/etc/postfix/local_dsn_filter

           /etc/postfix/local_dsn_filter:
               /^(2\S+ delivered to file).+/    $1
               /^(2\S+ delivered to command).+/ $1

       Notes:

       •      This feature will NOT override the soft_bounce safety net.

       •      This feature will change the enhanced status code and text that is logged to the mail‐
              log file, and that is reported to the sender in delivery confirmation or  non-delivery
              notifications.

       This feature is available in Postfix 3.0 and later.

**default**___**destination**___**concurrency**___**failed**___**cohort**___**limit** **(default:** **1)**
       How  many pseudo-cohorts must suffer connection or handshake failure before a specific desti‐
       nation is considered unavailable (and further delivery is suspended). Specify zero to disable
       this  feature. A destination's pseudo-cohort failure count is reset each time a delivery com‐
       pletes without connection or handshake failure for that specific destination.

       A pseudo-cohort is the number of deliveries equal to a destination's delivery concurrency.

       Use _transport__destination_concurrency_failed_cohort_limit  to  specify  a  transport-specific
       override, where _transport_ is the master.cf name of the message delivery transport.

       This  feature  is  available  in  Postfix 2.5. The default setting is compatible with earlier
       Postfix versions.

**default**___**destination**___**concurrency**___**limit** **(default:** **20)**
       The default maximal number of parallel deliveries to the same destination.  This is  the  de‐
       fault  limit  for  delivery via the [**lmtp**(8)](https://www.chedong.com/phpMan.php/man/lmtp/8/markdown), [**pipe**(8)](https://www.chedong.com/phpMan.php/man/pipe/8/markdown), [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) and [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agents.
       With per-destination recipient limit > 1, a destination is a domain, otherwise it is a recip‐
       ient.

       Use  _transport__destination_concurrency_limit  to specify a transport-specific override, where
       _transport_ is the master.cf name of the message delivery transport.

**default**___**destination**___**concurrency**___**negative**___**feedback** **(default:** **1)**
       The per-destination amount of delivery concurrency negative feedback, after a  delivery  com‐
       pletes  with  a connection or handshake failure. Feedback values are in the range 0..1 inclu‐
       sive. With negative feedback, concurrency is decremented at the beginning of  a  sequence  of
       length  1/feedback. This is unlike positive feedback, where concurrency is incremented at the
       end of a sequence of length 1/feedback.

       As of Postfix version 2.5, negative feedback cannot reduce delivery concurrency to zero.  In‐
       stead,  a destination is marked dead (further delivery suspended) after the failed pseudo-co‐
       hort count reaches $default_destination_concurrency_failed_cohort_limit (or $_transport__desti‐
       nation_concurrency_failed_cohort_limit).   To make the scheduler completely immune to connec‐
       tion or handshake failures, specify a zero feedback value and  a  zero  failed  pseudo-cohort
       limit.

       Specify one of the following forms:

       _number_

       _number_ / _number_
              Constant feedback. The value must be in the range 0..1 inclusive.  The default setting
              of "1" is compatible with Postfix versions before 2.5, where a destination's  delivery
              concurrency  is throttled down to zero (and further delivery suspended) after a single
              failed pseudo-cohort.

       _number_ / concurrency
              Variable feedback of "_number_ / (delivery concurrency)".  The _number_  must  be  in  the
              range  0..1  inclusive. With _number_ equal to "1", a destination's delivery concurrency
              is decremented by 1 after each failed pseudo-cohort.

       A pseudo-cohort is the number of deliveries equal to a destination's delivery concurrency.

       Use _transport__destination_concurrency_negative_feedback to specify a transport-specific over‐
       ride, where _transport_ is the master.cf name of the message delivery transport.

       This  feature  is  available  in  Postfix 2.5. The default setting is compatible with earlier
       Postfix versions.

**default**___**destination**___**concurrency**___**positive**___**feedback** **(default:** **1)**
       The per-destination amount of delivery concurrency positive feedback, after a  delivery  com‐
       pletes  without connection or handshake failure. Feedback values are in the range 0..1 inclu‐
       sive.  The concurrency increases until it reaches  the  per-destination  maximal  concurrency
       limit.  With  positive  feedback,  concurrency  is  incremented at the end of a sequence with
       length 1/feedback. This is unlike negative feedback, where concurrency is decremented at  the
       start of a sequence of length 1/feedback.

       Specify one of the following forms:

       _number_

       _number_ / _number_
              Constant feedback.  The value must be in the range 0..1 inclusive. The default setting
              of "1" is compatible with Postfix versions before 2.5, where a destination's  delivery
              concurrency doubles after each successful pseudo-cohort.

       _number_ / concurrency
              Variable  feedback  of  "_number_  / (delivery concurrency)".  The _number_ must be in the
              range 0..1 inclusive. With _number_ equal to "1", a destination's  delivery  concurrency
              is incremented by 1 after each successful pseudo-cohort.

       A pseudo-cohort is the number of deliveries equal to a destination's delivery concurrency.

       Use _transport__destination_concurrency_positive_feedback to specify a transport-specific over‐
       ride, where _transport_ is the master.cf name of the message delivery transport.

       This feature is available in Postfix 2.5 and later.

**default**___**destination**___**rate**___**delay** **(default:** **0s)**
       The default amount of delay that is inserted between individual  message  deliveries  to  the
       same  destination  and  over the same message delivery transport. Specify a non-zero value to
       rate-limit those message deliveries to at most one per $default_destination_rate_delay.

       The resulting behavior depends on the value of the  corresponding  per-destination  recipient
       limit.

       •      With a corresponding per-destination recipient limit > 1, the rate delay specifies the
              time between deliveries to the _same_ _domain_.  Different domains are delivered in paral‐
              lel, subject to the process limits specified in master.cf.

       •      With a corresponding per-destination recipient limit equal to 1, the rate delay speci‐
              fies the time between deliveries to the _same_ _recipient_. Different recipients  are  de‐
              livered in parallel, subject to the process limits specified in master.cf.

       To  enable  the  delay,  specify  a  non-zero  time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default  time  unit
       is s (seconds).

       NOTE:  the  delay  is  enforced  by the queue manager. The delay timer state does not survive
       "**postfix** **reload**" or "**postfix** **stop**".

       Use _transport__destination_rate_delay to specify a transport-specific override,  where  _trans__‐
       _port_ is the master.cf name of the message delivery transport.

       NOTE:  with  a  non-zero  _destination_rate_delay,  specify  a  _transport__destination_concur‐
       rency_failed_cohort_limit of 10 or more to prevent Postfix from deferring all  mail  for  the
       same destination after only one connection or handshake error.

       This feature is available in Postfix 2.5 and later.

**default**___**destination**___**recipient**___**limit** **(default:** **50)**
       The default maximal number of recipients per message delivery.  This is the default limit for
       delivery via the [**lmtp**(8)](https://www.chedong.com/phpMan.php/man/lmtp/8/markdown), [**pipe**(8)](https://www.chedong.com/phpMan.php/man/pipe/8/markdown), [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) and [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agents.

       Setting this parameter to a value of 1 affects email deliveries as follows:

       •      It changes the meaning of the corresponding per-destination  concurrency  limit,  from
              concurrency  of  deliveries  to  the _same_ _domain_ into concurrency of deliveries to the
              _same_ _recipient_.  Different recipients  are  delivered  in  parallel,  subject  to  the
              process limits specified in master.cf.

       •      It changes the meaning of the corresponding per-destination rate delay, from the delay
              between deliveries to the _same_ _domain_ into the delay between deliveries  to  the  _same_
              _recipient_.   Again,  different  recipients  are  delivered in parallel, subject to the
              process limits specified in master.cf.

       •      It changes the meaning of other corresponding per-destination settings  in  a  similar
              manner,  from  settings  for delivery to the _same_ _domain_ into settings for delivery to
              the _same_ _recipient_.

       Use _transport__destination_recipient_limit to specify  a  transport-specific  override,  where
       _transport_ is the master.cf name of the message delivery transport.

**default**___**extra**___**recipient**___**limit** **(default:** **1000)**
       The default value for the extra per-transport limit imposed on the number of in-memory recip‐
       ients.  This extra recipient space is reserved for the cases when the Postfix queue manager's
       scheduler  preempts  one  message with another and suddenly needs some extra recipients slots
       for the chosen message in order to avoid performance degradation.

       Use _transport__extra_recipient_limit to specify a transport-specific override, where _transport_
       is the master.cf name of the message delivery transport.

**default**___**filter**___**nexthop** **(default:** **empty)**
       When  a content_filter or FILTER request specifies no explicit next-hop destination, use $de‐
       fault_filter_nexthop instead; when that value is empty, use the domain in the  recipient  ad‐
       dress.  Specify "default_filter_nexthop = $myhostname" for compatibility with Postfix version
       2.6 and earlier, or specify an explicit next-hop destination with each  content_filter  value
       or FILTER action.

       This feature is available in Postfix 2.7 and later.

**default**___**minimum**___**delivery**___**slots** **(default:** **3)**
       How many recipients a message must have in order to invoke the Postfix queue manager's sched‐
       uling algorithm at all.  Messages which would never accumulate at least  this  many  delivery
       slots (subject to slot cost parameter as well) are never preempted.

       Use  _transport__minimum_delivery_slots  to specify a transport-specific override, where _trans__‐
       _port_ is the master.cf name of the message delivery transport.

**default**___**privs** **(default:** **nobody)**
       The default rights used by the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent for delivery to external file or  com‐
       mand.  These rights are used when delivery is requested from an [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) file that is owned
       by **root**, or when delivery is done on behalf of **root**. **DO** **NOT** **SPECIFY** **A** **PRIVILEGED** **USER** **OR**  **THE**
       **POSTFIX** **OWNER**.

**default**___**process**___**limit** **(default:** **100)**
       The  default  maximal  number  of  Postfix child processes that provide a given service. This
       limit can be overruled for specific services in the master.cf file.

**default**___**rbl**___**reply** **(default:** **see** **postconf** **-d** **output)**
       The default Postfix SMTP server response template for  a  request  that  is  rejected  by  an
       RBL-based  restriction.  This  template  can be overruled by specific entries in the optional
       rbl_reply_maps lookup table.

       This feature is available in Postfix 2.0 and later.

       The template does not support Postfix configuration parameter $name substitution. Instead, it
       supports exactly one level of $name substitution for the following attributes:

### $client
              The client hostname and IP address, formatted as name[address].

       **$client**___**address**
              The client IP address.

       **$client**___**name**
              The client hostname or "unknown". See reject_unknown_client_hostname for more details.

       **$reverse**___**client**___**name**
              The  client  hostname from address->name lookup, or "unknown".  See reject_unknown_re‐
              verse_client_hostname for more details.

       **$helo**___**name**
              The hostname given in HELO or EHLO command or empty string.

       **$rbl**___**class**
              The denylisted entity type: Client host, Helo command, Sender  address,  or  Recipient
              address.

       **$rbl**___**code**
              The  numerical SMTP response code, as specified with the maps_rbl_reject_code configu‐
              ration parameter. Note: The numerical SMTP response code is required, and must  appear
              at  the start of the reply. With Postfix version 2.3 and later this information may be
              followed by an RFC 3463 enhanced status code.

       **$rbl**___**domain**
              The RBL domain where $rbl_what is denylisted.

       **$rbl**___**reason**
              The reason why $rbl_what is denylisted, or an empty string.

       **$rbl**___**what**
              The entity that is denylisted (an IP address, a hostname, a domain name, or  an  email
              address whose domain was denylisted).

### $recipient
              The recipient address or <> in case of the null address.

       **$recipient**___**domain**
              The recipient domain or empty string.

       **$recipient**___**name**
              The recipient address localpart or <> in case of null address.

### $sender
              The sender address or <> in case of the null address.

       **$sender**___**domain**
              The sender domain or empty string.

       **$sender**___**name**
              The sender address localpart or <> in case of the null address.

### ${name?text}
              Expands to `text' if $name is not empty.

### ${name:text}
              Expands to `text' if $name is empty.

       Instead of $name you can also specify ${name} or $(name).

       Note:  when  an  enhanced status code is specified in an RBL reply template, it is subject to
       modification.  The following transformations are needed when the same RBL reply  template  is
       used for client, helo, sender, or recipient access restrictions.

       •      When  rejecting  a  sender address, the Postfix SMTP server will transform a recipient
              DSN status (e.g., 4.1.1-4.1.6) into the corresponding  sender  DSN  status,  and  vice
              versa.

       •      When  rejecting  non-address  information  (such  as  the HELO command argument or the
              client hostname/address), the Postfix SMTP server will transform a sender or recipient
              DSN status into a generic non-address DSN status (e.g., 4.0.0).

**default**___**recipient**___**limit** **(default:** **20000)**
       The  default  per-transport  upper limit on the number of in-memory recipients.  These limits
       take priority over the global qmgr_message_recipient_limit after the  message  has  been  as‐
       signed  to  the  respective transports.  See also default_extra_recipient_limit and qmgr_mes‐
       sage_recipient_minimum.

       Use _transport__recipient_limit to specify a transport-specific override,  where  _transport_  is
       the master.cf name of the message delivery transport.

**default**___**recipient**___**refill**___**delay** **(default:** **5s)**
       The default per-transport maximum delay between recipients refills.  When not all message re‐
       cipients fit into the memory at once, keep loading more of them at least once every this many
       seconds.   This  is  used to make sure the recipients are refilled in timely manner even when
       $default_recipient_refill_limit is too high for too slow deliveries.

       Use _transport__recipient_refill_delay to specify a transport-specific override,  where  _trans__‐
       _port_ is the master.cf name of the message delivery transport.

       This feature is available in Postfix 2.4 and later.

**default**___**recipient**___**refill**___**limit** **(default:** **100)**
       The  default  per-transport limit on the number of recipients refilled at once.  When not all
       message recipients fit into the memory at once, keep loading more of them in  batches  of  at
       least this many at a time.  See also $default_recipient_refill_delay, which may result in re‐
       cipient batches lower than this when this limit is too high for too slow deliveries.

       Use _transport__recipient_refill_limit to specify a transport-specific override,  where  _trans__‐
       _port_ is the master.cf name of the message delivery transport.

       This feature is available in Postfix 2.4 and later.

**default**___**transport** **(default:** **smtp)**
       The  default  mail  delivery  transport and next-hop destination for destinations that do not
       match  $mydestination,  $inet_interfaces,  $proxy_interfaces,  $virtual_alias_domains,  $vir‐
       tual_mailbox_domains,  or  $relay_domains.   This  information  can  be  overruled  with  the
       sender_dependent_default_transport_maps parameter and with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       In order of decreasing precedence, the  nexthop  destination  is  taken  from  $sender_depen‐
       dent_default_transport_maps,  $default_transport,  $sender_dependent_relayhost_maps,  $relay‐
       host, or from the recipient domain.

       Specify a string of the form _transport:nexthop_, where _transport_ is the name of a mail  deliv‐
       ery transport defined in master.cf.  The _:nexthop_ destination is optional; its syntax is doc‐
       umented in the manual page of the corresponding delivery agent. In the case of SMTP or  LMTP,
       specify  one  or  more  destinations  separated  by comma or whitespace (with Postfix 3.5 and
       later).

       Example:

       default_transport = uucp:relayhostname

**default**___**transport**___**rate**___**delay** **(default:** **0s)**
       The default amount of delay that is inserted between individual message deliveries  over  the
       same  message  delivery  transport,  regardless  of  destination. Specify a non-zero value to
       rate-limit those message deliveries to at most one per $default_transport_rate_delay.

       Use _transport__transport_rate_delay to specify a transport-specific override, where  the  ini‐
       tial _transport_ is the master.cf name of the message delivery transport.

       Example: throttle outbound SMTP mail to at most 3 deliveries per minute.

       /etc/postfix/main.cf:
           smtp_transport_rate_delay = 20s

       To  enable  the  delay,  specify  a  non-zero  time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default  time  unit
       is s (seconds).

       NOTE: the delay is enforced by the queue manager.

       This feature is available in Postfix 3.1 and later.

**default**___**verp**___**delimiters** **(default:** **+=)**
       The  two  default  VERP  delimiter characters. These are used when no explicit delimiters are
       specified with the SMTP XVERP command or with the "**sendmail** **-V**" command-line option.  Specify
       characters that are allowed by the verp_delimiter_filter setting.

       This feature is available in Postfix 1.1 and later.

**defer**___**code** **(default:** **450)**
       The numerical Postfix SMTP server response code when a remote SMTP client request is rejected
       by the "defer" restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

**defer**___**service**___**name** **(default:** **defer)**
       The name of the defer service. This service is implemented by the [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown) daemon and  main‐
       tains a record of failed delivery attempts and generates non-delivery notifications.

       This feature is available in Postfix 2.0 and later.

**defer**___**transports** **(default:** **empty)**
       The  names  of message delivery transports that should not deliver mail unless someone issues
       "**sendmail** **-q**" or equivalent. Specify zero or more names of  mail  delivery  transports  names
       that appear in the first field of master.cf.

       Example:

       defer_transports = smtp

**delay**___**logging**___**resolution**___**limit** **(default:** **2)**
       The  maximal  number  of digits after the decimal point when logging sub-second delay values.
       Specify a number in the range 0..6.

       Large delay values are rounded off to an integral number seconds; delay values below the  de‐
       lay_logging_resolution_limit  are  logged as "0", and delay values under 100s are logged with
       at most two-digit precision.

       The format of the "delays=a/b/c/d" logging is as follows:

       •      a = time from message arrival to last active queue entry

       •      b = time from last active queue entry to connection setup

       •      c = time in connection setup, including DNS, EHLO and STARTTLS

       •      d = time in message transmission

       This feature is available in Postfix 2.3 and later.

**delay**___**notice**___**recipient** **(default:** **postmaster)**
       The recipient of postmaster notifications with the message headers of mail that cannot be de‐
       livered within $delay_warning_time time units.

       See also: delay_warning_time, notify_classes.

**delay**___**warning**___**time** **(default:** **0h)**
       The  time after which the sender receives a copy of the message headers of mail that is still
       queued. The confirm_delay_cleared parameter  controls  sender  notification  when  the  delay
       clears up.

       To  enable  this  feature,  specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is h (hours).

       See also: delay_notice_recipient, notify_classes, confirm_delay_cleared.

**deliver**___**lock**___**attempts** **(default:** **20)**
       The  maximal  number  of attempts to acquire an exclusive lock on a mailbox file or [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown)
       logfile.

**deliver**___**lock**___**delay** **(default:** **1s)**
       The time between attempts to acquire an exclusive lock on a mailbox file  or  [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown)  log‐
       file.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**destination**___**concurrency**___**feedback**___**debug** **(default:** **no)**
       Make the queue manager's feedback algorithm verbose for performance analysis purposes.

       This feature is available in Postfix 2.5 and later.

**detect**___**8bit**___**encoding**___**header** **(default:** **yes)**
       Automatically detect 8BITMIME body content by looking at  Content-Transfer-Encoding:  message
       headers; historically, this behavior was hard-coded to be "always on".

       This feature is available in Postfix 2.5 and later.

**disable**___**dns**___**lookups** **(default:** **no)**
       Disable  DNS lookups in the Postfix SMTP and LMTP clients. When disabled, hosts are looked up
       with the getaddrinfo() system library routine which normally also looks in /etc/hosts.  As of
       Postfix 2.11, this parameter is deprecated; use smtp_dns_support_level instead.

       DNS lookups are enabled by default.

**disable**___**mime**___**input**___**processing** **(default:** **no)**
       Turn  off MIME processing while receiving mail. This means that no special treatment is given
       to Content-Type: message headers, and that all text after the initial message headers is con‐
       sidered to be part of the message body.

       This feature is available in Postfix 2.0 and later.

       Mime input processing is enabled by default, and is needed in order to recognize MIME headers
       in message content.

**disable**___**mime**___**output**___**conversion** **(default:** **no)**
       Disable the conversion of 8BITMIME format to 7BIT format.  Mime output conversion  is  needed
       when the destination does not advertise 8BITMIME support.

       This feature is available in Postfix 2.0 and later.

**disable**___**verp**___**bounces** **(default:** **no)**
       Disable sending one bounce report per recipient.

       The default, one per recipient, is what ezmlm needs.

       This feature is available in Postfix 1.1 and later.

**disable**___**vrfy**___**command** **(default:** **no)**
       Disable the SMTP VRFY command. This stops some techniques used to harvest email addresses.

       Example:

       disable_vrfy_command = no

**dns**___**ncache**___**ttl**___**fix**___**enable** **(default:** **no)**
       Enable  a  workaround for future libc incompatibility. The Postfix implementation of RFC 2308
       negative reply caching relies  on  the  promise  that  res_query()  and  res_search()  invoke
       res_send(),  which returns the server response in an application buffer even if the requested
       record does not exist. If this promise is broken, specify "yes" to enable a   workaround  for
       DNS reputation lookups.

       This feature is available in Postfix 3.1 and later.

**dnsblog**___**reply**___**delay** **(default:** **0s)**
       A debugging aid to artificially delay DNS responses.

       This feature is available in Postfix 2.8.

**dnsblog**___**service**___**name** **(default:** **dnsblog)**
       The  name  of  the  [**dnsblog**(8)](https://www.chedong.com/phpMan.php/man/dnsblog/8/markdown)  service  entry  in  master.cf.  This service performs DNS al‐
       low/denylist lookups.

       This feature is available in Postfix 2.8 and later.

**dnssec**___**probe** **(default:** **ns:.)**
       The DNS query type (default: "ns") and DNS query name (default: ".") that Postfix may use  to
       determine whether DNSSEC validation is available.

       Background:  DNSSEC  validation is needed for Postfix DANE support; this ensures that Postfix
       receives TLSA records with secure TLS server certificate info. When DNSSEC validation is  un‐
       available,  mail deliveries using _opportunistic_ DANE will not be protected by server certifi‐
       cate info in TLSA records, and mail deliveries using _mandatory_ DANE will not be made at all.

       By default, a Postfix process will send a DNSSEC probe after 1) the process made a DNS  query
       that  requested DNSSEC validation, 2) the process did not receive a DNSSEC validated response
       to this query or to an earlier query, and 3) the process did not already send a DNSSEC probe.

       When the DNSSEC probe has no response, or when the response is not DNSSEC validated,  Postfix
       logs a warning that DNSSEC validation may be unavailable.

       Example:

       warning: DNSSEC validation may be unavailable
       warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated
       warning: reason: dnssec_probe 'ns:.' received no response: Server failure

       Possible reasons why DNSSEC validation may be unavailable:

       •      The local /etc/resolv.conf file specifies a DNS resolver that does not validate DNSSEC
              signatures (that's $queue_directory/etc/resolv.conf when a Postfix daemon  runs  in  a
              chroot jail).

       •      The  local  system  library does not pass on the "DNSSEC validated" bit to Postfix, or
              Postfix does not know how to ask the library to do that.

       By default, the DNSSEC probe asks for the DNS root zone NS records, because resolvers  should
       always  have that information cached. If Postfix runs on a network where the DNS root zone is
       not reachable, specify a different probe, or specify an empty dnssec_probe value  to  disable
       the feature.

       This  feature  is  available  in Postfix 3.6 and later. It was backported to Postfix versions
       3.5.9, 3.4.19, 3.3.16. 3.2.21.

**dont**___**remove** **(default:** **0)**
       Don't remove queue files and save them to the "saved" mail queue.  This is a  debugging  aid.
       To  inspect  the envelope information and content of a Postfix queue file, use the [**postcat**(1)](https://www.chedong.com/phpMan.php/man/postcat/1/markdown)
       command.

**double**___**bounce**___**sender** **(default:** **double-bounce)**
       The sender address of postmaster notifications that are generated by  the  mail  system.  All
       mail to this address is silently discarded, in order to terminate mail bounce loops.

**duplicate**___**filter**___**limit** **(default:** **1000)**
       The  maximal number of addresses remembered by the address duplicate filter for [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) or
       [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown) alias expansion, or for [**showq**(8)](https://www.chedong.com/phpMan.php/man/showq/8/markdown) queue displays.

**empty**___**address**___**default**___**transport**___**maps**___**lookup**___**key** **(default:** **<>)**
       The sender_dependent_default_transport_maps search string that will be used  instead  of  the
       null sender address.

       This feature is available in Postfix 2.7 and later.

**empty**___**address**___**local**___**login**___**sender**___**maps**___**lookup**___**key** **(default:** **<>)**
       The  lookup  key to be used in local_login_sender_maps tables, instead of the null sender ad‐
       dress.

       This feature is available in Postfix 3.6 and later.

**empty**___**address**___**recipient** **(default:** **MAILER-DAEMON)**
       The recipient of mail addressed to the null address.  Postfix does not accept such  addresses
       in  SMTP  commands,  but  they may still be created locally as the result of configuration or
       software error.

**empty**___**address**___**relayhost**___**maps**___**lookup**___**key** **(default:** **<>)**
       The sender_dependent_relayhost_maps search string that will  be  used  instead  of  the  null
       sender address.

       This  feature  is  available  in  Postfix 2.5 and later. With earlier versions, sender_depen‐
       dent_relayhost_maps lookups were skipped for the null sender address.

**enable**___**errors**___**to** **(default:** **no)**
       Report mail delivery errors to the address specified with the non-standard Errors-To: message
       header,  instead of the envelope sender address (this feature is removed with Postfix version
       2.2, is turned off by default with Postfix version 2.1, and is always turned  on  with  older
       Postfix versions).

**enable**___**idna2003**___**compatibility** **(default:** **no)**
       Enable  'transitional' compatibility between IDNA2003 and IDNA2008, when converting UTF-8 do‐
       main names to/from the ASCII form that is used for DNS lookups. Specify "yes" for compatibil‐
       ity  with  Postfix <= 3.1 (not recommended). This affects the conversion of domain names that
       contain for example the German sz and  the  Greek  zeta.   See  <http://unicode.org/cldr/util>‐
       ity/idna.jsp for more examples.

       This feature is available in Postfix 3.2 and later.

**enable**___**long**___**queue**___**ids** **(default:** **no)**
       Enable long, non-repeating, queue IDs (queue file names).  The benefit of non-repeating names
       is simpler logfile analysis and easier queue migration (there is no need to  run  "postsuper"
       to change queue file names that don't match their message file inode number).

       Note: see below for how to convert long queue file names to Postfix <= 2.8.

       Changing the parameter value to "yes" has the following effects:

       •      Existing queue file names are not affected.

       •      New  queue files are created with names such as 3Pt2mN2VXxznjll.  These are encoded in
              a 52-character alphabet that contains  digits  (0-9),  upper-case  letters  (B-Z)  and
              lower-case letters (b-z). For safety reasons the vowels (AEIOUaeiou) are excluded from
              the alphabet.  The name format is: 6 or more characters for the  time  in  seconds,  4
              characters for the time in microseconds, the 'z'; the remainder is the file inode num‐
              ber encoded in the first 51 characters of the 52-character alphabet.

       •      New messages have a Message-ID header with _queueID_@_myhostname_.

       •      The mailq (postqueue -p) output has a wider Queue ID column.   The  number  of  white‐
              space-separated fields is not changed.

       •      The  hash_queue_depth  algorithm  uses the first characters of the queue file creation
              time in microseconds, after conversion into hexadecimal representation. This  produces
              the  same  queue  hashing  behavior  as  if  the queue file name was created with "en‐
              able_long_queue_ids = no".

       Changing the parameter value to "no" has the following effects:

       •      Existing long queue file names are renamed to the short form (while  running  "postfix
              reload" or "postsuper").

       •      New queue files are created with names such as C3CD21F3E90 from a hexadecimal alphabet
              that contains digits (0-9) and upper-case letters (A-F). The name format is: 5 charac‐
              ters for the time in microseconds; the remainder is the file inode number.

       •      New  messages  have  a Message-ID header with _YYYYMMDDHHMMSS.queueid_@_myhostname_, where
              _YYYYMMDDHHMMSS_ are the year, month, day, hour, minute and second.

       •      The mailq (postqueue -p) output has the same format as with Postfix <= 2.8.

       •      The hash_queue_depth algorithm uses the first characters of the queue file name,  with
              the hexadecimal representation of the file creation time in microseconds.

       Before migration to Postfix <= 2.8, the following commands are required to convert long queue
       file names into short names:

       # postfix stop
       # postconf enable_long_queue_ids=no
       # postsuper

       Repeat the postsuper command until it reports no more queue file name changes.

       This feature is available in Postfix 2.9 and later.

**enable**___**original**___**recipient** **(default:** **yes)**
       Enable support for the original recipient address after an address is rewritten to a  differ‐
       ent address (for example with aliasing or with canonical mapping).

       The original recipient address is used as follows:

       Final delivery
              With  "enable_original_recipient  =  yes", the original recipient address is stored in
              the **X-Original-To** message header. This header may be used to distinguish between  dif‐
              ferent recipients that share the same mailbox.

       Recipient deduplication
              With  "enable_original_recipient  = yes", the [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) daemon performs duplicate re‐
              cipient elimination based on the content of (original recipient,  maybe-rewritten  re‐
              cipient)  pairs.  Otherwise, the [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) daemon performs duplicate recipient elimi‐
              nation based only on the maybe-rewritten recipient address.

       Note: with Postfix <= 3.2 the "setting enable_original_recipient = **no**" breaks address verifi‐
       cation  for addresses that are aliased or otherwise rewritten (Postfix is unable to store the
       address verification result under the original probe destination  address;  instead,  it  can
       store the result only under the rewritten address).

       This  feature  is  available in Postfix 2.1 and later. Postfix version 2.0 behaves as if this
       parameter is always set to **yes**.  Postfix versions before 2.0 have no support for the original
       recipient address.

**enable**___**threaded**___**bounces** **(default:** **no)**
       Enable  non-delivery,  success,  and delay notifications that link to the original message by
       including a References: and In-Reply-To: header with the original Message-ID value. There are
       advantages and disadvantages to consider.

### advantage
              This  allows  mail readers to present a delivery status notification in the same email
              thread as the original message.

### disadvantage
              This makes it easy for users to mistakenly delete the whole email thread (all  related
              messages), instead of deleting only the non-delivery notification.

       This feature is available in Postfix 3.6 and later.

**error**___**notice**___**recipient** **(default:** **postmaster)**
       The  recipient  of  postmaster  notifications about mail delivery problems that are caused by
       policy, resource, software or protocol errors.  These notifications are enabled with the  no‐
       tify_classes parameter.

**error**___**service**___**name** **(default:** **error)**
       The  name of the [**error**(8)](https://www.chedong.com/phpMan.php/man/error/8/markdown) pseudo delivery agent. This service always returns mail as undeliv‐
       erable.

       This feature is available in Postfix 2.0 and later.

**execution**___**directory**___**expansion**___**filter** **(default:** **see** **postconf** **-d** **output)**
       Restrict the characters that the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent allows in $name expansions of  $com‐
       mand_execution_directory.  Characters outside the allowed set are replaced by underscores.

       This feature is available in Postfix 2.2 and later.

**expand**___**owner**___**alias** **(default:** **no)**
       When  delivering  to  an alias "_aliasname_" that has an "owner-_aliasname_" companion alias, set
       the envelope sender address to the expansion of the "owner-_aliasname_" alias.  Normally, Post‐
       fix sets the envelope sender address to the name of the "owner-_aliasname_" alias.

**export**___**environment** **(default:** **see** **postconf** **-d** **output)**
       The  list  of  environment  variables  that a Postfix process will export to non-Postfix pro‐
       cesses. The TZ variable is needed for sane time keeping on System-V-ish systems.

       Specify a list of names and/or name=value pairs, separated by whitespace or comma. Specify "{
       name=value  }" to protect whitespace or comma in parameter values (whitespace after the open‐
       ing "{" and before the closing "}" is ignored). The form name=value is supported with Postfix
       version 2.1 and later; the use of {} is supported with Postfix 3.0 and later.

       Example:

       export_environment = TZ PATH=/bin:/usr/bin

**extract**___**recipient**___**limit** **(default:** **10240)**
       The maximal number of recipient addresses that Postfix will extract from message headers when
       mail is submitted with "**sendmail** **-t**".

       This feature was removed in Postfix version 2.1.

**fallback**___**relay** **(default:** **empty)**
       Optional list of relay hosts for SMTP destinations that can't be found or that  are  unreach‐
       able. With Postfix 2.3 this parameter is renamed to smtp_fallback_relay.

       By  default,  mail is returned to the sender when a destination is not found, and delivery is
       deferred when a destination is unreachable.

       The  fallback  relays  must  be  SMTP  destinations.  Specify  a  domain,  host,   host:port,
       [host]:port, [address] or [address]:port; the form [host] turns off MX lookups.  If you spec‐
       ify multiple SMTP destinations, Postfix will try them in the specified order.

       Note: before Postfix 2.2, do not use the fallback_relay feature  when  relaying  mail  for  a
       backup or primary MX domain. Mail would loop between the Postfix MX host and the fallback_re‐
       lay host when the final destination is unavailable.

       •      In main.cf specify "relay_transport = relay",

       •      In master.cf specify "-o fallback_relay =" (i.e., empty) at the end of the  relay  en‐
              try.

       •      In  transport  maps,  specify "relay:_nexthop..._"  as the right-hand side for backup or
              primary MX domain entries.

       Postfix version 2.2 and later will not use the fallback_relay feature for  destinations  that
       it is MX host for.

**fallback**___**transport** **(default:** **empty)**
       Optional  message  delivery  transport  that the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent should use for names
       that are not found in the [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) or UNIX password database.

       The precedence of [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is:  aliases,  .forward  files,
       mailbox_transport_maps,  mailbox_transport, mailbox_command_maps, mailbox_command, home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

**fallback**___**transport**___**maps** **(default:** **empty)**
       Optional lookup tables with per-recipient message delivery transports for recipients that the
       [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent could not find in the [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) or UNIX password database.

       The  precedence  of  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps, mailbox_transport, mailbox_command_maps, mailbox_command,  home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

       For  safety  reasons, this feature does not allow $number substitutions in regular expression
       maps.

       This feature is available in Postfix 2.3 and later.

**fast**___**flush**___**domains** **(default:** **$relay**___**domains)**
       Optional list of destinations that are eligible for per-destination logfiles with  mail  that
       is queued to those destinations.

       By  default,  Postfix  maintains "fast flush" logfiles only for destinations that the Postfix
       SMTP server is willing to relay to (i.e. the default  is:  "fast_flush_domains  =  $relay_do‐
       mains"; see the relay_domains parameter in the [**postconf**(5)](https://www.chedong.com/phpMan.php/man/postconf/5/markdown) manual).

       Specify a list of hosts or domains, "/file/name" patterns or "type:table" lookup tables, sep‐
       arated by commas and/or whitespace.  Continue long lines  by  starting  the  next  line  with
       whitespace.  A  "/file/name" pattern is replaced by its contents; a "type:table" lookup table
       is matched when the domain or its parent domain appears as lookup key.

       Pattern matching of domain names is controlled by the presence or absence of  "fast_flush_do‐
       mains" in the parent_domain_matches_subdomains parameter value.

       Specify "fast_flush_domains =" (i.e., empty) to disable the feature altogether.

**fast**___**flush**___**purge**___**time** **(default:** **7d)**
       The time after which an empty per-destination "fast flush" logfile is deleted.

       You  can specify the time as a number, or as a number followed by a letter that indicates the
       time unit: s=seconds, m=minutes, h=hours, d=days, w=weeks.  The default time unit is days.

**fast**___**flush**___**refresh**___**time** **(default:** **12h)**
       The time after which a non-empty but unread per-destination "fast flush" logfile needs to  be
       refreshed.   The  contents  of a logfile are refreshed by requesting delivery of all messages
       listed in the logfile.

       You can specify the time as a number, or as a number followed by a letter that indicates  the
       time unit: s=seconds, m=minutes, h=hours, d=days, w=weeks.  The default time unit is hours.

**fault**___**injection**___**code** **(default:** **0)**
       Force  specific  internal tests to fail, to test the handling of errors that are difficult to
       reproduce otherwise.

**flush**___**service**___**name** **(default:** **flush)**
       The name of the [**flush**(8)](https://www.chedong.com/phpMan.php/man/flush/8/markdown) service. This service maintains per-destination  logfiles  with  the
       queue file names of mail that is queued for those destinations.

       This feature is available in Postfix 2.0 and later.

**fork**___**attempts** **(default:** **5)**
       The maximal number of attempts to fork() a child process.

**fork**___**delay** **(default:** **1s)**
       The delay between attempts to fork() a child process.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**forward**___**expansion**___**filter** **(default:** **see** **postconf** **-d** **output)**
       Restrict the characters that the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent allows in $name expansions of  $for‐
       ward_path.  Characters outside the allowed set are replaced by underscores.

**forward**___**path** **(default:** **see** **postconf** **-d** **output)**
       The  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery agent search list for finding a .forward file with user-specified de‐
       livery methods. The first file that is found is used.

       The forward_path value is not subject to Postfix configuration parameter $name expansion. In‐
       stead,  the  following  $name  expansions are done on forward_path before the search actually
       happens.  The result of $name expansion is filtered with the character set that is  specified
       with the forward_expansion_filter parameter.

       **$user**  The recipient's username.

       **$shell** The recipient's login shell pathname.

       **$home**  The recipient's home directory.

### $recipient
              The full recipient address.

### $extension
              The optional recipient address extension.

### $domain
              The recipient domain.

       **$local** The entire recipient localpart.

       **$recipient**___**delimiter**
              The  address extension delimiter that was found in the recipient address (Postfix 2.11
              and later), or the system-wide recipient address extension delimiter (Postfix 2.10 and
              earlier).

### ${name?value}
              Expands to _value_ when _$name_ is non-empty.

### ${name:value}
              Expands to _value_ when _$name_ is empty.

       Instead of $name you can also specify ${name} or $(name).

       Examples:

       forward_path = /var/forward/$user
       forward_path =
           /var/forward/$user/.forward$recipient_delimiter$extension,
           /var/forward/$user/.forward

**frozen**___**delivered**___**to** **(default:** **yes)**
       Update  the  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery agent's idea of the Delivered-To: address (see prepend_deliv‐
       ered_header) only once, at the start of a delivery attempt; do not update  the  Delivered-To:
       address while expanding aliases or .forward files.

       This feature is available in Postfix 2.3 and later. With older Postfix releases, the behavior
       is as if this parameter is set to "no". The old setting can be expensive with  deeply  nested
       aliases  or .forward files. When an alias or .forward file changes the Delivered-To: address,
       it ties up one queue file and one cleanup process instance while mail is being forwarded.

**hash**___**queue**___**depth** **(default:** **1)**
       The number of subdirectory levels for queue directories listed with the hash_queue_names  pa‐
       rameter.  Queue  hashing  is  implemented  by creating one or more levels of directories with
       one-character names.  Originally, these directory names were equal to the first characters of
       the  queue  file  name,  with the hexadecimal representation of the file creation time in mi‐
       croseconds.

       With long queue file names, queue hashing produces the same results as with short names.  The
       file  creation  time  in microseconds is converted into hexadecimal form before the result is
       used for queue hashing.  The base 16 encoding gives finer control over the number  of  subdi‐
       rectories than is possible with the base 52 encoding of long queue file names.

       After changing the hash_queue_names or hash_queue_depth parameter, execute the command "**post**‐‐
       **fix** **reload**".

**hash**___**queue**___**names** **(default:** **deferred,** **defer)**
       The names of queue directories that are split across multiple subdirectory levels.

       Before Postfix version 2.2, the default list  of  hashed  queues  was  significantly  larger.
       Claims  about improvements in file system technology suggest that hashing of the incoming and
       active queues is no longer needed. Fewer hashed directories  speed  up  the  time  needed  to
       restart Postfix.

       After changing the hash_queue_names or hash_queue_depth parameter, execute the command "**post**‐‐
       **fix** **reload**".

**header**___**address**___**token**___**limit** **(default:** **10240)**
       The maximal number of address tokens are allowed in an address  message  header.  Information
       that exceeds the limit is discarded.  The limit is enforced by the [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) server.

**header**___**checks** **(default:** **empty)**
       Optional  lookup tables for content inspection of primary non-MIME message headers, as speci‐
       fied in the **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) manual page.

**header**___**from**___**format** **(default:** **standard)**
       The format of the Postfix-generated **From:** header. This  setting  affects  the  appearance  of
       'full  name'  information  when  a  local program such as /bin/mail submits a message without
       From: header through the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command.

       Specify one of the following:

### standard (default)
              Produce a header formatted as "**From:** _name_ **<**_address_**>**".  This is the default as of Post‐
              fix 3.3.

### obsolete
              Produce  a  header  formatted as "**From:** _address_ **(**_name_**)**". This is the behavior prior to
              Postfix 3.3.

       Notes:

       •      Postfix generates the format "**From:** _address_" when _name_ information is  unavailable  or
              the  envelope  sender  address is empty. This is the same behavior as prior to Postfix
              3.3.

       •      In the **standard** form, the _name_ will be quoted if it contains **specials**  as  defined  in
              RFC 5322, or the "!%" address operators.

       •      The Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command gets _name_ information from the **-F** command-line option,
              from the **NAME** environment variable, or from the UNIX password file.

       This feature is available in Postfix 3.3 and later.

**header**___**size**___**limit** **(default:** **102400)**
       The maximal amount of memory in bytes for storing a message header.  If a header  is  larger,
       the excess is discarded.  The limit is enforced by the [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) server.

**helpful**___**warnings** **(default:** **yes)**
       Log warnings about problematic configuration settings, and provide helpful suggestions.

       This feature is available in Postfix 2.0 and later.

**home**___**mailbox** **(default:** **empty)**
       Optional pathname of a mailbox file relative to a [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) user's home directory.

       Specify a pathname ending in "/" for qmail-style delivery.

       The  precedence  of  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps, mailbox_transport, mailbox_command_maps, mailbox_command,  home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

       Examples:

       home_mailbox = Mailbox
       home_mailbox = Maildir/

**hopcount**___**limit** **(default:** **50)**
       The maximal number of Received:  message headers that is allowed in the primary message head‐
       ers. A message that exceeds the limit is bounced, in order to stop a mailer loop.

**html**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The location of Postfix HTML files that describe how to build, configure or  operate  a  spe‐
       cific Postfix subsystem or feature.

**ignore**___**mx**___**lookup**___**error** **(default:** **no)**
       Ignore  DNS  MX lookups that produce no response.  By default, the Postfix SMTP client defers
       delivery and tries again after some delay.  This behavior is required by the SMTP standard.

       Specify "ignore_mx_lookup_error = yes" to force a DNS A record lookup instead. This  violates
       the SMTP standard and can result in mis-delivery of mail.

**import**___**environment** **(default:** **see** **postconf** **-d** **output)**
       The  list  of  environment  parameters  that  a privileged Postfix process will import from a
       non-Postfix parent process, or name=value environment overrides.  Unprivileged utilities will
       enforce  the  name=value  overrides, but otherwise will not change their process environment.
       Examples of relevant parameters:

       **TZ**     May be needed for sane time keeping on most System-V-ish systems.

       **DISPLAY**
              Needed for debugging Postfix daemons with an X-windows debugger.

       **XAUTHORITY**
              Needed for debugging Postfix daemons with an X-windows debugger.

       **MAIL**___**CONFIG**
              Needed to make "**postfix** **-c**" work.

       Specify a list of names and/or name=value pairs, separated by whitespace or comma. Specify "{
       name=value  }" to protect whitespace or comma in parameter values (whitespace after the open‐
       ing "{" and before the closing "}" is ignored). The form name=value is supported with Postfix
       version 2.1 and later; the use of {} is supported with Postfix 3.0 and later.

**in**___**flow**___**delay** **(default:** **1s)**
       Time  to pause before accepting a new message, when the message arrival rate exceeds the mes‐
       sage delivery rate. This feature is turned on by default (it's disabled on SCO UNIX due to an
       SCO bug).

       With  the default 100 Postfix SMTP server process limit, "in_flow_delay = 1s" limits the mail
       inflow to 100 messages per second above the number of messages delivered per second.

       Specify 0 to disable the feature. Valid delays are 0..10.

**inet**___**interfaces** **(default:** **all)**
       The network interface addresses that this mail system receives mail on. Specify "all" to  re‐
       ceive  mail on all network interfaces (default), and "loopback-only" to receive mail on loop‐
       back network interfaces only (Postfix version 2.2 and later).  The  parameter  also  controls
       delivery of mail to user@[ip.address].

       Note 1: you need to stop and start Postfix when this parameter changes.

       Note 2: address information may be enclosed inside [], but this form is not required here.

       When  inet_interfaces  specifies just one IPv4 and/or IPv6 address that is not a loopback ad‐
       dress, the Postfix SMTP client will use this address as the IP source  address  for  outbound
       mail. Support for IPv6 is available in Postfix version 2.2 and later.

       On a multi-homed firewall with separate Postfix instances listening on the "inside" and "out‐
       side" interfaces, this can prevent each instance from being able to reach remote SMTP servers
       on  the  "other side" of the firewall. Setting smtp_bind_address to 0.0.0.0 avoids the poten‐
       tial problem for IPv4, and setting smtp_bind_address6 to :: solves the problem for IPv6.

       A better solution for multi-homed firewalls is to leave inet_interfaces at the default  value
       and  instead  use  explicit IP addresses in the master.cf SMTP server definitions.  This pre‐
       serves the Postfix SMTP client's loop detection, by ensuring that each side of  the  firewall
       knows  that the other IP address is still the same host. Setting $inet_interfaces to a single
       IPv4 and/or IPV6 address is primarily useful with virtual hosting of domains on secondary  IP
       addresses,  when  each  IP address serves a different domain (and has a different $myhostname
       setting).

       See also the proxy_interfaces parameter, for network addresses that are forwarded to  Postfix
       by way of a proxy or address translator.

       Examples:

       inet_interfaces = all (DEFAULT)
       inet_interfaces = loopback-only (Postfix version 2.2 and later)
       inet_interfaces = 127.0.0.1
       inet_interfaces = 127.0.0.1, [::1] (Postfix version 2.2 and later)
       inet_interfaces = 192.168.1.2, 127.0.0.1

**inet**___**protocols** **(default:** **see** **'postconf** **-d** **output')**
       The  Internet  protocols  Postfix  will  attempt to use when making or accepting connections.
       Specify one or more of "ipv4" or "ipv6", separated by whitespace or commas. The form "all" is
       equivalent  to  "ipv4,  ipv6" or "ipv4", depending on whether the operating system implements
       IPv6.

       With Postfix 2.8 and earlier the default is "ipv4". For backwards  compatibility  with  these
       releases,  the  Postfix 2.9 and later upgrade procedure appends an explicit "inet_protocols =
       ipv4" setting to main.cf when no explicit setting is present. This  compatibility  workaround
       will be phased out as IPv6 deployment becomes more common.

       This feature is available in Postfix 2.2 and later.

       Note: you MUST stop and start Postfix after changing this parameter.

       On systems that pre-date IPV6_V6ONLY support (RFC 3493), an IPv6 server will also accept IPv4
       connections, even when IPv4 is turned off with the inet_protocols parameter.  On systems with
       IPV6_V6ONLY  support,  Postfix  will  use separate server sockets for IPv6 and IPv4, and each
       will accept only connections for the corresponding protocol.

       When IPv4 support is enabled via the inet_protocols parameter, Postfix will look up DNS  type
       A records, and will convert IPv4-in-IPv6 client IP addresses (::ffff:1.2.3.4) to their origi‐
       nal IPv4 form (1.2.3.4).  The latter is needed on hosts  that  pre-date  IPV6_V6ONLY  support
       (RFC 3493).

       When  IPv6 support is enabled via the inet_protocols parameter, Postfix will do DNS type AAAA
       record lookups.

       When both IPv4 and IPv6 support are enabled, the Postfix SMTP client will choose the protocol
       as  specified with the smtp_address_preference parameter. Postfix versions before 2.8 attempt
       to connect via IPv6 before attempting to use IPv4.

       Examples:

       inet_protocols = ipv4
       inet_protocols = all (DEFAULT)
       inet_protocols = ipv6
       inet_protocols = ipv4, ipv6

**info**___**log**___**address**___**format** **(default:** **external)**
       The email address form that will be used in non-debug logging (info, warning,  etc.).  As  of
       Postfix 3.5 when an address localpart contains spaces or other special characters, the local‐
       part will be quoted, for example:

               from=<"name with spaces"@example.com>

       Older Postfix versions would log the internal (unquoted) form:

               from=<name with <spaces@example.com>>

       The external and internal forms are identical for the vast majority of email  addresses  that
       contain no spaces or other special characters in the localpart.

       The  logging  in external form is consistent with the address form that Postfix 3.2 and later
       prefer for most table lookups. This is therefore the more useful form for non-debug logging.

       Specify "**info**___**log**___**address**___**format** **=** **internal**" for backwards compatibility.

       Postfix uses the unquoted form internally, because an attacker can specify an  email  address
       in  different  forms  by playing games with quotes and backslashes. An attacker should not be
       able to use such games to circumvent Postfix access policies.

       This feature is available in Postfix 3.5 and later.

**initial**___**destination**___**concurrency** **(default:** **5)**
       The initial per-destination concurrency level for parallel delivery to the same  destination.
       With per-destination recipient limit > 1, a destination is a domain, otherwise it is a recip‐
       ient.

       Use _transport__initial_destination_concurrency to specify a transport-specific override, where
       _transport_ is the master.cf name of the message delivery transport (Postfix 2.5 and later).

       Warning: with concurrency of 1, one bad message can be enough to block all mail to a site.

**internal**___**mail**___**filter**___**classes** **(default:** **empty)**
       What  categories  of Postfix-generated mail are subject to before-queue content inspection by
       non_smtpd_milters, header_checks and body_checks.  Specify zero or  more  of  the  following,
       separated by whitespace or comma.

       **bounce** Inspect the content of delivery status notifications.

       **notify** Inspect the content of postmaster notifications by the [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) and [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) processes.

       NOTE:  It's  generally  not safe to enable content inspection of Postfix-generated email mes‐
       sages. The user is warned.

       This feature is available in Postfix 2.3 and later.

**invalid**___**hostname**___**reject**___**code** **(default:** **501)**
       The numerical Postfix SMTP server response code when the client HELO or EHLO command  parame‐
       ter is rejected by the reject_invalid_helo_hostname restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

**ipc**___**idle** **(default:** **version** **dependent)**
       The  time after which a client closes an idle internal communication channel.  The purpose is
       to allow Postfix daemon processes to terminate voluntarily after they become  idle.  This  is
       used, for example, by the Postfix address resolving and rewriting clients.

       With Postfix 2.4 the default value was reduced from 100s to 5s.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**ipc**___**timeout** **(default:** **3600s)**
       The time limit for sending or receiving information over an internal  communication  channel.
       The  purpose  is to break out of deadlock situations. If the time limit is exceeded the soft‐
       ware aborts with a fatal error.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**ipc**___**ttl** **(default:** **1000s)**
       The  time  after which a client closes an active internal communication channel.  The purpose
       is to allow Postfix daemon processes to terminate voluntarily  after  reaching  their  client
       limit.  This is used, for example, by the Postfix address resolving and rewriting clients.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

       This feature is available in Postfix 2.1 and later.

**known**___**tcp**___**ports** **(default:** **lmtp=24,** **smtp=25,** **smtps=submissions=465,** **submission=587)**
       Optional setting that avoids lookups in the [**services**(5)](https://www.chedong.com/phpMan.php/man/services/5/markdown) database.  This  feature  was  imple‐
       mented to address inconsistencies in the name of the port "465" service. The ABNF is:

           known_tcp_ports = empty | name-to-port *("," name-to-port)
           name-to-port = 1*(service-name "=') port-number

       Whitespace is optional but it cannot appear inside a service name or port number.

       This feature is available in Postfix 3.6 and later.

**line**___**length**___**limit** **(default:** **2048)**
       Upon input, long lines are chopped up into pieces of at most this length; upon delivery, long
       lines are reconstructed.

**lmdb**___**map**___**size** **(default:** **16777216)**
       The initial OpenLDAP LMDB database size limit in bytes.  Each time a database  becomes  full,
       its size limit is doubled.

       This feature is available in Postfix 2.11 and later.

**lmtp**___**address**___**preference** **(default:** **ipv6)**
       The  LMTP-specific version of the smtp_address_preference configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.8 and later.

**lmtp**___**address**___**verify**___**target** **(default:** **rcpt)**
       The LMTP-specific version of the  smtp_address_verify_target  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 3.0 and later.

**lmtp**___**assume**___**final** **(default:** **no)**
       When a remote LMTP server announces no DSN support, assume that the server performs final de‐
       livery, and send "delivered" delivery status notifications instead of "relayed". The  default
       setting  is  backwards compatible to avoid the infinitesimal possibility of breaking existing
       LMTP-based content filters.

**lmtp**___**balance**___**inet**___**protocols** **(default:** **yes)**
       The LMTP-specific version of the  smtp_balance_inet_protocols  configuration  parameter.  See
       there for details.

       This feature is available in Postfix 3.3 and later.

**lmtp**___**bind**___**address** **(default:** **empty)**
       The  LMTP-specific  version  of the smtp_bind_address configuration parameter.  See there for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**bind**___**address6** **(default:** **empty)**
       The LMTP-specific version of the smtp_bind_address6 configuration parameter.  See  there  for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**body**___**checks** **(default:** **empty)**
       The  LMTP-specific version of the smtp_body_checks configuration parameter. See there for de‐
       tails.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**cache**___**connection** **(default:** **yes)**
       Keep Postfix LMTP client connections open for up to $max_idle seconds. When the  LMTP  client
       receives a request for the same connection the connection is reused.

       This parameter is available in Postfix version 2.2 and earlier.  With Postfix version 2.3 and
       later, see lmtp_connection_cache_on_demand, lmtp_connection_cache_destinations, or  lmtp_con‐
       nection_reuse_time_limit.

       The  effectiveness  of  cached  connections  will  be determined by the number of remote LMTP
       servers in use, and the concurrency limit specified for the Postfix LMTP client. Cached  con‐
       nections are closed under any of the following conditions:

       •      The  Postfix LMTP client idle time limit is reached.  This limit is specified with the
              Postfix max_idle configuration parameter.

       •      A delivery request specifies a different destination than the one currently cached.

       •      The per-process limit on the number of delivery requests is reached.   This  limit  is
              specified with the Postfix max_use configuration parameter.

       •      Upon the onset of another delivery request, the remote LMTP server associated with the
              current session does not respond to the RSET command.

       Most of these limitations have been with the Postfix a connection cache that is shared  among
       multiple LMTP client programs.

**lmtp**___**cname**___**overrides**___**servername** **(default:** **yes)**
       The  LMTP-specific  version  of  the smtp_cname_overrides_servername configuration parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**connect**___**timeout** **(default:** **0s)**
       The Postfix LMTP client time limit for completing a TCP connection, or zero (use the  operat‐
       ing  system  built-in  time  limit).  When no connection can be made within the deadline, the
       LMTP client tries the next address on the mail exchanger list.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

       Example:

       lmtp_connect_timeout = 30s

**lmtp**___**connection**___**cache**___**destinations** **(default:** **empty)**
       The  LMTP-specific version of the smtp_connection_cache_destinations configuration parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**connection**___**cache**___**on**___**demand** **(default:** **yes)**
       The LMTP-specific version of  the  smtp_connection_cache_on_demand  configuration  parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**connection**___**cache**___**time**___**limit** **(default:** **2s)**
       The  LMTP-specific  version  of the smtp_connection_cache_time_limit configuration parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**connection**___**reuse**___**count**___**limit** **(default:** **0)**
       The LMTP-specific version of the smtp_connection_reuse_count_limit  configuration  parameter.
       See there for details.

       This feature is available in Postfix 2.11 and later.

**lmtp**___**connection**___**reuse**___**time**___**limit** **(default:** **300s)**
       The  LMTP-specific  version  of the smtp_connection_reuse_time_limit configuration parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**data**___**done**___**timeout** **(default:** **600s)**
       The Postfix LMTP client time limit for sending the LMTP ".", and  for  receiving  the  remote
       LMTP  server response.  When no response is received within the deadline, a warning is logged
       that the mail may be delivered multiple times.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**lmtp**___**data**___**init**___**timeout** **(default:** **120s)**
       The  Postfix  LMTP client time limit for sending the LMTP DATA command, and for receiving the
       remote LMTP server response.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**lmtp**___**data**___**xfer**___**timeout** **(default:** **180s)**
       The Postfix LMTP client time limit for sending the LMTP message content.  When the connection
       stalls for more than $lmtp_data_xfer_timeout the LMTP client terminates the transfer.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**lmtp**___**defer**___**if**___**no**___**mx**___**address**___**found** **(default:** **no)**
       The  LMTP-specific  version of the smtp_defer_if_no_mx_address_found configuration parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**delivery**___**status**___**filter** **(default:** **empty)**
       The LMTP-specific version of the smtp_delivery_status_filter  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 3.0 and later.

**lmtp**___**destination**___**concurrency**___**limit** **(default:** **$default**___**destination**___**concurrency**___**limit)**
       The maximal number of parallel deliveries to the same destination via the lmtp message deliv‐
       ery transport. This limit is enforced by the queue manager. The  message  delivery  transport
       name is the first field in the entry in the master.cf file.

**lmtp**___**destination**___**recipient**___**limit** **(default:** **$default**___**destination**___**recipient**___**limit)**
       The  maximal  number  of recipients per message for the lmtp message delivery transport. This
       limit is enforced by the queue manager. The message delivery  transport  name  is  the  first
       field in the entry in the master.cf file.

       Setting  this  parameter  to  a  value  of  1 changes the meaning of lmtp_destination_concur‐
       rency_limit from concurrency per domain into concurrency per recipient.

**lmtp**___**discard**___**lhlo**___**keyword**___**address**___**maps** **(default:** **empty)**
       Lookup tables, indexed by the remote LMTP server address, with case insensitive lists of LHLO
       keywords  (pipelining,  starttls, auth, etc.) that the Postfix LMTP client will ignore in the
       LHLO response from a remote LMTP server. See lmtp_discard_lhlo_keywords for details. The  ta‐
       ble is not indexed by hostname for consistency with smtpd_discard_ehlo_keyword_address_maps.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**discard**___**lhlo**___**keywords** **(default:** **empty)**
       A  case insensitive list of LHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
       LMTP client will ignore in the LHLO response from a remote LMTP server.

       This feature is available in Postfix 2.3 and later.

       Notes:

       •      Specify the **silent-discard** pseudo keyword to prevent this action from being logged.

       •      Use the lmtp_discard_lhlo_keyword_address_maps feature to discard LHLO keywords selec‐
              tively.

**lmtp**___**dns**___**reply**___**filter** **(default:** **empty)**
       Optional  filter  for  Postfix LMTP client DNS lookup results.  See smtp_dns_reply_filter for
       details including an example.

       This feature is available in Postfix 3.0 and later.

**lmtp**___**dns**___**resolver**___**options** **(default:** **empty)**
       The LMTP-specific version of  the  smtp_dns_resolver_options  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.8 and later.

**lmtp**___**dns**___**support**___**level** **(default:** **empty)**
       The  LMTP-specific  version of the smtp_dns_support_level configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.11 and later.

**lmtp**___**enforce**___**tls** **(default:** **no)**
       The LMTP-specific version of the smtp_enforce_tls configuration parameter.  See there for de‐
       tails.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**fallback**___**relay** **(default:** **empty)**
       Optional  list  of relay hosts for LMTP destinations that can't be found or that are unreach‐
       able.  In main.cf elements are separated by whitespace or commas.

       By default, mail is returned to the sender when a destination is not found, and  delivery  is
       deferred when a destination is unreachable.

       The  fallback  relays  must  be TCP destinations, specified without a leading "inet:" prefix.
       Specify a host or host:port.  Since MX lookups do not apply with LMTP, there is  no  need  to
       use  the "[host]" or "[host]:port" forms.  If you specify multiple LMTP destinations, Postfix
       will try them in the specified order.

       This feature is available in Postfix 3.1 and later.

**lmtp**___**generic**___**maps** **(default:** **empty)**
       The LMTP-specific version of the smtp_generic_maps configuration parameter.   See  there  for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**header**___**checks** **(default:** **empty)**
       The  LMTP-specific  version  of the smtp_header_checks configuration parameter. See there for
       details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**host**___**lookup** **(default:** **dns)**
       The LMTP-specific version of the smtp_host_lookup configuration parameter.  See there for de‐
       tails.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**lhlo**___**name** **(default:** **$myhostname)**
       The hostname to send in the LMTP LHLO command.

       The default value is the machine hostname.  Specify a hostname or [ip.add.re.ss].

       This  information  can  be  specified  in the main.cf file for all LMTP clients, or it can be
       specified in the master.cf file for a specific client, for example:

           /etc/postfix/master.cf:
               mylmtp ... lmtp -o lmtp_lhlo_name=foo.bar.com

       This feature is available in Postfix 2.3 and later.

**lmtp**___**lhlo**___**timeout** **(default:** **300s)**
       The Postfix LMTP client time limit for sending the LHLO command, and for receiving  the  ini‐
       tial remote LMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**lmtp**___**line**___**length**___**limit** **(default:** **990)**
       The LMTP-specific version of the smtp_line_length_limit configuration parameter.   See  there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**mail**___**timeout** **(default:** **300s)**
       The  Postfix  LMTP client time limit for sending the MAIL FROM command, and for receiving the
       remote LMTP server response.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**lmtp**___**mime**___**header**___**checks** **(default:** **empty)**
       The  LMTP-specific  version of the smtp_mime_header_checks configuration parameter. See there
       for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**mx**___**address**___**limit** **(default:** **5)**
       The LMTP-specific version of the smtp_mx_address_limit configuration  parameter.   See  there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**mx**___**session**___**limit** **(default:** **2)**
       The  LMTP-specific  version  of the smtp_mx_session_limit configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**nested**___**header**___**checks** **(default:** **empty)**
       The LMTP-specific version of the smtp_nested_header_checks configuration parameter. See there
       for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**per**___**record**___**deadline** **(default:** **no)**
       The LMTP-specific version of the smtp_per_record_deadline configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.9 and later.

**lmtp**___**pix**___**workaround**___**delay**___**time** **(default:** **10s)**
       The LMTP-specific version of the smtp_pix_workaround_delay_time configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**pix**___**workaround**___**maps** **(default:** **empty)**
       The LMTP-specific version of the smtp_pix_workaround_maps configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.4 and later.

**lmtp**___**pix**___**workaround**___**threshold**___**time** **(default:** **500s)**
       The LMTP-specific version of the smtp_pix_workaround_threshold_time configuration  parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**pix**___**workarounds** **(default:** **empty)**
       The  LMTP-specific version of the smtp_pix_workaround configuration parameter.  See there for
       details.

       This feature is available in Postfix 2.4 and later.

**lmtp**___**quit**___**timeout** **(default:** **300s)**
       The Postfix LMTP client time limit for sending the QUIT command, and for receiving the remote
       LMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**lmtp**___**quote**___**rfc821**___**envelope** **(default:** **yes)**
       The LMTP-specific version of the  smtp_quote_rfc821_envelope  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**randomize**___**addresses** **(default:** **yes)**
       The LMTP-specific version of the smtp_randomize_addresses configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**rcpt**___**timeout** **(default:** **300s)**
       The Postfix LMTP client time limit for sending the RCPT TO command, and for receiving the re‐
       mote LMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**lmtp**___**reply**___**filter** **(default:** **empty)**
       The LMTP-specific version of the smtp_reply_filter configuration parameter.   See  there  for
       details.

       This feature is available in Postfix 2.7 and later.

**lmtp**___**rset**___**timeout** **(default:** **20s)**
       The Postfix LMTP client time limit for sending the RSET command, and for receiving the remote
       LMTP server response. The LMTP client sends RSET in  order  to  finish  a  recipient  address
       probe, or to verify that a cached connection is still alive.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**lmtp**___**sasl**___**auth**___**cache**___**name** **(default:** **empty)**
       The LMTP-specific version of  the  smtp_sasl_auth_cache_name  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**sasl**___**auth**___**cache**___**time** **(default:** **90d)**
       The  LMTP-specific  version  of  the  smtp_sasl_auth_cache_time configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**sasl**___**auth**___**enable** **(default:** **no)**
       Enable SASL authentication in the Postfix LMTP client.

**lmtp**___**sasl**___**auth**___**soft**___**bounce** **(default:** **yes)**
       The LMTP-specific version of the  smtp_sasl_auth_soft_bounce  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**sasl**___**mechanism**___**filter** **(default:** **empty)**
       The  LMTP-specific  version  of  the smtp_sasl_mechanism_filter configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**sasl**___**password**___**maps** **(default:** **empty)**
       Optional Postfix LMTP client lookup tables with one username:password entry per host  or  do‐
       main.   If  a  remote  host  or  domain has no username:password entry, then the Postfix LMTP
       client will not attempt to authenticate to the remote host.

**lmtp**___**sasl**___**path** **(default:** **empty)**
       Implementation-specific information that is passed through to the SASL plug-in implementation
       that  is  selected with **lmtp**___**sasl**___**type**.  Typically this specifies the name of a configuration
       file or rendezvous point.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**sasl**___**security**___**options** **(default:** **noplaintext,** **noanonymous)**
       SASL security options; as of Postfix 2.3 the list of available features depends on  the  SASL
       client implementation that is selected with **lmtp**___**sasl**___**type**.

       The following security features are defined for the **cyrus** client SASL implementation:

### noplaintext
              Disallow authentication methods that use plaintext passwords.

### noactive
              Disallow authentication methods that are vulnerable to non-dictionary active attacks.

### nodictionary
              Disallow authentication methods that are vulnerable to passive dictionary attack.

### noanonymous
              Disallow anonymous logins.

       Example:

       lmtp_sasl_security_options = noplaintext

**lmtp**___**sasl**___**tls**___**security**___**options** **(default:** **$lmtp**___**sasl**___**security**___**options)**
       The LMTP-specific version of the smtp_sasl_tls_security_options configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**sasl**___**tls**___**verified**___**security**___**options** **(default:** **$lmtp**___**sasl**___**tls**___**security**___**options)**
       The LMTP-specific version of the smtp_sasl_tls_verified_security_options configuration param‐
       eter.  See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**sasl**___**type** **(default:** **cyrus)**
       The SASL plug-in type that the Postfix LMTP client should use for authentication.  The avail‐
       able types are listed with the "**postconf** **-A**" command.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**send**___**dummy**___**mail**___**auth** **(default:** **no)**
       The LMTP-specific version of  the  smtp_send_dummy_mail_auth  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.9 and later.

**lmtp**___**send**___**xforward**___**command** **(default:** **no)**
       Send  an  XFORWARD  command  to the remote LMTP server when the LMTP LHLO server response an‐
       nounces XFORWARD support.  This allows an [**lmtp**(8)](https://www.chedong.com/phpMan.php/man/lmtp/8/markdown) delivery agent,  used  for  content  filter
       message  injection,  to  forward  the  name,  address, protocol and HELO name of the original
       client to the content filter and downstream queuing LMTP server.  Before you change the value
       to yes, it is best to make sure that your content filter supports this command.

       This feature is available in Postfix 2.1 and later.

**lmtp**___**sender**___**dependent**___**authentication** **(default:** **no)**
       The  LMTP-specific  version of the smtp_sender_dependent_authentication configuration parame‐
       ter.  See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**skip**___**5xx**___**greeting** **(default:** **yes)**
       The LMTP-specific version of the smtp_skip_5xx_greeting configuration parameter.   See  there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**skip**___**quit**___**response** **(default:** **no)**
       Wait for the response to the LMTP QUIT command.

**lmtp**___**starttls**___**timeout** **(default:** **300s)**
       The  LMTP-specific  version  of the smtp_starttls_timeout configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tcp**___**port** **(default:** **24)**
       The default TCP port that the Postfix LMTP client connects to.  Specify a symbolic name  (see
       [**services**(5)](https://www.chedong.com/phpMan.php/man/services/5/markdown)) or a numeric port.

**lmtp**___**tls**___**CAfile** **(default:** **empty)**
       The  LMTP-specific version of the smtp_tls_CAfile configuration parameter.  See there for de‐
       tails.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**CApath** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_CApath configuration parameter.  See there for  de‐
       tails.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**block**___**early**___**mail**___**reply** **(default:** **empty)**
       The  LMTP-specific  version  of  the smtp_tls_block_early_mail_reply configuration parameter.
       See there for details.

       This feature is available in Postfix 2.7 and later.

**lmtp**___**tls**___**cert**___**file** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_cert_file configuration parameter.  See  there  for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**chain**___**files** **(default:** **empty)**
       The  LMTP-specific version of the smtp_tls_chain_files configuration parameter. See there for
       details.

       This feature is available in Postfix 3.4 and later.

**lmtp**___**tls**___**ciphers** **(default:** **medium)**
       The LMTP-specific version of the smtp_tls_ciphers configuration parameter. See there for  de‐
       tails.

       This feature is available in Postfix 2.6 and later.

**lmtp**___**tls**___**connection**___**reuse** **(default:** **no)**
       The LMTP-specific version of the smtp_tls_connection_reuse configuration parameter. See there
       for details.

       This feature is available in Postfix 3.4 and later.

**lmtp**___**tls**___**dcert**___**file** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_dcert_file configuration parameter.  See there  for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**dkey**___**file** **(default:** **$lmtp**___**tls**___**dcert**___**file)**
       The  LMTP-specific  version of the smtp_tls_dkey_file configuration parameter.  See there for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**eccert**___**file** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_eccert_file configuration parameter.  See there for
       details.

       This  feature is available in Postfix 2.6 and later, when Postfix is compiled and linked with
       OpenSSL 1.0.0 or later.

**lmtp**___**tls**___**eckey**___**file** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_eckey_file configuration parameter.  See there  for
       details.

       This  feature is available in Postfix 2.6 and later, when Postfix is compiled and linked with
       OpenSSL 1.0.0 or later.

**lmtp**___**tls**___**enforce**___**peername** **(default:** **yes)**
       The LMTP-specific version of  the  smtp_tls_enforce_peername  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**exclude**___**ciphers** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_exclude_ciphers configuration parameter.  See there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**fingerprint**___**cert**___**match** **(default:** **empty)**
       The LMTP-specific version of  the  smtp_tls_fingerprint_cert_match  configuration  parameter.
       See there for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**tls**___**fingerprint**___**digest** **(default:** **see** **postconf** **-d** **output)**
       The  LMTP-specific  version  of the smtp_tls_fingerprint_digest configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.5 and later.

**lmtp**___**tls**___**force**___**insecure**___**host**___**tlsa**___**lookup** **(default:** **no)**
       The LMTP-specific version of the smtp_tls_force_insecure_host_tlsa_lookup  configuration  pa‐
       rameter.  See there for details.

       This feature is available in Postfix 2.11 and later.

**lmtp**___**tls**___**key**___**file** **(default:** **$lmtp**___**tls**___**cert**___**file)**
       The  LMTP-specific  version  of the smtp_tls_key_file configuration parameter.  See there for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**loglevel** **(default:** **0)**
       The LMTP-specific version of the smtp_tls_loglevel configuration parameter.   See  there  for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**mandatory**___**ciphers** **(default:** **medium)**
       The  LMTP-specific  version  of  the smtp_tls_mandatory_ciphers configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**mandatory**___**exclude**___**ciphers** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_mandatory_exclude_ciphers configuration  parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**mandatory**___**protocols** **(default:** **see** **postconf** **-d** **output)**
       The  LMTP-specific  version  of the smtp_tls_mandatory_protocols configuration parameter. See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**note**___**starttls**___**offer** **(default:** **no)**
       The LMTP-specific version of the smtp_tls_note_starttls_offer configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**per**___**site** **(default:** **empty)**
       The  LMTP-specific  version  of the smtp_tls_per_site configuration parameter.  See there for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**policy**___**maps** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_policy_maps configuration parameter. See there  for
       details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**protocols** **(default:** **see** **postconf** **-d** **output)**
       The  LMTP-specific  version  of the smtp_tls_protocols configuration parameter. See there for
       details.

       This feature is available in Postfix 2.6 and later.

**lmtp**___**tls**___**scert**___**verifydepth** **(default:** **9)**
       The LMTP-specific version of the  smtp_tls_scert_verifydepth  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**secure**___**cert**___**match** **(default:** **nexthop)**
       The  LMTP-specific  version  of  the  smtp_tls_secure_cert_match configuration parameter. See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**security**___**level** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_security_level configuration parameter.  See  there
       for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**servername** **(default:** **empty)**
       The  LMTP-specific  version of the smtp_tls_servername configuration parameter. See there for
       details.

       This feature is available in Postfix 3.4 and later.

**lmtp**___**tls**___**session**___**cache**___**database** **(default:** **empty)**
       The LMTP-specific version of the smtp_tls_session_cache_database configuration parameter. See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**session**___**cache**___**timeout** **(default:** **3600s)**
       The LMTP-specific version of the smtp_tls_session_cache_timeout configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**tls**___**trust**___**anchor**___**file** **(default:** **empty)**
       The LMTP-specific version of the  smtp_tls_trust_anchor_file  configuration  parameter.   See
       there for details.

       This feature is available in Postfix 2.11 and later.

**lmtp**___**tls**___**verify**___**cert**___**match** **(default:** **hostname)**
       The  LMTP-specific  version  of  the  smtp_tls_verify_cert_match configuration parameter. See
       there for details.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**use**___**tls** **(default:** **no)**
       The LMTP-specific version of the smtp_use_tls configuration parameter.   See  there  for  de‐
       tails.

       This feature is available in Postfix 2.3 and later.

**lmtp**___**xforward**___**timeout** **(default:** **300s)**
       The  Postfix  LMTP  client time limit for sending the XFORWARD command, and for receiving the
       remote LMTP server response.

       In case of problems the client does NOT try the next address on the mail exchanger list.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

       This feature is available in Postfix 2.1 and later.

**local**___**command**___**shell** **(default:** **empty)**
       Optional shell program for [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery to non-Postfix command.  By default, non-Postfix
       commands are executed directly; commands are given to given to the default shell  (typically,
       /bin/sh) only when they contain shell meta characters or shell built-in commands.

       "sendmail's  restricted shell" (smrsh) is what most people will use in order to restrict what
       programs can be run from e.g. .forward files (smrsh is part of the Sendmail distribution).

       Note: when a shell program is specified, it is invoked even  when  the  command  contains  no
       shell built-in commands or meta characters.

       Example:

       local_command_shell = /some/where/smrsh -c
       local_command_shell = /bin/bash -c

**local**___**delivery**___**status**___**filter** **(default:** **$default**___**delivery**___**status**___**filter)**
       Optional filter for the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent to change the status code or explanatory text
       of successful or unsuccessful deliveries.  See default_delivery_status_filter for details.

       This feature is available in Postfix 3.0 and later.

**local**___**destination**___**concurrency**___**limit** **(default:** **2)**
       The maximal number of parallel deliveries via the local mail delivery transport to  the  same
       recipient  (when  "local_destination_recipient_limit  = 1") or the maximal number of parallel
       deliveries to the same local domain  (when  "local_destination_recipient_limit  >  1").  This
       limit  is  enforced  by  the  queue manager. The message delivery transport name is the first
       field in the entry in the master.cf file.

       A low limit of 2 is recommended, just in case someone has an expensive  shell  command  in  a
       .forward  file  or in an alias (e.g., a mailing list manager).  You don't want to run lots of
       those at the same time.

**local**___**destination**___**recipient**___**limit** **(default:** **1)**
       The maximal number of recipients per message delivery via the local mail delivery  transport.
       This limit is enforced by the queue manager. The message delivery transport name is the first
       field in the entry in the master.cf file.

       Setting this parameter to a value  >  1  changes  the  meaning  of  local_destination_concur‐
       rency_limit from concurrency per recipient into concurrency per domain.

**local**___**header**___**rewrite**___**clients** **(default:** **permit**___**inet**___**interfaces)**
       Rewrite  message  header addresses in mail from these clients and update incomplete addresses
       with the domain name in $myorigin or $mydomain; either don't  rewrite  message  headers  from
       other clients at all, or rewrite message headers and update incomplete addresses with the do‐
       main specified in the remote_header_rewrite_domain parameter.

       See the append_at_myorigin and append_dot_mydomain parameters for details of how domain names
       are appended to incomplete addresses.

       Specify a list of zero or more of the following:

       **permit**___**inet**___**interfaces**
              Append  the  domain  name in $myorigin or $mydomain when the client IP address matches
              $inet_interfaces. This is enabled by default.

       **permit**___**mynetworks**
              Append the domain name in $myorigin or $mydomain when the client  IP  address  matches
              any  network  or  network address listed in $mynetworks. This setting will not prevent
              remote mail header address rewriting when mail from a remote client is forwarded by  a
              neighboring system.

       **permit**___**sasl**___**authenticated**
              Append  the  domain name in $myorigin or $mydomain when the client is successfully au‐
              thenticated via the RFC 4954 (AUTH) protocol.

       **permit**___**tls**___**clientcerts**
              Append the domain name in $myorigin or $mydomain when the remote SMTP client TLS  cer‐
              tificate  fingerprint  or  public key fingerprint (Postfix 2.9 and later) is listed in
              $relay_clientcerts.   The  fingerprint  digest  algorithm  is  configurable  via   the
              smtpd_tls_fingerprint_digest  parameter  (hard-coded  as  md5 prior to Postfix version
              2.5).
              The default algorithm is **sha256** with Postfix >= 3.6 and the **compatibility**___**level** set to
              3.6  or  higher. With Postfix <= 3.5, the default algorithm is **md5**.  The best-practice
              algorithm is now **sha256**. Recent advances in hash function cryptanalysis  have  led  to
              md5  and  sha1  being deprecated in favor of sha256.  However, as long as there are no
              known "second pre-image" attacks against the older algorithms, their use in this  con‐
              text, though not recommended, is still likely safe.

       **permit**___**tls**___**all**___**clientcerts**
              Append  the domain name in $myorigin or $mydomain when the remote SMTP client TLS cer‐
              tificate is successfully verified, regardless of whether it is listed on  the  server,
              and regardless of the certifying authority.

       **check**___**address**___**map** _type:table_

       _type:table_
              Append  the  domain  name in $myorigin or $mydomain when the client IP address matches
              the specified lookup table.  The lookup result is ignored, and  no  subnet  lookup  is
              done. This is suitable for, e.g., pop-before-smtp lookup tables.

       Examples:

       The  Postfix  <  2.2 backwards compatible setting: always rewrite message headers, and always
       append my own domain to incomplete header addresses.

           local_header_rewrite_clients = static:all

       The purist (and default) setting: rewrite headers only in mail from Postfix sendmail  and  in
       SMTP mail from this machine.

           local_header_rewrite_clients = permit_inet_interfaces

       The intermediate setting: rewrite header addresses and append $myorigin or $mydomain informa‐
       tion only with mail from Postfix sendmail,  from  local  clients,  or  from  authorized  SMTP
       clients.

       Note: this setting will not prevent remote mail header address rewriting when mail from a re‐
       mote client is forwarded by a neighboring system.

           local_header_rewrite_clients = permit_mynetworks,
               permit_sasl_authenticated permit_tls_clientcerts
               check_address_map hash:/etc/postfix/pop-before-smtp

**local**___**login**___**sender**___**maps** **(default:** **static:*)**
       A list of lookup tables that are searched by the UNIX login name, and that return a  list  of
       allowed  envelope  sender patterns separated by space or comma. These sender patterns are en‐
       forced by the Postfix [**postdrop**(1)](https://www.chedong.com/phpMan.php/man/postdrop/1/markdown) command. The default is  backwards-compatible:  every  user
       may specify any sender envelope address.

       When  no UNIX login name is available, the [**postdrop**(1)](https://www.chedong.com/phpMan.php/man/postdrop/1/markdown) command will prepend "**uid:**" to the nu‐
       merical UID and use that instead.

       This feature ignores address extensions in the user-specified envelope sender address.

       The following sender patterns are special; these cannot be used as part of a longer pattern.

        *****     This pattern allows any envelope sender address.

        **<>**    This pattern allows the empty envelope sender address. See the empty_address_local_lo‐
              gin_sender_maps_lookup_key configuration parameter.

        **@**_domain_
              This pattern allows an envelope sender address when the '**@**' and _domain_ part match.

       Examples:

       /etc/postfix/main.cf:
           # Allow root and postfix full control, anyone else can only
           # send mail as themselves. Use "uid:" followed by the numerical
           # UID when the UID has no entry in the UNIX password file.
           local_login_sender_maps =
               inline:{ { root = *}, { postfix = * } },
               pcre:/etc/postfix/login_senders

       /etc/postfix/login_senders:
          # Allow both the bare username and the user@domain forms.
           /(.+)/ $1 $<1@example.com>/

       This feature is available in Postfix 3.6 and later.

**local**___**recipient**___**maps** **(default:** **proxy:unix:passwd.byname** **$alias**___**maps)**
       Lookup  tables  with all names or addresses of local recipients: a recipient address is local
       when its domain matches $mydestination, $inet_interfaces or $proxy_interfaces.  Specify  @do‐
       main as a wild-card for domains that do not have a valid recipient list.  Technically, tables
       listed with $local_recipient_maps are used as lists: Postfix needs to know only if  a  lookup
       string is found or not, but it does not use the result from table lookup.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       If this parameter is non-empty (the default), then the Postfix SMTP server will  reject  mail
       for unknown local users.

       To  turn  off  local  recipient  checking  in the Postfix SMTP server, specify "local_recipi‐
       ent_maps =" (i.e. empty).

       The default setting assumes that you use the default Postfix local delivery agent  for  local
       delivery. You need to update the local_recipient_maps setting if:

       •      You redefine the local delivery agent in master.cf.

       •      You redefine the "local_transport" setting in main.cf.

       •      You use the "luser_relay", "mailbox_transport", or "fallback_transport" feature of the
              Postfix [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent.

       Details are described in the LOCAL_RECIPIENT_README file.

       Beware: if the Postfix SMTP server runs chrooted, you need to access the passwd file via  the
       [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown)  service, in order to overcome chroot access restrictions. The alternative, main‐
       taining a copy of the system password file in the chroot jail is not practical.

       Examples:

       local_recipient_maps =

**local**___**transport** **(default:** **local:$myhostname)**
       The default mail delivery transport and next-hop destination for final  delivery  to  domains
       listed  with  mydestination,  and for [ipaddress] destinations that match $inet_interfaces or
       $proxy_interfaces.  This information can be overruled with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       By default, local mail is delivered to the transport called "local", which is just  the  name
       of a service that is defined the master.cf file.

       Specify  a string of the form _transport:nexthop_, where _transport_ is the name of a mail deliv‐
       ery transport defined in master.cf.  The _:nexthop_ destination is optional; its syntax is doc‐
       umented in the manual page of the corresponding delivery agent.

       Beware:  if  you  override  the  default local delivery agent then you need to review the LO‐
       CAL_RECIPIENT_README document, otherwise the SMTP server may reject mail  for  local  recipi‐
       ents.

**luser**___**relay** **(default:** **empty)**
       Optional catch-all destination for unknown [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) recipients.  By default, mail for unknown
       recipients in domains that match $mydestination, $inet_interfaces or $proxy_interfaces is re‐
       turned as undeliverable.

       The  luser_relay value is not subject to Postfix configuration parameter $name expansion. In‐
       stead, the following $name expansions are done:

### $domain
              The recipient domain.

### $extension
              The recipient address extension.

       **$home**  The recipient's home directory.

       **$local** The entire recipient address localpart.

### $recipient
              The full recipient address.

       **$recipient**___**delimiter**
              The address extension delimiter that was found in the recipient address (Postfix  2.11
              and later), or the system-wide recipient address extension delimiter (Postfix 2.10 and
              earlier).

       **$shell** The recipient's login shell.

       **$user**  The recipient username.

### ${name?value}
              Expands to _value_ when _$name_ has a non-empty value.

### ${name:value}
              Expands to _value_ when _$name_ has an empty value.

       Instead of $name you can also specify ${name} or $(name).

       Note: luser_relay works only for the Postfix [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent.

       Note: if you use this feature for accounts not in the UNIX password file, then you must spec‐
       ify  "local_recipient_maps  ="  (i.e.  empty) in the main.cf file, otherwise the Postfix SMTP
       server will reject mail for non-UNIX accounts with "User unknown in local recipient table".

       Examples:

       luser_relay = $<user@other.host>
       luser_relay = $<local@other.host>
       luser_relay = admin+$local

**mail**___**name** **(default:** **Postfix)**
       The mail system name that is displayed in Received: headers, in the SMTP greeting banner, and
       in bounced mail.

**mail**___**owner** **(default:** **postfix)**
       The UNIX system account that owns the Postfix queue and most Postfix daemon processes.  Spec‐
       ify the name of an unprivileged user account that does not share a  user  or  group  ID  with
       other  accounts,  and  that  owns  no other files or processes on the system.  In particular,
       don't specify nobody or daemon.  PLEASE USE A DEDICATED USER ID AND GROUP ID.

       When this parameter value is changed you need to re-run "**postfix** **set-permissions**" (with Post‐
       fix version 2.0 and earlier: "**/etc/postfix/post-install** **set-permissions**".

**mail**___**release**___**date** **(default:** **see** **postconf** **-d** **output)**
       The Postfix release date, in "YYYYMMDD" format.

**mail**___**spool**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The  directory  where  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) UNIX-style mailboxes are kept. The default setting depends on
       the system type. Specify a name ending in / for maildir-style delivery.

       Note: maildir delivery is done with  the  privileges  of  the  recipient.   If  you  use  the
       mail_spool_directory  setting  for maildir style delivery, then you must create the top-level
       maildir directory in advance. Postfix will not create it.

       Examples:

       mail_spool_directory = /var/mail
       mail_spool_directory = /var/spool/mail

**mail**___**version** **(default:** **see** **postconf** **-d** **output)**
       The version of the mail system. Stable releases are named _major_._minor_._patchlevel_.  Experimen‐
       tal  releases  also include the release date. The version string can be used in, for example,
       the SMTP greeting banner.

**mailbox**___**command** **(default:** **empty)**
       Optional external command that the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent should use for  mailbox  delivery.
       The  command  is  run  with the user ID and the primary group ID privileges of the recipient.
       Exception: command delivery for root executes with $default_privs privileges.  This is not  a
       problem, because 1) mail for root should always be aliased to a real user and 2) don't log in
       as root, use "su" instead.

       The following environment variables are exported to the command:

       **CLIENT**___**ADDRESS**
              Remote client network address. Available in Postfix version 2.2 and later.

       **CLIENT**___**HELO**
              Remote client EHLO command parameter. Available in Postfix version 2.2 and later.

       **CLIENT**___**HOSTNAME**
              Remote client hostname. Available in Postfix version 2.2 and later.

       **CLIENT**___**PROTOCOL**
              Remote client protocol. Available in Postfix version 2.2 and later.

       **DOMAIN** The domain part of the recipient address.

       **EXTENSION**
              The optional address extension.

       **HOME**   The recipient home directory.

       **LOCAL**  The recipient address localpart.

       **LOGNAME**
              The recipient's username.

       **ORIGINAL**___**RECIPIENT**
              The entire recipient address, before any address rewriting or aliasing.

       **RECIPIENT**
              The full recipient address.

       **SASL**___**METHOD**
              SASL authentication method specified in the remote client AUTH command.  Available  in
              Postfix version 2.2 and later.

       **SASL**___**SENDER**
              SASL  sender  address  specified  in the remote client MAIL FROM command. Available in
              Postfix version 2.2 and later.

       **SASL**___**USER**
              SASL username specified in the remote client AUTH command.  Available in Postfix  ver‐
              sion 2.2 and later.

       **SENDER** The full sender address.

       **SHELL**  The recipient's login shell.

       **USER**   The recipient username.

       Unlike other Postfix configuration parameters, the mailbox_command parameter is not subjected
       to $name substitutions. This is to make it easier to specify shell syntax  (see  example  be‐
       low).

       If  you  can, avoid shell meta characters because they will force Postfix to run an expensive
       shell process. If you're delivering via "procmail" then running a shell won't make a  notice‐
       able difference in the total cost.

       Note:  if you use the mailbox_command feature to deliver mail system-wide, you must set up an
       alias that forwards mail for root to a real user.

       The precedence of [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is:  aliases,  .forward  files,
       mailbox_transport_maps,  mailbox_transport, mailbox_command_maps, mailbox_command, home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

       Examples:

       mailbox_command = /some/where/procmail
       mailbox_command = /some/where/procmail -a "$EXTENSION"
       mailbox_command = /some/where/maildrop -d "$USER"
               -f "$SENDER" "$EXTENSION"

**mailbox**___**command**___**maps** **(default:** **empty)**
       Optional lookup tables with per-recipient external commands to use for [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  mailbox  de‐
       livery.  Behavior is as with mailbox_command.

       The  precedence  of  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps, mailbox_transport, mailbox_command_maps, mailbox_command,  home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

**mailbox**___**delivery**___**lock** **(default:** **see** **postconf** **-d** **output)**
       How to lock a UNIX-style [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) mailbox before attempting delivery.  For a list  of  avail‐
       able file locking methods, use the "**postconf** **-l**" command.

       This setting is ignored with **maildir** style delivery, because such deliveries are safe without
       explicit locks.

       Note: The **dotlock** method requires that the recipient UID or GID has write access to the  par‐
       ent directory of the mailbox file.

       Note: the default setting of this parameter is system dependent.

**mailbox**___**size**___**limit** **(default:** **51200000)**
       The  maximal size of any [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) individual mailbox or maildir file, or zero (no limit).  In
       fact, this limits the size of any file that is written  to  upon  local  delivery,  including
       files written by external commands that are executed by the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent.

       This limit must not be smaller than the message size limit.

**mailbox**___**transport** **(default:** **empty)**
       Optional  message  delivery transport that the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent should use for mailbox
       delivery to all local recipients, whether or not they are found in the UNIX passwd database.

       The precedence of [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is:  aliases,  .forward  files,
       mailbox_transport_maps,  mailbox_transport, mailbox_command_maps, mailbox_command, home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

**mailbox**___**transport**___**maps** **(default:** **empty)**
       Optional lookup tables with per-recipient message delivery transports  to  use  for  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)
       mailbox delivery, whether or not the recipients are found in the UNIX passwd database.

       The  precedence  of  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps, mailbox_transport, mailbox_command_maps, mailbox_command,  home_mail‐
       box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       For safety reasons, this feature does not allow $number substitutions in  regular  expression
       maps.

       This feature is available in Postfix 2.3 and later.

**maillog**___**file** **(default:** **empty)**
       The  name of an optional logfile that is written by the Postfix [**postlogd**(8)](https://www.chedong.com/phpMan.php/man/postlogd/8/markdown) service. An empty
       value selects logging to [**syslogd**(8)](https://www.chedong.com/phpMan.php/man/syslogd/8/markdown).  Specify "/dev/stdout" to  select  logging  to  standard
       output. Stdout logging requires that Postfix is started with "postfix start-fg".

       Note  1:  The  maillog_file  parameter value must contain a prefix that is specified with the
       maillog_file_prefixes parameter.

       Note 2: Some Postfix non-daemon programs may still log information to [**syslogd**(8)](https://www.chedong.com/phpMan.php/man/syslogd/8/markdown), before they
       have processed their configuration parameters and command-line options.

       This feature is available in Postfix 3.4 and later.

**maillog**___**file**___**compressor** **(default:** **gzip)**
       The  program to run after rotating $maillog_file with "postfix logrotate". The command is run
       with the rotated logfile name as its first argument.

       This feature is available in Postfix 3.4 and later.

**maillog**___**file**___**prefixes** **(default:** **/var,** **/dev/stdout)**
       A list of allowed prefixes for a maillog_file value. This is a safety feature to contain  the
       damage  from a single configuration mistake. Specify one or more prefix strings, separated by
       comma or whitespace.

       This feature is available in Postfix 3.4 and later.

**maillog**___**file**___**rotate**___**suffix** **(default:** **%Y%m%d-%H%M%S)**
       The format of the suffix to append to $maillog_file while rotating  the  file  with  "postfix
       logrotate".  See  [**strftime**(3)](https://www.chedong.com/phpMan.php/man/strftime/3/markdown) for syntax. The default suffix, YYYYMMDD-HHMMSS, allows logs to
       be rotated frequently.

       This feature is available in Postfix 3.4 and later.

**mailq**___**path** **(default:** **see** **postconf** **-d** **output)**
       Sendmail compatibility feature that specifies where  the  Postfix  [**mailq**(1)](https://www.chedong.com/phpMan.php/man/mailq/1/markdown)  command  is  in‐
       stalled. This command can be used to list the Postfix mail queue.

**manpage**___**directory** **(default:** **see** **postconf** **-d** **output)**
       Where the Postfix manual pages are installed.

**maps**___**rbl**___**domains** **(default:** **empty)**
       Obsolete feature: use the reject_rbl_client feature instead.

**maps**___**rbl**___**reject**___**code** **(default:** **554)**
       The  numerical Postfix SMTP server response code when a remote SMTP client request is blocked
       by   the    reject_rbl_client,    reject_rhsbl_client,    reject_rhsbl_reverse_client,    re‐
       ject_rhsbl_sender or reject_rhsbl_recipient restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

**masquerade**___**classes** **(default:** **envelope**___**sender,** **header**___**sender,** **header**___**recipient)**
       What addresses are subject to address masquerading.

       By  default,  address  masquerading  is  limited  to envelope sender addresses, and to header
       sender and header recipient addresses.  This allows you to use address masquerading on a mail
       gateway while still being able to forward mail to users on individual machines.

       Specify zero or more of: envelope_sender, envelope_recipient, header_sender, header_recipient

**masquerade**___**domains** **(default:** **empty)**
       Optional list of domains whose subdomain structure will be stripped off in email addresses.

       The list is processed left to right, and processing stops at the first match.  Thus,

           masquerade_domains = foo.example.com example.com

       strips     "<user@any.thing.foo.example.com>"    to    "<user@foo.example.com>",    but    strips
       "<user@any.thing.else.example.com>" to "<user@example.com>".

       A domain name prefixed with ! means do not masquerade this domain or its subdomains. Thus,

           masquerade_domains = !foo.example.com example.com

       does  not  change  "<user@any.thing.foo.example.com>"  or  "<user@foo.example.com>",  but  strips
       "<user@any.thing.else.example.com>" to "<user@example.com>".

       Note: with Postfix version 2.2, message header address masquerading happens only when message
       header address rewriting is enabled:

       •      The message is received with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command,

       •      The  message  is  received  from  a  network  client  that  matches  $local_header_re‐
              write_clients,

       •      The message is received from the network, and the remote_header_rewrite_domain parame‐
              ter specifies a non-empty value.

       To get the behavior before  Postfix  version  2.2,  specify  "local_header_rewrite_clients  =
       static:all".

       Example:

       masquerade_domains = $mydomain

**masquerade**___**exceptions** **(default:** **empty)**
       Optional  list  of user names that are not subjected to address masquerading, even when their
       addresses match $masquerade_domains.

       By default, address masquerading makes no exceptions.

       Specify a list of user names, "/file/name" or  "type:table"  patterns,  separated  by  commas
       and/or  whitespace.  The  list  is  matched  left to right, and the search stops on the first
       match. A "/file/name" pattern is replaced by its contents; a  "type:table"  lookup  table  is
       matched when a name matches a lookup key (the lookup result is ignored).  Continue long lines
       by starting the next line with whitespace. Specify "!pattern" to  exclude  a  name  from  the
       list. The form "!/file/name" is supported only in Postfix version 2.4 and later.

       Examples:

       masquerade_exceptions = root, mailer-daemon
       masquerade_exceptions = root

**master**___**service**___**disable** **(default:** **empty)**
       Selectively  disable  [**master**(8)](https://www.chedong.com/phpMan.php/man/master/8/markdown)  listener  ports by service type or by service name and type.
       Specify a list of service types ("inet", "unix", "fifo", or "pass")  or  "name/type"  tuples,
       where  "name"  is  the first field of a master.cf entry and "type" is a service type. As with
       other Postfix matchlists, a search stops at the first match.  Specify "!pattern" to exclude a
       service from the list. By default, all [**master**(8)](https://www.chedong.com/phpMan.php/man/master/8/markdown) listener ports are enabled.

       Note:  this  feature does not support "/file/name" or "type:table" patterns, nor does it sup‐
       port wildcards such as "*" or "all". This is intentional.

       Examples:

       # With Postfix 2.6..2.10 use '.' instead of '/'.
       # Turn on all [**master**(8)](https://www.chedong.com/phpMan.php/man/master/8/markdown) listener ports (the default).
       master_service_disable =
       # Turn off only the main SMTP listener port.
       master_service_disable = smtp/inet
       # Turn off all TCP/IP listener ports.
       master_service_disable = inet
       # Turn off all TCP/IP listener ports except "foo".
       master_service_disable = !foo/inet, inet

       This feature is available in Postfix 2.6 and later.

**max**___**idle** **(default:** **100s)**
       The maximum amount of time that an idle Postfix daemon process waits for an incoming  connec‐
       tion  before terminating voluntarily.  This parameter is ignored by the Postfix queue manager
       and by other long-lived Postfix daemon processes.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**max**___**use** **(default:** **100)**
       The  maximal number of incoming connections that a Postfix daemon process will service before
       terminating voluntarily.  This parameter is ignored by the Postfix queue manager and by other
       long-lived Postfix daemon processes.

**maximal**___**backoff**___**time** **(default:** **4000s)**
       The maximal time between attempts to deliver a deferred message.

       This  parameter  should be set to a value greater than or equal to $minimal_backoff_time. See
       also $queue_run_delay.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**maximal**___**queue**___**lifetime** **(default:** **5d)**
       Consider a message as undeliverable, when delivery fails with a temporary error, and the time
       in the queue has reached the maximal_queue_lifetime limit.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is d (days).

       Specify 0 when mail delivery should be tried only once.

**message**___**drop**___**headers** **(default:** **bcc,** **content-length,** **resent-bcc,** **return-path)**
       Names   of   message   headers   that  the  [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown)  daemon  will  remove  after  applying
       **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) and before invoking Milter applications.  The default setting is  compatible
       with Postfix < 3.0.

       Specify a list of header names, separated by comma or space.  Names are matched in a case-in‐
       sensitive manner.  The list of supported header names is limited only by available memory.

       This feature is available in Postfix 3.0 and later.

**message**___**reject**___**characters** **(default:** **empty)**
       The set of characters that Postfix will reject in message content.  The usual  C-like  escape
       sequences are recognized: \a \b \f \n \r \t \v \_ddd_ (up to three octal digits) and \\.

       Note  1:  this  feature  does not recognize text that requires MIME decoding. It inspects raw
       message content, just like header_checks and body_checks.

       Note 2: this feature is disabled with "receive_override_options = no_header_body_checks".

       Example:

       message_reject_characters = \0

       This feature is available in Postfix 2.3 and later.

**message**___**size**___**limit** **(default:** **10240000)**
       The maximal size in bytes of a message, including envelope information.

       Note: be careful when making changes.  Excessively small values will result in  the  loss  of
       non-delivery notifications, when a bounce message size exceeds the local or remote MTA's mes‐
       sage size limit.

**message**___**strip**___**characters** **(default:** **empty)**
       The set of characters that Postfix will remove from message content.  The usual C-like escape
       sequences are recognized: \a \b \f \n \r \t \v \_ddd_ (up to three octal digits) and \\.

       Note  1:  this  feature  does not recognize text that requires MIME decoding. It inspects raw
       message content, just like header_checks and body_checks.

       Note 2: this feature is disabled with "receive_override_options = no_header_body_checks".

       Example:

       message_strip_characters = \0

       This feature is available in Postfix 2.3 and later.

**meta**___**directory** **(default:** **see** **'postconf** **-d'** **output)**
       The location of non-executable files that are shared among multiple Postfix  instances,  such
       as  postfix-files,  dynamicmaps.cf,  and  the multi-instance template files main.cf.proto and
       master.cf.proto.  This directory should contain only Postfix-related files.   Typically,  the
       meta_directory parameter has the same default as the config_directory parameter (/etc/postfix
       or /usr/local/etc/postfix).

       For backwards compatibility with Postfix versions 2.6..2.11, specify "meta_directory =  $dae‐
       mon_directory"  in main.cf before installing or upgrading Postfix, or specify "meta_directory
       = /path/name" on the "make makefiles", "make install" or "make upgrade" command line.

       This feature is available in Postfix 3.0 and later.

**milter**___**command**___**timeout** **(default:** **30s)**
       The time limit for sending an SMTP command to a Milter (mail filter) application, and for re‐
       ceiving the response.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default  time  unit
       is s (seconds).

       This feature is available in Postfix 2.3 and later.

**milter**___**connect**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The  macros  that  are  sent to Milter (mail filter) applications after completion of an SMTP
       connection. See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**connect**___**timeout** **(default:** **30s)**
       The time limit for connecting to a Milter (mail filter) application, and for negotiating pro‐
       tocol options.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default  time  unit
       is s (seconds).

       This feature is available in Postfix 2.3 and later.

**milter**___**content**___**timeout** **(default:** **300s)**
       The time limit for sending message content to a Milter (mail filter) application, and for re‐
       ceiving the response.

       Specify a non-zero time value (an integral value plus  an  optional  one-letter  suffix  that
       specifies the time unit).

       Time  units:  s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit
       is s (seconds).

       This feature is available in Postfix 2.3 and later.

**milter**___**data**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The macros that are sent to version 4 or higher Milter (mail filter) applications  after  the
       SMTP DATA command. See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**default**___**action** **(default:** **tempfail)**
       The  default  action  when  a  Milter (mail filter) response is unavailable (for example, bad
       Postfix configuration or Milter failure). Specify one of the following:

       accept Proceed as if the mail filter was not present.

       reject Reject all further commands in this session with a permanent status code.

       tempfail
              Reject all further commands in this session with a temporary status code.

       quarantine
              Like "accept", but freeze the message in the "hold" queue. Available with Postfix  2.6
              and later.

       This feature is available in Postfix 2.3 and later.

**milter**___**end**___**of**___**data**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The  macros that are sent to Milter (mail filter) applications after the message end-of-data.
       See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**end**___**of**___**header**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The macros that are sent to Milter (mail filter) applications after the end  of  the  message
       header. See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.5 and later.

**milter**___**header**___**checks** **(default:** **empty)**
       Optional  lookup tables for content inspection of message headers that are produced by Milter
       applications.  See the **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) manual page available actions. Currently, PREPEND  is
       not implemented.

       The  following example sends all mail that is marked as SPAM to a spam handling machine. Note
       that matches are case-insensitive by default.

       /etc/postfix/main.cf:
           milter_header_checks = pcre:/etc/postfix/milter_header_checks

       /etc/postfix/milter_header_checks:
           /^X-SPAM-FLAG:\s+YES/ FILTER mysmtp:sanitizer.example.com:25

       The milter_header_checks mechanism could also be used for allowlisting. For example it  could
       be used to skip heavy content inspection for DKIM-signed mail from known friendly domains.

       This feature is available in Postfix 2.7, and as an optional patch for Postfix 2.6.

**milter**___**helo**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The  macros  that  are  sent to Milter (mail filter) applications after the SMTP HELO or EHLO
       command. See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**macro**___**daemon**___**name** **(default:** **$myhostname)**
       The {daemon_name} macro value for Milter (mail filter) applications.  See MILTER_README for a
       list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**macro**___**defaults** **(default:** **empty)**
       Optional list of _name=value_ pairs that specify default values for arbitrary macros that Post‐
       fix may send to Milter applications.  These defaults are used when there is no  corresponding
       information from the message delivery context.

       Specify _name=value_ or _{name}=value_ pairs separated by comma or whitespace.  Enclose a pair in
       "{}" when a value contains comma or whitespace (this form ignores whitespace  after  the  en‐
       closing "{", around the "=", and before the enclosing "}").

       This feature is available in Postfix 3.1 and later.

**milter**___**macro**___**v** **(default:** **$mail**___**name** **$mail**___**version)**
       The  {v}  macro value for Milter (mail filter) applications.  See MILTER_README for a list of
       available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**mail**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The macros that are sent to Milter (mail filter) applications after the SMTP MAIL  FROM  com‐
       mand. See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**protocol** **(default:** **6)**
       The  mail  filter  protocol version and optional protocol extensions for communication with a
       Milter application; prior to Postfix 2.6 the default protocol is 2. Postfix sends  this  ver‐
       sion  number  during the initial protocol handshake.  It should match the version number that
       is expected by the mail filter application (or by its Milter library).

       Protocol versions:

       2      Use Sendmail 8 mail filter protocol version 2 (default with Sendmail version  8.11  ..
              8.13 and Postfix version 2.3 ..  2.5).

       3      Use Sendmail 8 mail filter protocol version 3.

       4      Use Sendmail 8 mail filter protocol version 4.

       6      Use  Sendmail 8 mail filter protocol version 6 (default with Sendmail version 8.14 and
              Postfix version 2.6).

       Protocol extensions:

       no_header_reply
              Specify this when the Milter application will not reply for  each  individual  message
              header.

       This feature is available in Postfix 2.3 and later.

**milter**___**rcpt**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The macros that are sent to Milter (mail filter) applications after the SMTP RCPT TO command.
       See MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

**milter**___**unknown**___**command**___**macros** **(default:** **see** **postconf** **-d** **output)**
       The macros that are sent to version 3 or higher Milter (mail filter)  applications  after  an
       unknown  SMTP command.  See MILTER_README for a list of available macro names and their mean‐
       ings.

       This feature is available in Postfix 2.3 and later.

**mime**___**boundary**___**length**___**limit** **(default:** **2048)**
       The maximal length of MIME multipart boundary strings. The MIME processor is unable  to  dis‐
       tinguish between boundary strings that do not differ in the first $mime_boundary_length_limit
       characters.

       This feature is available in Postfix 2.0 and later.

**mime**___**header**___**checks** **(default:** **$header**___**checks)**
       Optional lookup tables for content inspection of MIME related message headers,  as  described
       in the **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) manual page.

       This feature is available in Postfix 2.0 and later.

**mime**___**nesting**___**limit** **(default:** **100)**
       The  maximal  recursion level that the MIME processor will handle.  Postfix refuses mail that
       is nested deeper than the specified limit.

       This feature is available in Postfix 2.0 and later.

**minimal**___**backoff**___**time** **(default:** **300s)**
       The minimal time between attempts to deliver a deferred message; prior to Postfix 2.4 the de‐
       fault value was 1000s.

       This  parameter  also  limits  the time an unreachable destination is kept in the short-term,
       in-memory, destination status cache.

       This parameter should be set greater than or  equal  to  $queue_run_delay.  See  also  $maxi‐
       mal_backoff_time.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**multi**___**instance**___**directories** **(default:** **empty)**
       An optional list of non-default Postfix configuration directories; these  directories  belong
       to  additional  Postfix  instances  that share the Postfix executable files and documentation
       with the default Postfix instance, and that are started, stopped, etc., together with the de‐
       fault Postfix instance.  Specify a list of pathnames separated by comma or whitespace.

       When  $multi_instance_directories  is  empty,  the [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) command runs in single-instance
       mode and operates on a single Postfix instance only. Otherwise, the [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown)  command  runs
       in  multi-instance  mode  and invokes the multi-instance manager specified with the multi_in‐
       stance_wrapper parameter. The multi-instance manager in turn executes [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) commands for
       the default instance and for all Postfix instances in $multi_instance_directories.

       Currently, this parameter setting is ignored except for the default main.cf file.

       This feature is available in Postfix 2.6 and later.

**multi**___**instance**___**enable** **(default:** **no)**
       Allow  this  Postfix  instance to be started, stopped, etc., by a multi-instance manager.  By
       default, new instances are created in a safe state that prevents them from being started  in‐
       advertently.  This parameter is reserved for the multi-instance manager.

       This feature is available in Postfix 2.6 and later.

**multi**___**instance**___**group** **(default:** **empty)**
       The optional instance group name of this Postfix instance. A group identifies closely-related
       Postfix instances that the multi-instance manager can start, stop, etc., as a unit.  This pa‐
       rameter is reserved for the multi-instance manager.

       This feature is available in Postfix 2.6 and later.

**multi**___**instance**___**name** **(default:** **empty)**
       The optional instance name of this Postfix instance. This name becomes also the default value
       for the syslog_name parameter.

       This feature is available in Postfix 2.6 and later.

**multi**___**instance**___**wrapper** **(default:** **empty)**
       The pathname of a multi-instance manager command that the [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) command invokes when the
       multi_instance_directories parameter value is non-empty. The pathname may be followed by ini‐
       tial command arguments separated by whitespace; shell metacharacters such as quotes  are  not
       supported in this context.

       The [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) command invokes the manager command with the [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) non-option command ar‐
       guments on the manager command line, and with all installation configuration  parameters  ex‐
       ported  into the manager command process environment. The manager command in turn invokes the
       [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) command for individual Postfix instances as "postfix -c _config_directory_ _command_".

       This feature is available in Postfix 2.6 and later.

**multi**___**recipient**___**bounce**___**reject**___**code** **(default:** **550)**
       The numerical Postfix SMTP server response code when a remote SMTP client request is  blocked
       by the reject_multi_recipient_bounce restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

       This feature is available in Postfix 2.1 and later.

### mydestination (default: $myhostname, localhost.$mydomain, localhost)
       The  list  of domains that are delivered via the $local_transport mail delivery transport. By
       default this is the Postfix  [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery  agent  which  looks  up  all  recipients  in
       /etc/passwd  and  /etc/aliases. The SMTP server validates recipient addresses with $local_re‐
       cipient_maps and rejects non-existent recipients. See also the local domain class in the  AD‐
       DRESS_CLASS_README file.

       The default mydestination value specifies names for the local machine only.  On a mail domain
       gateway, you should also include $mydomain.

       The  $local_transport  delivery   method   is   also   selected   for   mail   addressed   to
       user@[the.net.work.address]  of the mail system (the IP addresses specified with the inet_in‐
       terfaces and proxy_interfaces parameters).

       Warnings:

       •      Do not specify the names of virtual domains - those domains are  specified  elsewhere.
              See VIRTUAL_README for more information.

       •      Do not specify the names of domains that this machine is backup MX host for. See STAN‐
              DARD_CONFIGURATION_README for how to set up backup MX hosts.

       •      By default, the Postfix SMTP server rejects mail for recipients not  listed  with  the
              local_recipient_maps  parameter.   See the [**postconf**(5)](https://www.chedong.com/phpMan.php/man/postconf/5/markdown) manual for a description of the
              local_recipient_maps and unknown_local_recipient_reject_code parameters.

       Specify a list of host or domain names, "/file/name" or "type:table" patterns,  separated  by
       commas  and/or whitespace. A "/file/name" pattern is replaced by its contents; a "type:table"
       lookup table is matched when a name matches a lookup key  (the  lookup  result  is  ignored).
       Continue long lines by starting the next line with whitespace.

       Examples:

       mydestination = $myhostname, localhost.$mydomain $mydomain
       mydestination = $myhostname, localhost.$mydomain www.$mydomain, ftp.$mydomain

### mydomain (default: see postconf -d output)
       The  internet  domain  name of this mail system.  The default is to use $myhostname minus the
       first component, or "localdomain" (Postfix 2.3 and later).  $mydomain is used  as  a  default
       value for many other configuration parameters.

       Example:

       mydomain = domain.tld

### myhostname (default: see postconf -d output)
       The  internet  hostname of this mail system. The default is to use the fully-qualified domain
       name (FQDN) from gethostname(), or to use the non-FQDN result from gethostname()  and  append
       ".$mydomain".   $myhostname  is  used as a default value for many other configuration parame‐
       ters.

       Example:

       myhostname = host.example.com

### mynetworks (default: see postconf -d output)
       The list of "trusted" remote SMTP clients that have more privileges than "strangers".

       In particular, "trusted" SMTP clients are allowed to relay mail  through  Postfix.   See  the
       smtpd_relay_restrictions parameter description in the [**postconf**(5)](https://www.chedong.com/phpMan.php/man/postconf/5/markdown) manual.

       You  can specify the list of "trusted" network addresses by hand or you can let Postfix do it
       for you (which is the default).  See the description of the  mynetworks_style  parameter  for
       more information.

       If you specify the mynetworks list by hand, Postfix ignores the mynetworks_style setting.

       Specify  a  list of network addresses or network/netmask patterns, separated by commas and/or
       whitespace. Continue long lines by starting the next line with whitespace.

       The netmask specifies the number of bits in the network part of a host address.  You can also
       specify  "/file/name"  or  "type:table"  patterns.  A "/file/name" pattern is replaced by its
       contents; a "type:table" lookup table is matched when a table entry matches a  lookup  string
       (the lookup result is ignored).

       The  list  is matched left to right, and the search stops on the first match.  Specify "!pat‐
       tern" to exclude an address or network block from the list. The form  "!/file/name"  is  sup‐
       ported only in Postfix version 2.4 and later.

       Note  1:  Pattern matching of domain names is controlled by the or absence of "mynetworks" in
       the parent_domain_matches_subdomains parameter value.

       Note 2: IP version 6 address information must be specified inside [] in the mynetworks value,
       and  in files specified with "/file/name".  IP version 6 addresses contain the ":" character,
       and would otherwise be confused with a "type:table" pattern.

       Examples:

       mynetworks = 127.0.0.0/8 168.100.189.0/28
       mynetworks = !192.168.0.1, 192.168.0.0/28
       mynetworks = 127.0.0.0/8 168.100.189.0/28 [::1]/128 [2001:240:587::]/64
       mynetworks = $config_directory/mynetworks
       mynetworks = hash:/etc/postfix/network_table

**mynetworks**___**style** **(default:** **Postfix** **>=** **3.0:** **host,** **Postfix** **<** **3.0:** **subnet)**
       The method to generate the default value for the mynetworks parameter.  This is the  list  of
       trusted networks for relay access control etc.

       •      Specify "mynetworks_style = host" when Postfix should "trust" only the local machine.

       •      Specify "mynetworks_style = subnet" when Postfix should "trust" remote SMTP clients in
              the same IP subnetworks as the local machine.  On Linux,  this  works  correctly  only
              with interfaces specified with the "ifconfig" command.

       •      Specify  "mynetworks_style = class" when Postfix should "trust" remote SMTP clients in
              the same IP class A/B/C networks as the local machine.  Caution: this may cause  Post‐
              fix  to  "trust"  your entire provider's network.  Instead, specify an explicit mynet‐
              works list by hand, as described with the mynetworks configuration parameter.

### myorigin (default: $myhostname)
       The domain name that locally-posted mail appears to come from, and that locally  posted  mail
       is  delivered to. The default, $myhostname, is adequate for small sites.  If you run a domain
       with multiple machines, you should (1) change this to $mydomain and (2) set up a  domain-wide
       alias database that aliases each user to <user@that.users.mailhost>.

       Example:

       myorigin = $mydomain

**nested**___**header**___**checks** **(default:** **$header**___**checks)**
       Optional  lookup  tables  for content inspection of non-MIME message headers in attached mes‐
       sages, as described in the **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) manual page.

       This feature is available in Postfix 2.0 and later.

**newaliases**___**path** **(default:** **see** **postconf** **-d** **output)**
       Sendmail compatibility feature that specifies the location of the [**newaliases**(1)](https://www.chedong.com/phpMan.php/man/newaliases/1/markdown) command. This
       command can be used to rebuild the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) database.

**non**___**fqdn**___**reject**___**code** **(default:** **504)**
       The  numerical  Postfix  SMTP  server reply code when a client request is rejected by the re‐
       ject_non_fqdn_helo_hostname, reject_non_fqdn_sender or reject_non_fqdn_recipient restriction.

**non**___**smtpd**___**milters** **(default:** **empty)**
       A list of Milter (mail filter) applications for new mail that does not arrive via the Postfix
       [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown)  server.  This  includes local submission via the [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command line, new mail
       that arrives via the Postfix [**qmqpd**(8)](https://www.chedong.com/phpMan.php/man/qmqpd/8/markdown) server, and old mail that is re-injected into the queue
       with "postsuper -r".  Specify space or comma as separator. See the MILTER_README document for
       details.

       This feature is available in Postfix 2.3 and later.

**notify**___**classes** **(default:** **resource,** **software)**
       The list of error classes that are reported to the postmaster. The default is to report  only
       the  most  serious problems. The paranoid may wish to turn on the policy (UCE and mail relay‐
       ing) and protocol error (broken mail software) reports.

       NOTE: postmaster notifications may contain confidential information such as SASL passwords or
       message  content.   It is the system administrator's responsibility to treat such information
       with care.

       The error classes are:

       **bounce** (also implies **2bounce**)
              Send the postmaster copies of the headers of bounced mail,  and  send  transcripts  of
              SMTP sessions when Postfix rejects mail. The notification is sent to the address spec‐
              ified with the bounce_notice_recipient configuration parameter (default: postmaster).

### 2bounce
              Send undeliverable bounced mail to the postmaster. The notification is sent to the ad‐
              dress  specified  with  the 2bounce_notice_recipient configuration parameter (default:
              postmaster).

       **data**   Send the postmaster a transcript of the SMTP session with an error because a  critical
              data  file was unavailable. The notification is sent to the address specified with the
              error_notice_recipient configuration parameter (default: postmaster).
              This feature is available in Postfix 2.9 and later.

       **delay**  Send the postmaster copies of the headers of delayed  mail  (see  delay_warning_time).
              The notification is sent to the address specified with the delay_notice_recipient con‐
              figuration parameter (default: postmaster).

       **policy** Send the postmaster a transcript of the SMTP session when a  client  request  was  re‐
              jected because of (UCE) policy. The notification is sent to the address specified with
              the error_notice_recipient configuration parameter (default: postmaster).

### protocol
              Send the postmaster a transcript of the SMTP session in case of client or server  pro‐
              tocol  errors.  The  notification  is sent to the address specified with the error_no‐
              tice_recipient configuration parameter (default: postmaster).

### resource
              Inform the postmaster of mail not delivered due to resource problems.   The  notifica‐
              tion  is  sent  to the address specified with the error_notice_recipient configuration
              parameter (default: postmaster).

### software
              Inform the postmaster of mail not delivered due to software problems.   The  notifica‐
              tion  is  sent  to the address specified with the error_notice_recipient configuration
              parameter (default: postmaster).

       Examples:

       notify_classes = bounce, delay, policy, protocol, resource, software
       notify_classes = 2bounce, resource, software

**nullmx**___**reject**___**code** **(default:** **556)**
       The numerical reply code when the Postfix SMTP server rejects a sender or  recipient  address
       because its domain has a nullmx DNS record (an MX record with an empty hostname). This is one
       of the possible replies from the  restrictions  reject_unknown_sender_domain  and  reject_un‐
       known_recipient_domain.

       This feature is available in Postfix 3.0 and later.

**openssl**___**path** **(default:** **openssl)**
       The  location  of  the OpenSSL command line program [**openssl**(1)](https://www.chedong.com/phpMan.php/man/openssl/1/markdown).  This is used by the "**postfix**
       **tls**" command to create private keys, certificate signing requests, self-signed  certificates,
       and  to  compute  public  key digests for DANE TLSA records.  In multi-instance environments,
       this parameter is always determined from the configuration of the default Postfix instance.

       Example:

           /etc/postfix/main.cf:
               # NetBSD pkgsrc:
               openssl_path = /usr/pkg/bin/openssl
               # Local build:
               openssl_path = /usr/local/bin/openssl

       This feature is available in Postfix 3.1 and later.

**owner**___**request**___**special** **(default:** **yes)**
       Enable special treatment for owner-_listname_ entries in the [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown) file, and  don't  split
       owner-_listname_ and _listname_-request address localparts when the recipient_delimiter is set to
       "-".  This feature is useful for mailing lists.

**parent**___**domain**___**matches**___**subdomains** **(default:** **see** **postconf** **-d** **output)**
       A list of Postfix features where the pattern "example.com" also matches subdomains  of  exam‐
       ple.com,  instead of requiring an explicit ".example.com" pattern.  This is planned backwards
       compatibility:  eventually, all Postfix features are expected  to  require  explicit  ".exam‐
       ple.com" style patterns when you really want to match subdomains.

       The following Postfix feature names are supported.

       Postfix version 1.0 and later
              debug_peer_list,  fast_flush_domains, mynetworks, permit_mx_backup_networks, relay_do‐
              mains, transport_maps

       Postfix version 1.1 and later
              qmqpd_authorized_clients, smtpd_access_maps,

       Postfix version 2.8 and later
              postscreen_access_list

       Postfix version 3.0 and later
              smtpd_client_event_limit_exceptions

**permit**___**mx**___**backup**___**networks** **(default:** **empty)**
       Restrict the use of the permit_mx_backup SMTP access feature to only domains whose primary MX
       hosts  match  the listed networks.  The parameter value syntax is the same as with the mynet‐
       works parameter; note, however, that the default value is empty.

       Pattern matching of  domain  names  is  controlled  by  the  presence  or  absence  of  "per‐
       mit_mx_backup_networks" in the parent_domain_matches_subdomains parameter value.

**pickup**___**service**___**name** **(default:** **pickup)**
       The  name  of  the  [**pickup**(8)](https://www.chedong.com/phpMan.php/man/pickup/8/markdown)  service. This service picks up local mail submissions from the
       Postfix maildrop queue.

       This feature is available in Postfix 2.0 and later.

**pipe**___**delivery**___**status**___**filter** **(default:** **$default**___**delivery**___**status**___**filter)**
       Optional filter for the [**pipe**(8)](https://www.chedong.com/phpMan.php/man/pipe/8/markdown) delivery agent to change the delivery status code or explana‐
       tory  text  of successful or unsuccessful deliveries.  See default_delivery_status_filter for
       details.

       This feature is available in Postfix 3.0 and later.

**plaintext**___**reject**___**code** **(default:** **450)**
       The numerical Postfix SMTP server response code  when  a  request  is  rejected  by  the  **re**‐‐
       **ject**___**plaintext**___**session** restriction.

       This feature is available in Postfix 2.3 and later.

**postlog**___**service**___**name** **(default:** **postlog)**
       The name of the [**postlogd**(8)](https://www.chedong.com/phpMan.php/man/postlogd/8/markdown) service entry in master.cf.  This service appends logfile records
       to the file specified with the maillog_file parameter.

       This feature is available in Postfix 3.4 and later.

**postlogd**___**watchdog**___**timeout** **(default:** **10s)**
       How much time a [**postlogd**(8)](https://www.chedong.com/phpMan.php/man/postlogd/8/markdown) process may take to process a request before it is terminated  by
       a built-in watchdog timer. This is a safety mechanism that prevents [**postlogd**(8)](https://www.chedong.com/phpMan.php/man/postlogd/8/markdown) from becoming
       non-responsive due to a bug in Postfix itself or in system software. This limit cannot be set
       under 10s.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).  Time units: s (seconds), m  (minutes),  h  (hours),  d  (days),  w
       (weeks).

       This feature is available in Postfix 3.4 and later.

**postmulti**___**control**___**commands** **(default:** **reload** **flush)**
       The  [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) commands that the [**postmulti**(1)](https://www.chedong.com/phpMan.php/man/postmulti/1/markdown) instance manager treats as "control" commands,
       that operate on running instances. For these commands, disabled instances are skipped.

       This feature is available in Postfix 2.6 and later.

**postmulti**___**start**___**commands** **(default:** **start)**
       The [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) commands that the [**postmulti**(1)](https://www.chedong.com/phpMan.php/man/postmulti/1/markdown) instance manager treats  as  "start"  commands.
       For  these  commands,  disabled instances are "checked" rather than "started", and failure to
       "start" a member instance of an instance group will abort the start-up of later instances.

       This feature is available in Postfix 2.6 and later.

**postmulti**___**stop**___**commands** **(default:** **see** **postconf** **-d** **output)**
       The [**postfix**(1)](https://www.chedong.com/phpMan.php/man/postfix/1/markdown) commands that the [**postmulti**(1)](https://www.chedong.com/phpMan.php/man/postmulti/1/markdown) instance manager treats as "stop" commands. For
       these  commands,  disabled  instances are skipped, and enabled instances are processed in re‐
       verse order.

       This feature is available in Postfix 2.6 and later.

**postscreen**___**access**___**list** **(default:** **permit**___**mynetworks)**
       Permanent allow/denylist for remote SMTP client IP addresses.   [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)  searches  this
       list  immediately  after a remote SMTP client connects.  Specify a comma- or whitespace-sepa‐
       rated list of commands (in upper or lower case) or lookup tables. The search stops  upon  the
       first command that fires for the client IP address.

        **permit**___**mynetworks**
              Allowlist the client and terminate the search if the client IP address matches $mynet‐
              works.  Do not subject the client to any before/after 220 greeting  tests.   Pass  the
              connection immediately to a Postfix SMTP server process.
              Pattern  matching  of  domain  names  is  controlled  by  the  presence  or absence of
              "postscreen_access_list" in the parent_domain_matches_subdomains parameter value.

### type:table
              Query the specified lookup table. Each table lookup result is an access  list,  except
              that access lists inside a table cannot specify type:table entries.
              To  discourage  the use of hash, btree, etc. tables, there is no support for substring
              matching like [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown). Use CIDR tables instead.

### permit
              Allowlist the client and terminate the search. Do not subject the client  to  any  be‐
              fore/after  220  greeting  tests.  Pass  the  connection immediately to a Postfix SMTP
              server process.

### reject
              Denylist the client and terminate the search. Subject the client to the action config‐
              ured with the postscreen_denylist_action configuration parameter.

        **dunno** All [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) access lists implicitly have this command at the end.
              When  **dunno** is executed inside a lookup table, return from the lookup table and evalu‐
              ate the next command.
              When  **dunno**  is executed outside a lookup table, terminate the search, and subject the
              client to the configured before/after 220 greeting tests.

       Example:

       /etc/postfix/main.cf:
           postscreen_access_list = permit_mynetworks,
               cidr:/etc/postfix/postscreen_access.cidr
           # Postfix < 3.6 use postscreen_blacklist_action.
           postscreen_denylist_action = enforce

       /etc/postfix/postscreen_access.cidr:
           # Rules are evaluated in the order as specified.
           # Denylist 192.168.* except 192.168.0.1.
           192.168.0.1         dunno
           192.168.0.0/16      reject

       This feature is available in Postfix 2.8.

**postscreen**___**allowlist**___**interfaces** **(default:** **static:all)**
       A  list of local [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server IP addresses where a non-allowlisted remote SMTP client
       can obtain [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)'s temporary allowlist status. This status  is  required  before  the
       client  can  talk  to  a  Postfix  SMTP  server  process.   By  default,  a client can obtain
       [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)'s allowlist status on any local [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server IP address.

       When [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) listens on both  primary  and  backup  MX  addresses,  the  postscreen_al‐
       lowlist_interfaces  parameter  can  be configured to give the temporary allowlist status only
       when a client connects to a primary MX address. Once a client is allowlisted it can talk to a
       Postfix  SMTP  server  on any address. Thus, clients that connect only to backup MX addresses
       will never become allowlisted, and will never be allowed to talk to  a  Postfix  SMTP  server
       process.

       Specify  a  list of network addresses or network/netmask patterns, separated by commas and/or
       whitespace. The netmask specifies the number of bits in the network part of a  host  address.
       Continue long lines by starting the next line with whitespace.

       You  can  also  specify "/file/name" or "type:table" patterns.  A "/file/name" pattern is re‐
       placed by its contents; a "type:table" lookup table is matched when a table entry  matches  a
       lookup string (the lookup result is ignored).

       The  list  is  matched left to right, and the search stops on the first match. Specify "!pat‐
       tern" to exclude an address or network block from the list.

       Note: IP version 6 address information must be specified  inside  []  in  the  postscreen_al‐
       lowlist_interfaces  value,  and in files specified with "/file/name".  IP version 6 addresses
       contain the ":" character, and would otherwise be confused with a "type:table" pattern.

       Example:

       /etc/postfix/main.cf:
           # Don't allowlist connections to the backup IP address.
           # Postfix < 3.6 use postscreen_whitelist_interfaces.
           postscreen_allowlist_interfaces = !168.100.189.8, static:all

       This feature is available in Postfix 3.6 and later.

       Available as postscreen_whitelist_interfaces in Postfix 2.9 - 3.5.

**postscreen**___**bare**___**newline**___**action** **(default:** **ignore)**
       The action that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) takes when a remote SMTP client sends a bare newline character,
       that is, a newline not preceded by carriage return.  Specify one of the following:

       **ignore** Ignore  the  failure  of this test. Allow other tests to complete.  Do _not_ repeat this
              test before some the result from some other test expires.  This option is  useful  for
              testing and collecting statistics without blocking mail permanently.

### enforce
              Allow  other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
              and log the helo/sender/recipient information.  Repeat this test  the  next  time  the
              client connects.

       **drop**   Drop  the connection immediately with a 521 SMTP reply. Repeat this test the next time
              the client connects.

       This feature is available in Postfix 2.8.

**postscreen**___**bare**___**newline**___**enable** **(default:** **no)**
       Enable "bare newline" SMTP protocol tests in the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server. These tests are expen‐
       sive:  a remote SMTP client must disconnect after it passes the test, before it can talk to a
       real Postfix SMTP server.

       This feature is available in Postfix 2.8.

**postscreen**___**bare**___**newline**___**ttl** **(default:** **30d)**
       The amount of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a  successful  "bare  newline"
       SMTP  protocol  test. During this time, the client IP address is excluded from this test. The
       default is long because a remote SMTP client must disconnect after it passes the test, before
       it can talk to a real Postfix SMTP server.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).  Time units: s (seconds), m  (minutes),  h  (hours),  d  (days),  w
       (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**blacklist**___**action** **(default:** **ignore)**
       Renamed to postscreen_denylist_action in Postfix 3.6.

       This feature is available in Postfix 2.8 - 3.5.

**postscreen**___**cache**___**cleanup**___**interval** **(default:** **12h)**
       The  amount  of  time  between [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) cache cleanup runs.  Cache cleanup increases the
       load on the cache database and should therefore not be run frequently. This feature  requires
       that  the  cache database supports the "delete" and "sequence" operators.  Specify a zero in‐
       terval to disable cache cleanup.

       After each cache cleanup run, the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) daemon logs the number of entries  that  were
       retained  and  dropped. A cleanup run is logged as "partial" when the daemon terminates early
       after "**postfix** **reload**", "**postfix** **stop**", or no requests for $max_idle seconds.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**cache**___**map** **(default:** **btree:$data**___**directory/postscreen**___**cache)**
       Persistent storage for the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server decisions.

       To  share   a   [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)   cache   between   multiple   [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)   instances,   use
       "postscreen_cache_map  =  proxy:btree:/path/to/file".   This  requires Postfix version 2.9 or
       later; earlier [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) implementations don't support cache cleanup.  For  an  alternative
       approach see the **memcache**___**[table**(5)](https://www.chedong.com/phpMan.php/man/table/5/markdown) manpage.

       This feature is available in Postfix 2.8.

**postscreen**___**cache**___**retention**___**time** **(default:** **7d)**
       The  amount of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will cache an expired temporary allowlist entry before
       it is removed. This prevents clients from being logged as "NEW" just because their cache  en‐
       try  expired an hour ago. It also prevents the cache from filling up with clients that passed
       some deep protocol test once and never came back.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**client**___**connection**___**count**___**limit** **(default:** **$smtpd**___**client**___**connection**___**count**___**limit)**
       How many simultaneous connections any  remote  SMTP  client  is  allowed  to  have  with  the
       [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)  daemon.  By  default,  this limit is the same as with the Postfix SMTP server.
       Note  that  the  triage  process  can  take  several  seconds,  with  the   time   spent   in
       postscreen_greet_wait  delay,  and  with the time spent talking to the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) built-in
       dummy SMTP protocol engine.

       This feature is available in Postfix 2.8.

**postscreen**___**command**___**count**___**limit** **(default:** **20)**
       The limit on the total number of commands per SMTP session for [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)'s built-in  SMTP
       protocol  engine.  This SMTP engine defers or rejects all attempts to deliver mail, therefore
       there is no need to enforce separate limits on the number of junk  commands  and  error  com‐
       mands.

       This feature is available in Postfix 2.8.

**postscreen**___**command**___**filter** **(default:** **$smtpd**___**command**___**filter)**
       A  mechanism  to  transform  commands from remote SMTP clients.  See smtpd_command_filter for
       further details.

       This feature is available in Postfix 2.8 and later.

**postscreen**___**command**___**time**___**limit** **(default:** **normal:** **300s,** **overload:** **10s)**
       The time limit to read an entire command line with [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)'s built-in SMTP protocol en‐
       gine.

       This feature is available in Postfix 2.8.

**postscreen**___**denylist**___**action** **(default:** **ignore)**
       The  action that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) takes when a remote SMTP client is permanently denylisted with
       the postscreen_access_list parameter.  Specify one of the following:

### ignore (default)
              Ignore  this result. Allow other tests to complete.  Repeat this test  the  next  time
              the  client  connects.   This  option  is useful for testing and collecting statistics
              without blocking mail.

### enforce
              Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP  reply,
              and  log  the  helo/sender/recipient  information.  Repeat this test the next time the
              client connects.

       **drop**   Drop the connection immediately with a 521 SMTP reply. Repeat this test the next  time
              the client connects.

       This feature is available in Postfix 3.6 and later.

       Available as postscreen_blacklist_action in Postfix 2.8 - 3.5.

**postscreen**___**disable**___**vrfy**___**command** **(default:** **$disable**___**vrfy**___**command)**
       Disable  the SMTP VRFY command in the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) daemon.  See disable_vrfy_command for de‐
       tails.

       This feature is available in Postfix 2.8.

**postscreen**___**discard**___**ehlo**___**keyword**___**address**___**maps** **(default:** **$smtpd**___**discard**___**ehlo**___**keyword**___**address**___**maps)**
       Lookup tables, indexed by the remote SMTP client address, with case insensitive lists of EHLO
       keywords  (pipelining,  starttls,  auth, etc.) that the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server will not send in
       the EHLO response to a remote SMTP client. See smtpd_discard_ehlo_keywords for details.   The
       table is not searched by hostname for robustness reasons.

       This feature is available in Postfix 2.8 and later.

**postscreen**___**discard**___**ehlo**___**keywords** **(default:** **$smtpd**___**discard**___**ehlo**___**keywords)**
       A  case  insensitive  list  of  EHLO  keywords  (pipelining,  starttls,  auth, etc.) that the
       [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server will not send in  the  EHLO  response  to  a  remote  SMTP  client.  See
       smtpd_discard_ehlo_keywords for details.

       This feature is available in Postfix 2.8 and later.

**postscreen**___**dnsbl**___**action** **(default:** **ignore)**
       The action that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) takes when a remote SMTP client's combined DNSBL score is equal
       to  or  greater  than  a  threshold  (as  defined   with   the   postscreen_dnsbl_sites   and
       postscreen_dnsbl_threshold parameters).  Specify one of the following:

### ignore (default)
              Ignore  the failure of this test. Allow other tests to complete.  Repeat this test the
              next time the client connects.  This option is useful for testing and collecting  sta‐
              tistics without blocking mail.

### enforce
              Allow  other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
              and log the helo/sender/recipient information.  Repeat this test  the  next  time  the
              client connects.

       **drop**   Drop  the connection immediately with a 521 SMTP reply. Repeat this test the next time
              the client connects.

       This feature is available in Postfix 2.8.

**postscreen**___**dnsbl**___**allowlist**___**threshold** **(default:** **0)**
       Allow a remote SMTP client to skip "before" and "after 220 greeting" protocol tests, based on
       its combined DNSBL score as defined with the postscreen_dnsbl_sites parameter.

       Specify   a   negative   value   to   enable   this   feature.   When  a  client  passes  the
       postscreen_dnsbl_allowlist_threshold without having failed other tests, all pending  or  dis‐
       abled tests are flagged as completed with a time-to-live value equal to postscreen_dnsbl_ttl.
       When a test was already completed, its time-to-live value is updated  if  it  was  less  than
       postscreen_dnsbl_ttl.

       This feature is available in Postfix 3.6 and later.

       Available as postscreen_dnsbl_whitelist_threshold in Postfix 2.11 - 3.5.

**postscreen**___**dnsbl**___**max**___**ttl** **(default:** **${postscreen**___**dnsbl**___**ttl?{$postscreen**___**dnsbl**___**ttl}:{1}}h)**
       The maximum amount of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a successful DNS-based
       reputation test before a client IP address is required to pass that test again.  If  the  DNS
       reply  specifies a shorter TTL value, that value will be used unless it would be smaller than
       postscreen_dnsbl_min_ttl.

       Specify a non-zero time value (an integral value plus  an  optional  one-letter  suffix  that
       specifies  the  time  unit).   Time  units:  s (seconds), m (minutes), h (hours), d (days), w
       (weeks).

       This feature is available in Postfix 3.1. The default setting  is  backwards-compatible  with
       older Postfix versions.

**postscreen**___**dnsbl**___**min**___**ttl** **(default:** **60s)**
       The minimum amount of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a successful DNS-based
       reputation test before a client IP address is required to pass that test again.  If  the  DNS
       reply  specifies  a  larger TTL value, that value will be used unless it would be larger than
       postscreen_dnsbl_max_ttl.

       Specify a non-zero time value (an integral value plus  an  optional  one-letter  suffix  that
       specifies  the  time  unit).   Time  units:  s (seconds), m (minutes), h (hours), d (days), w
       (weeks).

       This feature is available in Postfix 3.1.

**postscreen**___**dnsbl**___**reply**___**map** **(default:** **empty)**
       A mapping from actual DNSBL domain name which includes a secret password, to the DNSBL domain
       name that postscreen will reply with when it rejects mail.  When no mapping is found, the ac‐
       tual DNSBL domain will be used.

       For maximal stability it is best to use a file that is read into memory such as  pcre:,  reg‐
       exp:  or  texthash:  (texthash:  is  similar  to  hash:,  except  a)  there is no need to run
       [**postmap**(1)](https://www.chedong.com/phpMan.php/man/postmap/1/markdown) before the file can be used, and b) texthash: does not detect  changes  after  the
       file is read).

       Example:

       /etc/postfix/main.cf:
           postscreen_dnsbl_reply_map = texthash:/etc/postfix/dnsbl_reply

       /etc/postfix/dnsbl_reply:
          secret.zen.spamhaus.org      zen.spamhaus.org

       This feature is available in Postfix 2.8.

**postscreen**___**dnsbl**___**sites** **(default:** **empty)**
       Optional  list  of  DNS  allow/denylist domains, filters and weight factors. When the list is
       non-empty, the [**dnsblog**(8)](https://www.chedong.com/phpMan.php/man/dnsblog/8/markdown) daemon will query these domains with the  IP  addresses  of  remote
       SMTP  clients, and [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will update an SMTP client's DNSBL score with each non-error
       reply.

       Caution: when postscreen rejects mail, it  replies  with  the  DNSBL  domain  name.  Use  the
       postscreen_dnsbl_reply_map feature to hide "password" information in DNSBL domain names.

       When   a   client's  score  is  equal  to  or  greater  than  the  threshold  specified  with
       postscreen_dnsbl_threshold, [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) can  drop  the  connection  with  the  remote  SMTP
       client.

       Specify a list of domain=filter*weight entries, separated by comma or whitespace.

       •      When  no  "=filter"  is  specified,  [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use any non-error DNSBL reply.
              Otherwise, [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) uses only DNSBL replies that match the filter. The filter has
              the  form  d.d.d.d, where each d is a number, or a pattern inside [] that contains one
              or more ";"-separated numbers or number..number ranges.

       •      When no "*weight" is specified, [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)  increments  the  remote  SMTP  client's
              DNSBL score by 1.  Otherwise, the weight must be an integral number, and [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)
              adds the specified weight to the remote SMTP client's DNSBL score.  Specify a negative
              number for allowlisting.

       •      When one postscreen_dnsbl_sites entry produces multiple DNSBL responses, [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)
              applies the weight at most once.

       Examples:

       To use example.com as a high-confidence blocklist, and to block mail with example.net and ex‐
       ample.org only when both agree:

       postscreen_dnsbl_threshold = 2
       postscreen_dnsbl_sites = example.com*2, example.net, example.org

       To filter only DNSBL replies containing 127.0.0.4:

       postscreen_dnsbl_sites = example.com=127.0.0.4

       This feature is available in Postfix 2.8.

**postscreen**___**dnsbl**___**threshold** **(default:** **1)**
       The  inclusive  lower  bound  for  blocking a remote SMTP client, based on its combined DNSBL
       score as defined with the postscreen_dnsbl_sites parameter.

       This feature is available in Postfix 2.8.

**postscreen**___**dnsbl**___**timeout** **(default:** **10s)**
       The time limit for DNSBL or DNSWL lookups. This is separate from the  timeouts  in  the  **dns**‐‐
       [**blog**(8)](https://www.chedong.com/phpMan.php/man/blog/8/markdown) daemon which are defined by system [**resolver**(3)](https://www.chedong.com/phpMan.php/man/resolver/3/markdown) routines.

       This feature is available in Postfix 3.0.

**postscreen**___**dnsbl**___**ttl** **(default:** **1h)**
       The amount of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a successful DNS-based reputa‐
       tion test before a client IP address is required to pass that test again.

       Specify a non-zero time value (an integral value plus  an  optional  one-letter  suffix  that
       specifies  the  time  unit).   Time  units:  s (seconds), m (minutes), h (hours), d (days), w
       (weeks).

       This feature is available in Postfix 2.8-3.0. It was replaced by postscreen_dnsbl_max_ttl  in
       Postfix 3.1.

**postscreen**___**dnsbl**___**whitelist**___**threshold** **(default:** **0)**
       Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.

       This feature is available in Postfix 2.11 - 3.5.

**postscreen**___**enforce**___**tls** **(default:** **$smtpd**___**enforce**___**tls)**
       Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require that clients use
       TLS encryption.  See smtpd_postscreen_enforce_tls for details.

       This feature is available in Postfix 2.8 and  later.   Preferably,  use  postscreen_tls_secu‐
       rity_level instead.

**postscreen**___**expansion**___**filter** **(default:** **see** **postconf** **-d** **output)**
       List  of characters that are permitted in postscreen_reject_footer attribute expansions.  See
       smtpd_expansion_filter for further details.

       This feature is available in Postfix 2.8 and later.

**postscreen**___**forbidden**___**commands** **(default:** **$smtpd**___**forbidden**___**commands)**
       List of commands that the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server considers in violation of the  SMTP  protocol.
       See  smtpd_forbidden_commands for syntax, and postscreen_non_smtp_command_action for possible
       actions.

       This feature is available in Postfix 2.8.

**postscreen**___**greet**___**action** **(default:** **ignore)**
       The action that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) takes when a remote SMTP client speaks before its  turn  within
       the time specified with the postscreen_greet_wait parameter.  Specify one of the following:

### ignore (default)
              Ignore  the failure of this test. Allow other tests to complete.  Repeat this test the
              next time the client connects.  This option is useful for testing and collecting  sta‐
              tistics without blocking mail.

### enforce
              Allow  other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
              and log the helo/sender/recipient information.  Repeat this test  the  next  time  the
              client connects.

       **drop**   Drop  the connection immediately with a 521 SMTP reply. Repeat this test the next time
              the client connects.

       In either case, [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will not allowlist the remote SMTP client IP address.

       This feature is available in Postfix 2.8.

**postscreen**___**greet**___**banner** **(default:** **$smtpd**___**banner)**
       The _text_ in the optional "220-_text_..." server response that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) sends ahead of  the
       real  Postfix SMTP server's "220 text..." response, in an attempt to confuse bad SMTP clients
       so that they speak before their turn (pre-greet).  Specify an empty  value  to  disable  this
       feature.

       This feature is available in Postfix 2.8.

**postscreen**___**greet**___**ttl** **(default:** **1d)**
       The  amount  of  time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a successful PREGREET test.
       During this time, the client IP address is excluded from this test. The default is relatively
       short, because a good client can immediately talk to a real Postfix SMTP server.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).  Time units: s (seconds), m  (minutes),  h  (hours),  d  (days),  w
       (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**greet**___**wait** **(default:** **normal:** **6s,** **overload:** **2s)**
       The  amount  of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will wait for an SMTP client to send a command before
       its turn, and for DNS blocklist lookup results to arrive (default:  up  to  2  seconds  under
       stress, up to 6 seconds otherwise).

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**helo**___**required** **(default:** **$smtpd**___**helo**___**required)**
       Require that a remote SMTP client sends HELO or EHLO before commencing a MAIL transaction.

       This feature is available in Postfix 2.8.

**postscreen**___**non**___**smtp**___**command**___**action** **(default:** **drop)**
       The action that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) takes when a remote SMTP  client  sends  non-SMTP  commands  as
       specified with the postscreen_forbidden_commands parameter.  Specify one of the following:

       **ignore** Ignore  the  failure  of this test. Allow other tests to complete.  Do _not_ repeat this
              test before some the result from some other test expires.  This option is  useful  for
              testing and collecting statistics without blocking mail permanently.

### enforce
              Allow  other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
              and log the helo/sender/recipient information.  Repeat this test  the  next  time  the
              client connects.

       **drop**   Drop  the connection immediately with a 521 SMTP reply. Repeat this test the next time
              the client connects. This action is  the  same  as  with  the  Postfix  SMTP  server's
              smtpd_forbidden_commands feature.

       This feature is available in Postfix 2.8.

**postscreen**___**non**___**smtp**___**command**___**enable** **(default:** **no)**
       Enable  "non-SMTP  command"  tests  in the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server. These tests are expensive: a
       client must disconnect after it passes the test, before it can talk to a  real  Postfix  SMTP
       server.

       This feature is available in Postfix 2.8.

**postscreen**___**non**___**smtp**___**command**___**ttl** **(default:** **30d)**
       The  amount  of  time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a successful "non_smtp_com‐
       mand" SMTP protocol test. During this time, the client IP address is excluded from this test.
       The  default is long because a client must disconnect after it passes the test, before it can
       talk to a real Postfix SMTP server.

       Specify a non-zero time value (an integral value plus  an  optional  one-letter  suffix  that
       specifies  the  time  unit).   Time  units:  s (seconds), m (minutes), h (hours), d (days), w
       (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**pipelining**___**action** **(default:** **enforce)**
       The action that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) takes when a remote SMTP client sends multiple commands instead
       of sending one command and waiting for the server to respond.  Specify one of the following:

       **ignore** Ignore  the  failure  of this test. Allow other tests to complete.  Do _not_ repeat this
              test before some the result from some other test expires.  This option is  useful  for
              testing and collecting statistics without blocking mail permanently.

### enforce
              Allow  other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
              and log the helo/sender/recipient information.  Repeat this test  the  next  time  the
              client connects.

       **drop**   Drop  the connection immediately with a 521 SMTP reply. Repeat this test the next time
              the client connects.

       This feature is available in Postfix 2.8.

**postscreen**___**pipelining**___**enable** **(default:** **no)**
       Enable "pipelining" SMTP protocol tests in the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server. These tests  are  expen‐
       sive:  a  good  client must disconnect after it passes the test, before it can talk to a real
       Postfix SMTP server.

       This feature is available in Postfix 2.8.

**postscreen**___**pipelining**___**ttl** **(default:** **30d)**
       The amount of time that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) will use the result from a successful "pipelining" SMTP
       protocol  test.  During  this time, the client IP address is excluded from this test. The de‐
       fault is long because a good client must disconnect after it passes the test, before  it  can
       talk to a real Postfix SMTP server.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).  Time units: s (seconds), m  (minutes),  h  (hours),  d  (days),  w
       (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**post**___**queue**___**limit** **(default:** **$default**___**process**___**limit)**
       The  number  of  clients  that  can  be  waiting  for service from a real Postfix SMTP server
       process. When this queue is full, all clients will receive a 421 response.

       This feature is available in Postfix 2.8.

**postscreen**___**pre**___**queue**___**limit** **(default:** **$default**___**process**___**limit)**
       The number of non-allowlisted clients that can be waiting for a decision  whether  they  will
       receive service from a real Postfix SMTP server process. When this queue is full, all non-al‐
       lowlisted clients will receive a 421 response.

       This feature is available in Postfix 2.8.

**postscreen**___**reject**___**footer** **(default:** **$smtpd**___**reject**___**footer)**
       Optional information that is appended after a 4XX or 5XX [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server  response.  See
       smtpd_reject_footer for further details.

       This feature is available in Postfix 2.8 and later.

**postscreen**___**reject**___**footer**___**maps** **(default:** **$smtpd**___**reject**___**footer**___**maps)**
       Optional  lookup  table  for  information  that  is appended after a 4XX or 5XX [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown)
       server response. See smtpd_reject_footer_maps for further details.

       This feature is available in Postfix 3.4 and later.

**postscreen**___**tls**___**security**___**level** **(default:** **$smtpd**___**tls**___**security**___**level)**
       The SMTP TLS security level for the [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) server; when a non-empty  value  is  speci‐
       fied,  this  overrides the obsolete parameters postscreen_use_tls and postscreen_enforce_tls.
       See smtpd_tls_security_level for details.

       This feature is available in Postfix 2.8 and later.

**postscreen**___**upstream**___**proxy**___**protocol** **(default:** **empty)**
       The name of the proxy protocol used by an optional  before-postscreen  proxy  agent.  When  a
       proxy  agent  is  used,  this protocol conveys local and remote address and port information.
       Specify "postscreen_upstream_proxy_protocol = haproxy" to enable the haproxy  protocol;  ver‐
       sion 2 is supported with Postfix 3.5 and later.

       This feature is available in Postfix 2.10 and later.

**postscreen**___**upstream**___**proxy**___**timeout** **(default:** **5s)**
       The  time  limit for the proxy protocol specified with the postscreen_upstream_proxy_protocol
       parameter.

       This feature is available in Postfix 2.10 and later.

**postscreen**___**use**___**tls** **(default:** **$smtpd**___**use**___**tls)**
       Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not require  that
       clients use TLS encryption.

       This  feature  is  available  in Postfix 2.8 and later.  Preferably, use postscreen_tls_secu‐
       rity_level instead.

**postscreen**___**watchdog**___**timeout** **(default:** **10s)**
       How much time a [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) process may take to respond to a remote SMTP client command  or
       to perform a cache operation before it is terminated by a built-in watchdog timer.  This is a
       safety mechanism that prevents [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) from becoming non-responsive due  to  a  bug  in
       Postfix itself or in system software.  To avoid false alarms and unnecessary cache corruption
       this limit cannot be set under 10s.

       Specify a non-zero time value (an integral value plus  an  optional  one-letter  suffix  that
       specifies  the  time  unit).   Time  units:  s (seconds), m (minutes), h (hours), d (days), w
       (weeks).

       This feature is available in Postfix 2.8.

**postscreen**___**whitelist**___**interfaces** **(default:** **static:all)**
       Renamed to postscreen_allowlist_interfaces in Postfix 3.6.

       This feature is available in Postfix 2.9 - 3.5.

**prepend**___**delivered**___**header** **(default:** **command,** **file,** **forward)**
       The message delivery contexts where the Postfix [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery  agent  prepends  a  Deliv‐
       ered-To:  message header with the address that the mail was delivered to. This information is
       used for mail delivery loop detection.

       By default, the Postfix local delivery agent prepends a Delivered-To: header when  forwarding
       mail  and when delivering to file (mailbox) and command. Turning off the Delivered-To: header
       when forwarding mail is not recommended.

       Specify zero or more of **forward**, **file**, or **command**.

       Example:

       prepend_delivered_header = forward

**process**___**id** **(read-only)**
       The process ID of a Postfix command or daemon process.

**process**___**id**___**directory** **(default:** **pid)**
       The location of Postfix PID files relative to $queue_directory.  This is a read-only  parame‐
       ter.

**process**___**name** **(read-only)**
       The process name of a Postfix command or daemon process.

**propagate**___**unmatched**___**extensions** **(default:** **canonical,** **virtual)**
       What  address  lookup  tables copy an address extension from the lookup key to the lookup re‐
       sult.

       For example, with a [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown) mapping of "<_joe@example.com_> _=>_ <_joe.user@example.net_>", the  ad‐
       dress "_joe+<foo@example.com_>" would rewrite to "_joe.user+<foo@example.net_>".

       Specify  zero  or more of **canonical**, **virtual**, **alias**, **forward**, **include** or **generic**. These cause
       address extension propagation with [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown), [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown), and [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown)  maps,  with  **lo**‐‐
       [**cal**(8)](https://www.chedong.com/phpMan.php/man/cal/8/markdown) .forward and :include: file lookups, and with [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) generic maps, respectively.

       Note:  enabling  this  feature  for types other than **canonical** and **virtual** is likely to cause
       problems when mail is forwarded to other sites, especially with mail that is sent to a  mail‐
       ing list exploder address.

       Examples:

       propagate_unmatched_extensions = canonical, virtual, alias,
               forward, include
       propagate_unmatched_extensions = canonical, virtual

**proxy**___**interfaces** **(default:** **empty)**
       The  network  interface addresses that this mail system receives mail on by way of a proxy or
       network address translation unit.

       This feature is available in Postfix 2.0 and later.

       You must specify your "outside" proxy/NAT addresses when your system is a backup MX host  for
       other domains, otherwise mail delivery loops will happen when the primary MX host is down.

       Example:

       proxy_interfaces = 1.2.3.4

**proxy**___**read**___**maps** **(default:** **see** **postconf** **-d** **output)**
       The lookup tables that the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) server is allowed to access for the read-only service.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma.  Table ref‐
       erences that don't begin with proxy: are ignored.

       This feature is available in Postfix 2.0 and later.

**proxy**___**write**___**maps** **(default:** **see** **postconf** **-d** **output)**
       The lookup tables that the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) server is allowed to access for  the  read-write  ser‐
       vice. Postfix-owned local database files should be stored under the Postfix-owned data_direc‐
       tory.  Table references that don't begin with proxy: are ignored.

       This feature is available in Postfix 2.5 and later.

**proxymap**___**service**___**name** **(default:** **proxymap)**
       The name of the proxymap read-only table lookup service.  This  service  is  normally  imple‐
       mented by the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) daemon.

       This feature is available in Postfix 2.6 and later.

**proxywrite**___**service**___**name** **(default:** **proxywrite)**
       The  name of the proxywrite read-write table lookup service.  This service is normally imple‐
       mented by the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) daemon.

       This feature is available in Postfix 2.6 and later.

**qmgr**___**clog**___**warn**___**time** **(default:** **300s)**
       The minimal delay between warnings that a specific destination is clogging up the Postfix ac‐
       tive queue. Specify 0 to disable.

       This feature is enabled with the helpful_warnings parameter.

       This feature is available in Postfix 2.0 and later.

**qmgr**___**daemon**___**timeout** **(default:** **1000s)**
       How  much time a Postfix queue manager process may take to handle a request before it is ter‐
       minated by a built-in watchdog timer.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

       This feature is available in Postfix 2.8 and later.

**qmgr**___**fudge**___**factor** **(default:** **100)**
       Obsolete  feature:  the  percentage of delivery resources that a busy mail system will use up
       for delivery of a large mailing  list message.

       This feature exists only in the [**oqmgr**(8)](https://www.chedong.com/phpMan.php/man/oqmgr/8/markdown) old queue manager. The current queue manager  solves
       the problem in a better way.

**qmgr**___**ipc**___**timeout** **(default:** **60s)**
       The time limit for the queue manager to send or receive information over an internal communi‐
       cation channel.  The purpose is to break out of deadlock situations. If the time limit is ex‐
       ceeded the software either retries or aborts the operation.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

       This feature is available in Postfix 2.8 and later.

**qmgr**___**message**___**active**___**limit** **(default:** **20000)**
       The maximal number of messages in the active queue.

**qmgr**___**message**___**recipient**___**limit** **(default:** **20000)**
       The maximal number of recipients held in memory by the Postfix queue manager, and the maximal
       size of the short-term, in-memory "dead" destination status cache.

**qmgr**___**message**___**recipient**___**minimum** **(default:** **10)**
       The  minimal  number  of  in-memory  recipients for any message. This takes priority over any
       other in-memory recipient limits (i.e., the global qmgr_message_recipient_limit and  the  per
       transport _recipient_limit) if necessary. The minimum value allowed for this parameter is 1.

**qmqpd**___**authorized**___**clients** **(default:** **empty)**
       What remote QMQP clients are allowed to connect to the Postfix QMQP server port.

       By default, no client is allowed to use the service. This is because the QMQP server will re‐
       lay mail to any destination.

       Specify a list of client patterns. A list pattern specifies a host name, a  domain  name,  an
       internet  address,  or a network/mask pattern, where the mask specifies the number of bits in
       the network part.  When a pattern specifies a file name, its contents are substituted for the
       file  name;  when  a  pattern is a "type:table" table specification, table lookup is used in‐
       stead.

       Patterns are separated by whitespace and/or commas. In order to reverse the result, precede a
       pattern  with  an  exclamation point (!). The form "!/file/name" is supported only in Postfix
       version 2.4 and later.

       Pattern matching of domain names is controlled by the presence or  absence  of  "qmqpd_autho‐
       rized_clients" in the parent_domain_matches_subdomains parameter value.

       Example:

       qmqpd_authorized_clients = !192.168.0.1, 192.168.0.0/24

**qmqpd**___**client**___**port**___**logging** **(default:** **no)**
       Enable logging of the remote QMQP client port in addition to the hostname and IP address. The
       logging format is "host[address]:port".

       This feature is available in Postfix 2.5 and later.

**qmqpd**___**error**___**delay** **(default:** **1s)**
       How long the Postfix QMQP server will pause before sending a negative  reply  to  the  remote
       QMQP client. The purpose is to slow down confused or malicious clients.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**qmqpd**___**timeout** **(default:** **300s)**
       The time limit for sending or receiving information over the network.  If a read or write op‐
       eration blocks for more than $qmqpd_timeout seconds the Postfix QMQP server gives up and dis‐
       connects.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**queue**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The  location of the Postfix top-level queue directory. This is the root directory of Postfix
       daemon processes that run chrooted.

**queue**___**file**___**attribute**___**count**___**limit** **(default:** **100)**
       The maximal number of (name=value) attributes that may be stored in a Postfix queue file. The
       limit is enforced by the [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) server.

       This feature is available in Postfix 2.0 and later.

**queue**___**minfree** **(default:** **0)**
       The  minimal amount of free space in bytes in the queue file system that is needed to receive
       mail.  This is currently used by the Postfix SMTP server to decide if it will accept any mail
       at all.

       By  default, the Postfix SMTP server rejects MAIL FROM commands when the amount of free space
       is less than 1.5*$message_size_limit (Postfix version 2.1 and later).  To  specify  a  higher
       minimum  free  space  limit,  specify  a  queue_minfree  value  that  is  at  least 1.5*$mes‐
       sage_size_limit.

       With Postfix versions 2.0 and earlier, a queue_minfree value of zero means there is no  mini‐
       mum required amount of free space.

**queue**___**run**___**delay** **(default:** **300s)**
       The  time between deferred queue scans by the queue manager; prior to Postfix 2.4 the default
       value was 1000s.

       This parameter should be set less than or equal to  $minimal_backoff_time.  See  also  $maxi‐
       mal_backoff_time.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**queue**___**service**___**name** **(default:** **qmgr)**
       The name of the [**qmgr**(8)](https://www.chedong.com/phpMan.php/man/qmgr/8/markdown) service. This service manages the Postfix queue and schedules  deliv‐
       ery requests.

       This feature is available in Postfix 2.0 and later.

**rbl**___**reply**___**maps** **(default:** **empty)**
       Optional  lookup tables with RBL response templates. The tables are indexed by the RBL domain
       name. By default, Postfix uses the default template as specified with  the  default_rbl_reply
       configuration parameter. See there for a discussion of the syntax of RBL reply templates.

       This feature is available in Postfix 2.0 and later.

**readme**___**directory** **(default:** **see** **postconf** **-d** **output)**
       The  location of Postfix README files that describe how to build, configure or operate a spe‐
       cific Postfix subsystem or feature.

**receive**___**override**___**options** **(default:** **empty)**
       Enable or disable recipient validation, built-in content filtering, or address mapping. Typi‐
       cally,  these are specified in master.cf as command-line arguments for the [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown), [**qmqpd**(8)](https://www.chedong.com/phpMan.php/man/qmqpd/8/markdown)
       or [**pickup**(8)](https://www.chedong.com/phpMan.php/man/pickup/8/markdown) daemons.

       Specify zero or more of the following options.  The options override main.cf settings and are
       either  implemented  by [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown), [**qmqpd**(8)](https://www.chedong.com/phpMan.php/man/qmqpd/8/markdown), or [**pickup**(8)](https://www.chedong.com/phpMan.php/man/pickup/8/markdown) themselves, or they are forwarded to
       the cleanup server.

       **no**___**unknown**___**recipient**___**checks**
              Do not try to reject unknown recipients (SMTP server only).  This is typically  speci‐
              fied AFTER an external content filter.

       **no**___**address**___**mappings**
              Disable  canonical address mapping, virtual alias map expansion, address masquerading,
              and automatic BCC (blind carbon-copy) recipients. This is typically  specified  BEFORE
              an external content filter.

       **no**___**header**___**body**___**checks**
              Disable header/body_checks. This is typically specified AFTER an external content fil‐
              ter.

       **no**___**milters**
              Disable Milter (mail filter) applications. This is typically specified AFTER an exter‐
              nal content filter.

       Note:  when  the "BEFORE content filter" receive_override_options setting is specified in the
       main.cf file, specify the "AFTER content filter"  receive_override_options  setting  in  mas‐
       ter.cf (and vice versa).

       Examples:

       receive_override_options =
           no_unknown_recipient_checks, no_header_body_checks
       receive_override_options = no_address_mappings

       This feature is available in Postfix 2.1 and later.

**recipient**___**bcc**___**maps** **(default:** **empty)**
       Optional  BCC  (blind  carbon-copy) address lookup tables, indexed by recipient address.  The
       BCC address (multiple results are not supported) is added when mail enters  from  outside  of
       Postfix.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       The table search order is as follows:

       •      Look up the "user+<extension@domain.tld>" address including the optional address  exten‐
              sion.

       •      Look up the "<user@domain.tld>" address without the optional address extension.

       •      Look up the "user+extension" address local part when the recipient domain equals $myo‐
              rigin, $mydestination, $inet_interfaces or $proxy_interfaces.

       •      Look up the "user" address local part when the recipient domain equals $myorigin, $my‐
              destination, $inet_interfaces or $proxy_interfaces.

       •      Look up the "@domain.tld" part.

       Note:  with  Postfix  2.3  and later the BCC address is added as if it was specified with NO‐
       TIFY=NONE. The sender will not be notified when the BCC address is undeliverable, as long  as
       all down-stream software implements RFC 3461.

       Note:  with  Postfix 2.2 and earlier the sender will unconditionally be notified when the BCC
       address is undeliverable.

       Note: automatic BCC recipients are produced only for new mail.  To avoid mailer loops,  auto‐
       matic BCC recipients are not generated after Postfix forwards mail internally, or after Post‐
       fix generates mail itself.

       Example:

       recipient_bcc_maps = hash:/etc/postfix/recipient_bcc

       After a change, run "**postmap** **/etc/postfix/recipient**___**bcc**".

       This feature is available in Postfix 2.1 and later.

**recipient**___**canonical**___**classes** **(default:** **envelope**___**recipient,** **header**___**recipient)**
       What addresses are subject to recipient_canonical_maps address mapping.  By default,  recipi‐
       ent_canonical_maps  address mapping is applied to envelope recipient addresses, and to header
       recipient addresses.

       Specify one or more of: envelope_recipient, header_recipient

       This feature is available in Postfix 2.2 and later.

**recipient**___**canonical**___**maps** **(default:** **empty)**
       Optional address mapping lookup tables for envelope and header recipient addresses.  The  ta‐
       ble format and lookups are documented in [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown).

       Note: $recipient_canonical_maps is processed before $canonical_maps.

       Example:

       recipient_canonical_maps = hash:/etc/postfix/recipient_canonical

**recipient**___**delimiter** **(default:** **empty)**
       The  set of characters that can separate an email address localpart, user name, or a .forward
       file name from its extension.  For example, with  "recipient_delimiter  =  +",  the  software
       tries  user+<foo@example.com>  before trying <user@example.com>, user+foo before trying user, and
       .forward+foo before trying .forward.

       More formally, an email address localpart or user name is separated from its extension by the
       first  character that matches the recipient_delimiter set. The delimiter character and exten‐
       sion may then be used to generate an extended .forward file name. This implementation  recog‐
       nizes one delimiter character and one extension per email address localpart or email address.
       With Postfix 2.10 and earler, the recipient_delimiter specifies a single character.

       See [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown), [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown), [**relocated**(5)](https://www.chedong.com/phpMan.php/man/relocated/5/markdown) and [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown) for the effects  of  recipient_delim‐
       iter on lookups in aliases, canonical, virtual, and relocated maps, and see the propagate_un‐
       matched_extensions parameter for propagating an extension from one email address to another.

       When used in command_execution_directory, forward_path,  or  luser_relay,  ${recipient_delim‐
       iter}  is  replaced with the actual recipient delimiter that was found in the recipient email
       address (Postfix 2.11 and later), or it is replaced with the main.cf recipient_delimiter  pa‐
       rameter value (Postfix 2.10 and earlier).

       The  recipient_delimiter is not applied to the mailer-daemon address, the postmaster address,
       or the double-bounce address. With the default "owner_request_special = yes" setting, the re‐
       cipient_delimiter  is  also  not applied to addresses with the special "owner-" prefix or the
       special "-request" suffix.

       Examples:

       # Handle Postfix-style extensions.
       recipient_delimiter = +

       # Handle both Postfix and qmail extensions (Postfix 2.11 and later).
       recipient_delimiter = +-

       # Use .forward for mail without address extension, and for mail with
       # an unrecognized address extension.
       forward_path = $home/.forward${recipient_delimiter}${extension},
           $home/.forward

**reject**___**code** **(default:** **554)**
       The numerical Postfix SMTP server response code when a remote SMTP client request is rejected
       by the "reject" restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

**reject**___**tempfail**___**action** **(default:** **defer**___**if**___**permit)**
       The  Postfix SMTP server's action when a reject-type restriction fails due to a temporary er‐
       ror condition. Specify "defer" to defer the remote SMTP client request immediately. With  the
       default "defer_if_permit" action, the Postfix SMTP server continues to look for opportunities
       to reject mail, and defers the client request only if it would otherwise be accepted.

       For finer control, see: unverified_recipient_tempfail_action,  unverified_sender_tempfail_ac‐
       tion, unknown_address_tempfail_action, and unknown_helo_hostname_tempfail_action.

       This feature is available in Postfix 2.6 and later.

**relay**___**clientcerts** **(default:** **empty)**
       List  of  tables  with remote SMTP client-certificate fingerprints or public key fingerprints
       (Postfix 2.9 and later) for which the Postfix SMTP server will allow  access  with  the  per‐
       mit_tls_clientcerts  feature.   The  fingerprint  digest  algorithm  is  configurable via the
       smtpd_tls_fingerprint_digest parameter (hard-coded as md5 prior to Postfix version 2.5).

       The default algorithm is **sha256** with Postfix >= 3.6 and the **compatibility**___**level** set to 3.6 or
       higher.  With  Postfix  <= 3.5, the default algorithm is **md5**.  The best-practice algorithm is
       now **sha256**. Recent advances in hash function cryptanalysis have led to  md5  and  sha1  being
       deprecated in favor of sha256.  However, as long as there are no known "second pre-image" at‐
       tacks against the older algorithms, their use in this context,  though  not  recommended,  is
       still likely safe.

       Postfix lookup tables are in the form of (key, value) pairs.  Since we only need the key, the
       value   can   be   chosen   freely,   e.g.    the    name    of    the    user    or    host:
       D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home

       Example:

       relay_clientcerts = hash:/etc/postfix/relay_clientcerts

       For more fine-grained control, use check_ccert_access to select an appropriate [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) pol‐
       icy for each client.  See RESTRICTION_CLASS_README.

       This feature is available with Postfix version 2.2.

**relay**___**destination**___**concurrency**___**limit** **(default:** **$default**___**destination**___**concurrency**___**limit)**
       The maximal number of parallel deliveries to the same destination via the relay  message  de‐
       livery transport. This limit is enforced by the queue manager. The message delivery transport
       name is the first field in the entry in the master.cf file.

       This feature is available in Postfix 2.0 and later.

**relay**___**destination**___**recipient**___**limit** **(default:** **$default**___**destination**___**recipient**___**limit)**
       The maximal number of recipients per message for the relay message delivery  transport.  This
       limit  is  enforced  by  the  queue manager. The message delivery transport name is the first
       field in the entry in the master.cf file.

       Setting this parameter to a value of  1  changes  the  meaning  of  relay_destination_concur‐
       rency_limit from concurrency per domain into concurrency per recipient.

       This feature is available in Postfix 2.0 and later.

**relay**___**domains** **(default:** **Postfix** **>=** **3.0:** **empty,** **Postfix** **<** **3.0:** **$mydestination)**
       What destination domains (and subdomains thereof) this system will relay mail to. For details
       about how the relay_domains value is used, see the description of the permit_auth_destination
       and reject_unauth_destination SMTP recipient restrictions.

       Domains  that  match  $relay_domains  are  delivered  with the $relay_transport mail delivery
       transport. The SMTP server validates recipient addresses with $relay_recipient_maps  and  re‐
       jects  non-existent  recipients.  See  also  the  relay  domains  address  class  in  the AD‐
       DRESS_CLASS_README file.

       Note: Postfix will not automatically forward mail for domains that list this system as  their
       primary  or  backup  MX  host. See the permit_mx_backup restriction in the [**postconf**(5)](https://www.chedong.com/phpMan.php/man/postconf/5/markdown) manual
       page.

       Specify a list of host or domain names, "/file/name" patterns or "type:table" lookup  tables,
       separated  by  commas  and/or whitespace.  Continue long lines by starting the next line with
       whitespace. A "/file/name" pattern is replaced by its contents; a "type:table"  lookup  table
       is  matched when a (parent) domain appears as lookup key. Specify "!pattern" to exclude a do‐
       main from the list. The form "!/file/name" is supported  only  in  Postfix  version  2.4  and
       later.

       Pattern  matching of domain names is controlled by the presence or absence of "relay_domains"
       in the parent_domain_matches_subdomains parameter value.

**relay**___**domains**___**reject**___**code** **(default:** **554)**
       The numerical Postfix SMTP server response code when a client request is rejected by the  re‐
       ject_unauth_destination recipient restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

**relay**___**recipient**___**maps** **(default:** **empty)**
       Optional  lookup  tables  with  all valid addresses in the domains that match $relay_domains.
       Specify @domain as a wild-card for domains that have no valid recipient list,  and  become  a
       source  of backscatter mail: Postfix accepts spam for non-existent recipients and then floods
       innocent people with undeliverable mail.   Technically,  tables  listed  with  $relay_recipi‐
       ent_maps  are  used  as lists: Postfix needs to know only if a lookup string is found or not,
       but it does not use the result from table lookup.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       If  this parameter is non-empty, then the Postfix SMTP server will reject mail to unknown re‐
       lay users. This feature is off by default.

       See also the relay domains address class in the ADDRESS_CLASS_README file.

       Example:

       relay_recipient_maps = hash:/etc/postfix/relay_recipients

       This feature is available in Postfix 2.0 and later.

**relay**___**transport** **(default:** **relay)**
       The default mail delivery transport and next-hop destination for remote delivery  to  domains
       listed  with  $relay_domains.  In  order of decreasing precedence, the nexthop destination is
       taken from $relay_transport, $sender_dependent_relayhost_maps, $relayhost, or from the recip‐
       ient domain. This information can be overruled with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       Specify  a string of the form _transport:nexthop_, where _transport_ is the name of a mail deliv‐
       ery transport defined in master.cf.  The _:nexthop_ destination is optional; its syntax is doc‐
       umented in the manual page of the corresponding delivery agent.

       See also the relay domains address class in the ADDRESS_CLASS_README file.

       This feature is available in Postfix 2.0 and later.

### relayhost (default: empty)
       The  next-hop destination(s) for non-local mail; overrides non-local domains in recipient ad‐
       dresses. This information is overruled with relay_transport,  sender_dependent_default_trans‐
       port_maps,  default_transport,  sender_dependent_relayhost_maps and with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) ta‐
       ble.

       On an intranet, specify the organizational domain name. If  your  internal  DNS  uses  no  MX
       records, specify the name of the intranet gateway host instead.

       In  the  case of SMTP or LMTP delivery, specify one or more destinations in the form of a do‐
       main name, hostname, hostname:port,  [hostname]:port,  [hostaddress]  or  [hostaddress]:port,
       separated  by comma or whitespace.  The form [hostname] turns off MX lookups. Multiple desti‐
       nations are supported in Postfix 3.5 and later.

       If you're connected via UUCP, see the UUCP_README file for useful information.

       Examples:

       relayhost = $mydomain
       relayhost = [gateway.example.com]
       relayhost = mail1.example:587, mail2.example:587
       relayhost = [an.ip.add.ress]

**relocated**___**maps** **(default:** **empty)**
       Optional lookup tables with new contact information for users or domains that no  longer  ex‐
       ist.  The table format and lookups are documented in [**relocated**(5)](https://www.chedong.com/phpMan.php/man/relocated/5/markdown).

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       If you use this feature, run "**postmap** **/etc/postfix/relocated**" to build the necessary  DBM  or
       DB file after change, then "**postfix** **reload**" to make the changes visible.

       Examples:

       relocated_maps = dbm:/etc/postfix/relocated
       relocated_maps = hash:/etc/postfix/relocated

**remote**___**header**___**rewrite**___**domain** **(default:** **empty)**
       Don't rewrite message headers from remote clients at all when this parameter is empty; other‐
       wise, rewrite message headers and append the specified domain name to  incomplete  addresses.
       The local_header_rewrite_clients parameter controls what clients Postfix considers local.

       Examples:

       The  safe  setting:  append  "domain.invalid" to incomplete header addresses from remote SMTP
       clients, so that those addresses cannot be confused with local addresses.

           remote_header_rewrite_domain = domain.invalid

       The default, purist, setting: don't rewrite headers from remote clients at all.

           remote_header_rewrite_domain =

**require**___**home**___**directory** **(default:** **no)**
       Require that a [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) recipient's home directory exists before mail delivery is  attempted.
       By  default this test is disabled.  It can be useful for environments that import home direc‐
       tories to the mail server (IMPORTING HOME DIRECTORIES IS NOT RECOMMENDED).

**reset**___**owner**___**alias** **(default:** **no)**
       Reset the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent's idea of the owner-alias attribute, when  delivering  mail
       to a child alias that does not have its own owner alias.

       This feature is available in Postfix 2.8 and later. With older Postfix releases, the behavior
       is as if this parameter is set to "yes".

       As documented in [**aliases**(5)](https://www.chedong.com/phpMan.php/man/aliases/5/markdown), when an alias _name_ has a companion alias named owner-_name_,  this
       will  replace  the  envelope  sender address, so that delivery errors will be reported to the
       owner alias instead of the sender. This configuration is recommended for mailing lists.

       A less known property of the owner alias is that it also forces the [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown)  delivery  agent
       to  write local and remote addresses from alias expansion to a new queue file, instead of at‐
       tempting to deliver mail to local addresses as soon as they come out of alias expansion.

       Writing local addresses from alias expansion to a new queue file allows for  robust  handling
       of  temporary  delivery  errors: errors with one local member have no effect on deliveries to
       other members of the list.  On the other hand, delivery to local addresses as  soon  as  they
       come  out  of alias expansion is fragile: a temporary error with one local address from alias
       expansion will cause the entire alias to be expanded repeatedly until the error goes away, or
       until the message expires in the queue.  In that case, a problem with one list member results
       in multiple message deliveries to other list members.

       The default behavior of Postfix 2.8 and later is to keep the  owner-alias  attribute  of  the
       parent  alias,  when delivering mail to a child alias that does not have its own owner alias.
       Then, local addresses from that child alias will be written to a new queue file, and a tempo‐
       rary error with one local address will not affect delivery to other mailing list members.

       Unfortunately, older Postfix releases reset the owner-alias attribute when delivering mail to
       a child alias that does not have its own owner alias. To be precise, this resets only the de‐
       cision  to create a new queue file, not the decision to override the envelope sender address.
       The [**local**(8)](https://www.chedong.com/phpMan.php/man/local/8/markdown) delivery agent then attempts to deliver local addresses as soon as they come out
       of child alias expansion.  If delivery to any address from child alias expansion fails with a
       temporary error condition, the entire mailing list may be expanded repeatedly until the  mail
       expires  in  the  queue, resulting in multiple deliveries of the same message to mailing list
       members.

**resolve**___**dequoted**___**address** **(default:** **yes)**
       Resolve a recipient address safely instead of correctly, by looking inside quotes.

       By default, the Postfix address resolver does not quote the address localpart as per RFC 822,
       so  that  additional  @  or % or !  operators remain visible. This behavior is safe but it is
       also technically incorrect.

       If you specify "resolve_dequoted_address = no", then the Postfix resolver will not know about
       additional  @  etc.  operators in the address localpart. This opens opportunities for obscure
       mail relay attacks with user@domain@domain addresses when Postfix provides backup MX  service
       for Sendmail systems.

**resolve**___**null**___**domain** **(default:** **no)**
       Resolve  an address that ends in the "@" null domain as if the local hostname were specified,
       instead of rejecting the address as invalid.

       This feature is available in Postfix 2.1 and later.  Earlier versions always resolve the null
       domain as the local hostname.

       The Postfix SMTP server uses this feature to reject mail from or to addresses that end in the
       "@" null domain, and from addresses that rewrite into a form that ends in the  "@"  null  do‐
       main.

**resolve**___**numeric**___**domain** **(default:** **no)**
       Resolve "user@ipaddress" as "user@[ipaddress]", instead of rejecting the address as invalid.

       This feature is available in Postfix 2.3 and later.

**respectful**___**logging** **(default:** **see** **'postconf** **-d'** **output)**
       Avoid  logging  that implies white is better than black. Instead use 'allowlist', 'denylist',
       and variations of those words.

       This feature is available in Postfix 3.6 and later.

**rewrite**___**service**___**name** **(default:** **rewrite)**
       The name of the address rewriting service. This service rewrites addresses to  standard  form
       and resolves them to a (delivery method, next-hop host, recipient) triple.

       This feature is available in Postfix 2.0 and later.

**sample**___**directory** **(default:** **/etc/postfix)**
       The  name  of  the directory with example Postfix configuration files.  Starting with Postfix
       2.1, these files have been replaced with the [**postconf**(5)](https://www.chedong.com/phpMan.php/man/postconf/5/markdown) manual page.

**send**___**cyrus**___**sasl**___**authzid** **(default:** **no)**
       When authenticating to a remote SMTP or LMTP server with the default setting  "no",  send  no
       SASL  authoriZation ID (authzid); send only the SASL authentiCation ID (authcid) plus the au‐
       thcid's password.

       The non-default setting "yes" enables the behavior of older Postfix versions.   These  always
       send a SASL authzid that is equal to the SASL authcid, but this causes interoperability prob‐
       lems with some SMTP servers.

       This feature is available in Postfix 2.4.4 and later.

**sender**___**based**___**routing** **(default:** **no)**
       This parameter should not be used. It  was  replaced  by  sender_dependent_relayhost_maps  in
       Postfix version 2.3.

**sender**___**bcc**___**maps** **(default:** **empty)**
       Optional  BCC  (blind carbon-copy) address lookup tables, indexed by sender address.  The BCC
       address (multiple results are not supported) is added when mail enters from outside of  Post‐
       fix.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       The table search order is as follows:

       •      Look up the "user+<extension@domain.tld>" address including the optional address  exten‐
              sion.

       •      Look up the "<user@domain.tld>" address without the optional address extension.

       •      Look  up the "user+extension" address local part when the sender domain equals $myori‐
              gin, $mydestination, $inet_interfaces or $proxy_interfaces.

       •      Look up the "user" address local part when the sender domain equals $myorigin, $mydes‐
              tination, $inet_interfaces or $proxy_interfaces.

       •      Look up the "@domain.tld" part.

       Note:  with  Postfix  2.3  and later the BCC address is added as if it was specified with NO‐
       TIFY=NONE. The sender will not be notified when the BCC address is undeliverable, as long  as
       all down-stream software implements RFC 3461.

       Note:  with Postfix 2.2 and earlier the sender will be notified when the BCC address is unde‐
       liverable.

       Note: automatic BCC recipients are produced only for new mail.  To avoid mailer loops,  auto‐
       matic BCC recipients are not generated after Postfix forwards mail internally, or after Post‐
       fix generates mail itself.

       Example:

       sender_bcc_maps = hash:/etc/postfix/sender_bcc

       After a change, run "**postmap** **/etc/postfix/sender**___**bcc**".

       This feature is available in Postfix 2.1 and later.

**sender**___**canonical**___**classes** **(default:** **envelope**___**sender,** **header**___**sender)**
       What  addresses  are  subject  to  sender_canonical_maps  address   mapping.    By   default,
       sender_canonical_maps  address mapping is applied to envelope sender addresses, and to header
       sender addresses.

       Specify one or more of: envelope_sender, header_sender

       This feature is available in Postfix 2.2 and later.

**sender**___**canonical**___**maps** **(default:** **empty)**
       Optional address mapping lookup tables for envelope and header sender addresses.   The  table
       format and lookups are documented in [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown).

       Example:  you  want to rewrite the SENDER address "<user@ugly.domain>" to "<user@pretty.domain>",
       while still being able to send mail to the RECIPIENT address "<user@ugly.domain>".

       Note: $sender_canonical_maps is processed before $canonical_maps.

       Example:

       sender_canonical_maps = hash:/etc/postfix/sender_canonical

**sender**___**dependent**___**default**___**transport**___**maps** **(default:** **empty)**
       A sender-dependent override for the global default_transport parameter  setting.  The  tables
       are  searched by the envelope sender address and @domain. A lookup result of DUNNO terminates
       the search without overriding the global default_transport parameter setting.  This  informa‐
       tion is overruled with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       Note: this overrides default_transport, not transport_maps, and therefore the expected syntax
       is  that of default_transport, not the syntax of transport_maps.  Specifically, this does not
       support the transport_maps syntax for null transport, null nexthop, or null email addresses.

       For safety reasons, this feature does not allow $number substitutions in  regular  expression
       maps.

       This feature is available in Postfix 2.7 and later.

**sender**___**dependent**___**relayhost**___**maps** **(default:** **empty)**
       A  sender-dependent  override  for  the  global  relayhost  parameter setting. The tables are
       searched by the envelope sender address and @domain. A lookup result of DUNNO terminates  the
       search  without  overriding  the  global relayhost parameter setting (Postfix 2.6 and later).
       This information is overruled with relay_transport,  sender_dependent_default_transport_maps,
       default_transport and with the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       For safety reasons, this feature does not allow $number substitutions in  regular  expression
       maps.

       This feature is available in Postfix 2.3 and later.

**sendmail**___**fix**___**line**___**endings** **(default:** **always)**
       Controls  how  the Postfix sendmail command converts email message line endings from <CR><LF>
       into UNIX format (<LF>).

       **always** Always convert message lines ending in <CR><LF>. This  setting  is  the  default  with
              Postfix 2.9 and later.

       **strict** Convert  message  lines  ending  in  <CR><LF>  only  if  the  first input line ends in
              <CR><LF>. This setting is backwards-compatible with Postfix 2.8 and earlier.

       **never**  Never convert message lines ending in <CR><LF>. This setting exists  for  completeness
              only.

       This feature is available in Postfix 2.9 and later.

**sendmail**___**path** **(default:** **see** **postconf** **-d** **output)**
       A  Sendmail compatibility feature that specifies the location of the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) com‐
       mand. This command can be used to submit mail into the Postfix queue.

**service**___**name** **(read-only)**
       The master.cf service name of a Postfix daemon process. This can be used to  distinguish  the
       logging from different services that use the same program name.

       Example master.cf entries:

       # Distinguish inbound MTA logging from submission and smtps logging.
       smtp      inet  n       -       n       -       -       smtpd
       submission inet n       -       n       -       -       smtpd
           -o syslog_name=postfix/$service_name
       smtps     inet  n       -       n       -       -       smtpd
           -o syslog_name=postfix/$service_name

       # Distinguish outbound MTA logging from inbound relay logging.
       smtp      unix  -       -       n       -       -       smtp
       relay     unix  -       -       n       -       -       smtp
           -o syslog_name=postfix/$service_name

**service**___**throttle**___**time** **(default:** **60s)**
       How  long the Postfix [**master**(8)](https://www.chedong.com/phpMan.php/man/master/8/markdown) waits before forking a server that appears to be malfunction‐
       ing.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**setgid**___**group** **(default:** **postdrop)**
       The  group  ownership  of set-gid Postfix commands and of group-writable Postfix directories.
       When this parameter value is changed you need to re-run "**postfix** **set-permissions**" (with Post‐
       fix version 2.0 and earlier: "**/etc/postfix/post-install** **set-permissions**".

**shlib**___**directory** **(default:** **see** **'postconf** **-d'** **output)**
       The location of Postfix dynamically-linked libraries (libpostfix-*.so), and the default loca‐
       tion of Postfix database plugins (postfix-*.so) that have a relative pathname in  the  dynam‐
       icmaps.cf  file.   The  shlib_directory  parameter  defaults  to  "no"  when  Postfix dynami‐
       cally-linked libraries and database plugins are disabled at compile time, otherwise it  typi‐
       cally defaults to /usr/lib/postfix or /usr/local/lib/postfix.

       Notes:

       •      The  directory  specified  with  shlib_directory  should  contain only Postfix-related
              files. Postfix dynamically-linked libraries and database plugins  should  not  be  in‐
              stalled  in  a  "public"  system directory such as /usr/lib or /usr/local/lib. Linking
              Postfix dynamically-linked library files or database plugins into non-Postfix programs
              is not supported.  Postfix dynamically-linked libraries and database plugins implement
              a Postfix-internal API that changes without maintaining compatibility.

       •      You can change the shlib_directory value after Postfix is built. However, you may have
              to  run  ldconfig  or  equivalent to prevent Postfix programs from failing because the
              libpostfix-*.so files are not found.  No ldconfig command is needed if  you  keep  the
              libpostfix-*.so files in the compiled-in default $shlib_directory location.

       This feature is available in Postfix 3.0 and later.

**show**___**user**___**unknown**___**table**___**name** **(default:** **yes)**
       Display  the  name  of the recipient table in the "User unknown" responses.  The extra detail
       makes troubleshooting easier but also reveals information that is nobody else's business.

       This feature is available in Postfix 2.0 and later.

**showq**___**service**___**name** **(default:** **showq)**
       The name of the [**showq**(8)](https://www.chedong.com/phpMan.php/man/showq/8/markdown) service. This service produces mail queue status reports.

       This feature is available in Postfix 2.0 and later.

**smtp**___**address**___**preference** **(default:** **any)**
       The address type ("ipv6", "ipv4" or "any") that the Postfix SMTP client will try first,  when
       a  destination  has IPv6 and IPv4 addresses with equal MX preference. This feature has no ef‐
       fect unless the inet_protocols setting enables both IPv4 and IPv6.

       Postfix SMTP client address preference has evolved. With Postfix 2.8 the default  is  "ipv6";
       earlier implementations are hard-coded to prefer IPv6 over IPv4.

       Notes for mail delivery between sites that have both IPv4 and IPv6 connectivity:

       •      The  setting  "smtp_address_preference = ipv6" is unsafe.  It can fail to deliver mail
              when there is an outage that affects IPv6, while the destination  is  still  reachable
              over IPv4.

       •      The  setting  "smtp_address_preference = any" is safe. With this, mail will eventually
              be delivered even if there is an outage that affects IPv6 or IPv4, as long as it  does
              not affect both.

       This feature is available in Postfix 2.8 and later.

**smtp**___**address**___**verify**___**target** **(default:** **rcpt)**
       In the context of email address verification, the SMTP protocol stage that determines whether
       an email address is deliverable.  Specify one of "rcpt" or "data".  The latter is needed with
       remote  SMTP servers that reject recipients after the DATA command. Use transport_maps to ap‐
       ply this feature selectively:

           /etc/postfix/main.cf:
               transport_maps = hash:/etc/postfix/transport

           /etc/postfix/transport:
               smtp-domain-that-verifies-after-data    smtp-data-target:
               lmtp-domain-that-verifies-after-data    lmtp-data-target:

           /etc/postfix/master.cf:
               smtp-data-target    unix    -    -    n    -    -    smtp
                   -o smtp_address_verify_target=data
               lmtp-data-target    unix    -    -    n    -    -    lmtp
                   -o lmtp_address_verify_target=data

       Unselective use of the "data" target does no harm, but will result in unnecessary "lost  con‐
       nection after DATA" events at remote SMTP/LMTP servers.

       This feature is available in Postfix 3.0 and later.

**smtp**___**always**___**send**___**ehlo** **(default:** **yes)**
       Always send EHLO at the start of an SMTP session.

       With  "smtp_always_send_ehlo  =  no",  the  Postfix SMTP client sends EHLO only when the word
       "ESMTP" appears in the server greeting banner (example: 220 spike.porcupine.org  ESMTP  Post‐
       fix).

**smtp**___**balance**___**inet**___**protocols** **(default:** **yes)**
       When  a  remote destination resolves to a combination of IPv4 and IPv6 addresses, ensure that
       the Postfix SMTP client can try both address  types  before  it  runs  into  the  smtp_mx_ad‐
       dress_limit.

       This  avoids  an  interoperability  problem when a destination resolves to primarily IPv6 ad‐
       dresses, the smtp_address_limit feature eliminates most or all IPv4 addresses, and the desti‐
       nation is not reachable over IPv6.

       This feature is available in Postfix 3.3 and later.

**smtp**___**bind**___**address** **(default:** **empty)**
       An optional numerical network address that the Postfix SMTP client should bind to when making
       an IPv4 connection.

       This can be specified in the main.cf file for all SMTP clients, or it can be specified in the
       master.cf file for a specific client, for example:

           /etc/postfix/master.cf:
               smtp ... smtp -o smtp_bind_address=11.22.33.44

       Note  1:  when inet_interfaces specifies no more than one IPv4 address, and that address is a
       non-loopback address, it is automatically used as the smtp_bind_address.  This supports  vir‐
       tual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces docu‐
       mentation for more detail.

       Note 2: address information may be enclosed inside [], but this form is not required here.

**smtp**___**bind**___**address6** **(default:** **empty)**
       An optional numerical network address that the Postfix SMTP client should bind to when making
       an IPv6 connection.

       This feature is available in Postfix 2.2 and later.

       This can be specified in the main.cf file for all SMTP clients, or it can be specified in the
       master.cf file for a specific client, for example:

           /etc/postfix/master.cf:
               smtp ... smtp -o smtp_bind_address6=1:2:3:4:5:6:7:8

       Note 1: when inet_interfaces specifies no more than one IPv6 address, and that address  is  a
       non-loopback address, it is automatically used as the smtp_bind_address6.  This supports vir‐
       tual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces docu‐
       mentation for more detail.

       Note 2: address information may be enclosed inside [], but this form is not recommended here.

**smtp**___**body**___**checks** **(default:** **empty)**
       Restricted  **body**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown)  tables  for  the  Postfix SMTP client.  These tables are searched
       while mail is being delivered.  Actions that change the delivery time or destination are  not
       available.

       This feature is available in Postfix 2.5 and later.

**smtp**___**cname**___**overrides**___**servername** **(default:** **version** **dependent)**
       When  the  remote SMTP servername is a DNS CNAME, replace the servername with the result from
       CNAME expansion for the purpose of logging, SASL password lookup, TLS  policy  decisions,  or
       TLS certificate verification. The value "no" hardens Postfix smtp_tls_per_site hostname-based
       policies against false hostname information in DNS CNAME records,  and  makes  SASL  password
       file lookups more predictable. This is the default setting as of Postfix 2.3.

       When  DNS  CNAME  records  are  validated  with  secure DNS lookups (smtp_dns_support_level =
       dnssec), they are always allowed to override the above servername (Postfix 2.11 and later).

       This feature is available in Postfix 2.2.9 and later.

**smtp**___**connect**___**timeout** **(default:** **30s)**
       The Postfix SMTP client time limit for completing a TCP connection, or zero (use the  operat‐
       ing system built-in time limit).

       When  no  connection  can be made within the deadline, the Postfix SMTP client tries the next
       address on the mail exchanger list. Specify 0 to disable the time limit  (i.e.  use  whatever
       timeout is implemented by the operating system).

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtp**___**connection**___**cache**___**destinations** **(default:** **empty)**
       Permanently enable SMTP connection caching for the specified destinations.  With SMTP connec‐
       tion  caching, a connection is not closed immediately after completion of a mail transaction.
       Instead, the connection is kept open for  up  to  $smtp_connection_cache_time_limit  seconds.
       This allows connections to be reused for other deliveries, and can improve mail delivery per‐
       formance.

       Specify a comma or white space separated list of destinations or pseudo-destinations:

       •      if mail is sent without a relay host: a domain name (the right-hand side of  an  email
              address, without the [] around a numeric IP address),

       •      if  mail  is  sent  via a relay host: a relay host name (without [] or non-default TCP
              port), as specified in main.cf or in the transport map,

       •      if mail is sent via a UNIX-domain socket: a pathname (without the unix: prefix),

       •      a /file/name with domain names and/or relay host names as defined above,

       •      a "type:table" with domain names and/or relay host names on the left-hand  side.   The
              right-hand side result from "type:table" lookups is ignored.

       This feature is available in Postfix 2.2 and later.

**smtp**___**connection**___**cache**___**on**___**demand** **(default:** **yes)**
       Temporarily  enable  SMTP connection caching while a destination has a high volume of mail in
       the active queue.  With SMTP connection caching, a connection is not closed immediately after
       completion  of a mail transaction.  Instead, the connection is kept open for up to $smtp_con‐
       nection_cache_time_limit seconds.  This allows connections to be reused for other deliveries,
       and can improve mail delivery performance.

       This feature is available in Postfix 2.2 and later.

**smtp**___**connection**___**cache**___**time**___**limit** **(default:** **2s)**
       When SMTP connection caching is enabled, the amount of time that an unused SMTP client socket
       is kept open before it is closed.  Do not specify larger values without permission  from  the
       remote sites.

       This feature is available in Postfix 2.2 and later.

**smtp**___**connection**___**reuse**___**count**___**limit** **(default:** **0)**
       When  SMTP  connection  caching  is  enabled, the number of times that an SMTP session may be
       reused before it is closed, or zero (no limit).  With a reuse count limit of N, a  connection
       is used up to N+1 times.

       NOTE:  This feature is unsafe. When a high-volume destination has multiple inbound MTAs, then
       the slowest inbound MTA will attract the most connections to that destination.  This  limita‐
       tion does not exist with the smtp_connection_reuse_time_limit feature.

       This feature is available in Postfix 2.11.

**smtp**___**connection**___**reuse**___**time**___**limit** **(default:** **300s)**
       The  amount  of  time during which Postfix will use an SMTP connection repeatedly.  The timer
       starts when the connection is initiated (i.e. it includes the connect, greeting and helo  la‐
       tency, in addition to the latencies of subsequent mail delivery transactions).

       This feature addresses a performance stability problem with remote SMTP servers. This problem
       is not specific to Postfix: it can happen when any MTA sends large amounts of SMTP email to a
       site that has multiple MX hosts.

       The  problem  starts when one of a set of MX hosts becomes slower than the rest.  Even though
       SMTP clients connect to fast and slow MX hosts with equal probability, the slow MX host  ends
       up  with  more simultaneous inbound connections than the faster MX hosts, because the slow MX
       host needs more time to serve each client request.

       The slow MX host becomes a connection attractor.  If one MX host becomes N times slower  than
       the  rest,  it  dominates mail delivery latency unless there are more than N fast MX hosts to
       counter the effect. And if the number of MX hosts is smaller than N, the  mail  delivery  la‐
       tency  becomes  effectively  that  of  the  slowest MX host divided by the total number of MX
       hosts.

       The solution uses connection caching in a way that differs from Postfix version 2.2.  By lim‐
       iting  the amount of time during which a connection can be used repeatedly (instead of limit‐
       ing the number of deliveries over that connection), Postfix not only restores fairness in the
       distribution  of simultaneous connections across a set of MX hosts, it also favors deliveries
       over connections that perform well, which is exactly what we want.

       The default reuse time limit, 300s, is comparable to the various  smtp  transaction  timeouts
       which  are fair estimates of maximum excess latency for a slow delivery.  Note that hosts may
       accept thousands of messages over a single connection within  the  default  connection  reuse
       time  limit. This number is much larger than the default Postfix version 2.2 limit of 10 mes‐
       sages per cached connection. It may prove necessary to lower the limit  to  avoid  interoper‐
       ability issues with MTAs that exhibit bugs when many messages are delivered via a single con‐
       nection.  A lower reuse time limit risks losing the benefit of connection reuse when the  av‐
       erage connection and mail delivery latency exceeds the reuse time limit.

       This feature is available in Postfix 2.3 and later.

**smtp**___**data**___**done**___**timeout** **(default:** **600s)**
       The  Postfix  SMTP  client  time limit for sending the SMTP ".", and for receiving the remote
       SMTP server response.

       When no response is received within the deadline, a warning is logged that the  mail  may  be
       delivered multiple times.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtp**___**data**___**init**___**timeout** **(default:** **120s)**
       The Postfix SMTP client time limit for sending the SMTP DATA command, and for  receiving  the
       remote SMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtp**___**data**___**xfer**___**timeout** **(default:** **180s)**
       The Postfix SMTP client time limit for sending the SMTP message content.  When the connection
       makes  no progress for more than $smtp_data_xfer_timeout seconds the Postfix SMTP client ter‐
       minates the transfer.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**smtp**___**defer**___**if**___**no**___**mx**___**address**___**found** **(default:** **no)**
       Defer mail delivery when no MX record resolves to an IP address.

       The  default (no) is to return the mail as undeliverable. With older Postfix versions the de‐
       fault was to keep trying to deliver the mail until someone fixed the MX record or  until  the
       mail was too old.

       Note:  the  Postfix SMTP client always ignores MX records with equal or worse preference than
       the local MTA itself.

       This feature is available in Postfix 2.1 and later.

**smtp**___**delivery**___**status**___**filter** **(default:** **$default**___**delivery**___**status**___**filter)**
       Optional filter for the [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) delivery agent to change the delivery status code or explana‐
       tory  text  of successful or unsuccessful deliveries.  See default_delivery_status_filter for
       details.

       NOTE: This feature modifies Postfix SMTP client error or non-error messages that may  or  may
       not be derived from remote SMTP server responses.  In contrast, the smtp_reply_filter feature
       modifies remote SMTP server responses only.

**smtp**___**destination**___**concurrency**___**limit** **(default:** **$default**___**destination**___**concurrency**___**limit)**
       The maximal number of parallel deliveries to the same destination via the smtp message deliv‐
       ery  transport.  This  limit is enforced by the queue manager. The message delivery transport
       name is the first field in the entry in the master.cf file.

**smtp**___**destination**___**recipient**___**limit** **(default:** **$default**___**destination**___**recipient**___**limit)**
       The maximal number of recipients per message for the smtp message  delivery  transport.  This
       limit  is  enforced  by  the  queue manager. The message delivery transport name is the first
       field in the entry in the master.cf file.

       Setting this parameter to a value  of  1  changes  the  meaning  of  smtp_destination_concur‐
       rency_limit from concurrency per domain into concurrency per recipient.

**smtp**___**discard**___**ehlo**___**keyword**___**address**___**maps** **(default:** **empty)**
       Lookup tables, indexed by the remote SMTP server address, with case insensitive lists of EHLO
       keywords (pipelining, starttls, auth, etc.) that the Postfix SMTP client will ignore  in  the
       EHLO  response from a remote SMTP server. See smtp_discard_ehlo_keywords for details. The ta‐
       ble is not indexed by hostname for consistency with smtpd_discard_ehlo_keyword_address_maps.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       This feature is available in Postfix 2.2 and later.

**smtp**___**discard**___**ehlo**___**keywords** **(default:** **empty)**
       A  case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
       SMTP client will ignore in the EHLO response from a remote SMTP server.

       This feature is available in Postfix 2.2 and later.

       Notes:

       •      Specify the **silent-discard** pseudo keyword to prevent this action from being logged.

       •      Use the smtp_discard_ehlo_keyword_address_maps feature to discard EHLO keywords selec‐
              tively.

**smtp**___**dns**___**reply**___**filter** **(default:** **empty)**
       Optional  filter for Postfix SMTP client DNS lookup results.  Specify zero or more lookup ta‐
       bles.  The lookup tables are searched in the given order for a match with the DNS lookup  re‐
       sult, converted to the following form:

           _name_ _ttl_ _class_ _type_ _preference_ _value_

       The _class_ field is always "IN", the _preference_ field exists only for MX records, the names of
       hosts, domains, etc.  end in ".", and those names are in ASCII form (xn--mumble form  in  the
       case of UTF8 names).

       When  a  match  is found, the table lookup result specifies an action.  By default, the table
       query and the action name are case-insensitive.  Currently, only the **IGNORE** action is  imple‐
       mented.

       Notes:

       •      Postfix DNS reply filters have no effect on implicit DNS lookups through nsswitch.conf
              or equivalent mechanisms.

       •      The Postfix SMTP/LMTP client uses smtp_dns_reply_filter and lmtp_dns_reply_filter only
              to discover a remote SMTP or LMTP service (record types MX, A, AAAA, and TLSA).  These
              lookups are also made to  implement  the  features  reject_unverified_sender  and  re‐
              ject_unverified_recipient.

       •      The Postfix SMTP/LMTP client defers mail delivery when a filter removes all lookup re‐
              sults from a successful query.

       •      Postfix SMTP server uses smtpd_dns_reply_filter only to look up MX, A, AAAA,  and  TXT
              records   to   implement   the   features   reject_unknown_helo_hostname,   reject_un‐
              known_sender_domain,   reject_unknown_recipient_domain,    reject_rbl_*,    and    re‐
              ject_rhsbl_*.

       •      The  Postfix  SMTP server logs a warning or defers mail delivery when a filter removes
              all lookup results from a successful query.

       Example: ignore Google AAAA records in Postfix SMTP client DNS lookups, because Google  some‐
       times hard-rejects mail from IPv6 clients with valid PTR etc. records.

       /etc/postfix/main.cf:
           smtp_dns_reply_filter = pcre:/etc/postfix/smtp_dns_reply_filter

       /etc/postfix/smtp_dns_reply_filter:
           # /domain ttl IN AAAA address/ action, all case-insensitive.
           # Note: the domain name ends in ".".
           /^\S+\.google\.com\.\s+\S+\s+\S+\s+AAAA\s+/ IGNORE

       This feature is available in Postfix 3.0 and later.

**smtp**___**dns**___**resolver**___**options** **(default:** **empty)**
       DNS  Resolver options for the Postfix SMTP client.  Specify zero or more of the following op‐
       tions, separated by comma or whitespace.  Option names are case-sensitive. Some options refer
       to domain names that are specified in the file /etc/resolv.conf or equivalent.

       **res**___**defnames**
              Append  the current domain name to single-component names (those that do not contain a
              "." character). This can produce incorrect results, and  is  the  hard-coded  behavior
              prior to Postfix 2.8.

       **res**___**dnsrch**
              Search  for  host  names in the current domain and in parent domains. This can produce
              incorrect results and is therefore not recommended.

       This feature is available in Postfix 2.8 and later.

**smtp**___**dns**___**support**___**level** **(default:** **empty)**
       Level of DNS support in the Postfix SMTP client.  With "smtp_dns_support_level" left  at  its
       empty  default  value, the legacy "disable_dns_lookups" parameter controls whether DNS is en‐
       abled in the Postfix SMTP client, otherwise the legacy parameter is ignored.

       Specify one of the following:

### disabled
              Disable DNS lookups.  No MX lookups are performed and hostname to address lookups  are
              unconditionally "native".  This setting is not appropriate for hosts that deliver mail
              to the public Internet.   Some  obsolete  how-to  documents  recommend  disabling  DNS
              lookups  in  some configurations with content_filters.  This is no longer required and
              strongly discouraged.

### enabled
              Enable DNS lookups.  Nexthop destination domains not enclosed in "[]" will be  subject
              to MX lookups.  If "dns" and "native" are included in the "smtp_host_lookup" parameter
              value, DNS will be queried first to resolve MX-host A records,  followed  by  "native"
              lookups if no answer is found in DNS.

       **dnssec** Enable  DNSSEC lookups.  The "dnssec" setting differs from the "enabled" setting above
              in the following ways:

       •      Any MX lookups will set RES_USE_DNSSEC and RES_USE_EDNS0 to  request  DNSSEC-validated
              responses. If the MX response is DNSSEC-validated the corresponding hostnames are con‐
              sidered validated.

       •      The address lookups of validated hostnames are also  validated,  (provided  of  course
              "smtp_host_lookup" includes "dns", see below).

       •      Temporary failures in DNSSEC-enabled hostname-to-address resolution block any "native"
              lookups.  Additional "native" lookups only happen when DNSSEC lookups  hard-fail  (NO‐
              DATA or NXDOMAIN).

       The Postfix SMTP client considers non-MX "[nexthop]" and "[nexthop]:port" destinations equiv‐
       alent to statically-validated MX records of the form "nexthop.  IN MX 0 nexthop."  Therefore,
       with  "dnssec"  support turned on, validated hostname-to-address lookups apply to the nexthop
       domain of any "[nexthop]" or "[nexthop]:port"  destination.   This  is  also  true  for  LMTP
       "inet:host"  and  "inet:host:port"  destinations,  as  LMTP hostnames are never subject to MX
       lookups.

       The "dnssec" setting is recommended only if you plan to use the dane or dane-only  TLS  secu‐
       rity  level,  otherwise  enabling  DNSSEC  support  in Postfix offers no additional security.
       Postfix DNSSEC support relies on an upstream recursive nameserver that validates DNSSEC  sig‐
       natures.   Such  a  DNS server will always filter out forged DNS responses, even when Postfix
       itself is not configured to use DNSSEC.

       When using Postfix DANE support the "smtp_host_lookup" parameter  should  include  "dns",  as
       DANE is not applicable to hosts resolved via "native" lookups.

       As mentioned above, Postfix is not a validating stub resolver; it relies on the system's con‐
       figured DNSSEC-validating recursive nameserver to perform all DNSSEC validation.  Since  this
       nameserver's  DNSSEC-validated  responses  will  be fully trusted, it is strongly recommended
       that the MTA host have a local DNSSEC-validating recursive caching nameserver listening on  a
       loopback  address, and be configured to use only this nameserver for all lookups.  Otherwise,
       Postfix may remain subject to man-in-the-middle attacks that forge responses from the  recur‐
       sive nameserver

       DNSSEC  support  requires  a  version of Postfix compiled against a reasonably-modern DNS **re**‐‐
       [**solver**(3)](https://www.chedong.com/phpMan.php/man/solver/3/markdown) library that implements the RES_USE_DNSSEC and RES_USE_EDNS0 resolver options.

       This feature is available in Postfix 2.11 and later.

**smtp**___**enforce**___**tls** **(default:** **no)**
       Enforcement mode: require that remote SMTP servers use TLS encryption, and never send mail in
       the  clear.   This also requires that the remote SMTP server hostname matches the information
       in the remote server certificate, and that the remote SMTP server certificate was issued by a
       CA that is trusted by the Postfix SMTP client. If the certificate doesn't verify or the host‐
       name doesn't match, delivery is deferred and mail stays in the queue.

       The server hostname is matched against all names provided as dNSNames in the  SubjectAlterna‐
       tiveName.   If  no  dNSNames  are  specified, the CommonName is checked.  The behavior may be
       changed with the smtp_tls_enforce_peername option.

       This option is useful only if you are definitely sure that you will only connect  to  servers
       that  support  RFC  2487  _and_  that  provide valid server certificates.  Typical use is for
       clients that send all their email to a dedicated mailhub.

       This feature is available  in  Postfix  2.2  and  later.  With  Postfix  2.3  and  later  use
       smtp_tls_security_level instead.

**smtp**___**fallback**___**relay** **(default:** **$fallback**___**relay)**
       Optional  list  of relay hosts for SMTP destinations that can't be found or that are unreach‐
       able. With Postfix 2.2 and earlier this parameter is called fallback_relay.

       By default, mail is returned to the sender when a destination is not found, and  delivery  is
       deferred when a destination is unreachable.

       With  bulk  email  deliveries, it can be beneficial to run the fallback relay MTA on the same
       host, so that it can reuse the sender IP address.  This speeds up deliveries that are delayed
       by IP-based reputation systems (greylist, etc.).

       The   fallback  relays  must  be  SMTP  destinations.  Specify  a  domain,  host,  host:port,
       [host]:port, [address] or [address]:port; the form [host] turns off MX lookups.  If you spec‐
       ify multiple SMTP destinations, Postfix will try them in the specified order.

       To  prevent  mailer loops between MX hosts and fall-back hosts, Postfix version 2.2 and later
       will not use the fallback relays for destinations that it is MX host for (assuming DNS lookup
       is turned on).

**smtp**___**generic**___**maps** **(default:** **empty)**
       Optional  lookup  tables that perform address rewriting in the Postfix SMTP client, typically
       to transform a locally valid address into a globally valid address when sending  mail  across
       the  Internet.   This  is needed when the local machine does not have its own Internet domain
       name, but uses something like _localdomain.local_ instead.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       The  table  format  and  lookups  are documented in [**generic**(5)](https://www.chedong.com/phpMan.php/man/generic/5/markdown); examples are shown in the AD‐
       DRESS_REWRITING_README and STANDARD_CONFIGURATION_README documents.

       This feature is available in Postfix 2.2 and later.

**smtp**___**header**___**checks** **(default:** **empty)**
       Restricted **header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) tables for the Postfix SMTP client.  These  tables  are  searched
       while  mail is being delivered.  Actions that change the delivery time or destination are not
       available.

       This feature is available in Postfix 2.5 and later.

**smtp**___**helo**___**name** **(default:** **$myhostname)**
       The hostname to send in the SMTP HELO or EHLO command.

       The default value is the machine hostname.  Specify a hostname or [ip.add.re.ss].

       This information can be specified in the main.cf file for all SMTP  clients,  or  it  can  be
       specified in the master.cf file for a specific client, for example:

           /etc/postfix/master.cf:
               mysmtp ... smtp -o smtp_helo_name=foo.bar.com

       This feature is available in Postfix 2.0 and later.

**smtp**___**helo**___**timeout** **(default:** **300s)**
       The  Postfix  SMTP  client time limit for sending the HELO or EHLO command, and for receiving
       the initial remote SMTP server response.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**smtp**___**host**___**lookup** **(default:** **dns)**
       What  mechanisms the Postfix SMTP client uses to look up a host's IP address.  This parameter
       is ignored  when  DNS  lookups  are  disabled  (see:  disable_dns_lookups  and  smtp_dns_sup‐
       port_level).  The "dns" mechanism is always tried before "native" if both are listed.

       Specify one of the following:

       **dns**    Hosts can be found in the DNS (preferred).

       **native** Use the native naming service only (nsswitch.conf, or equivalent mechanism).

### dns, native
              Use the native service for hosts not found in the DNS.

       This feature is available in Postfix 2.1 and later.

**smtp**___**line**___**length**___**limit** **(default:** **998)**
       The  maximal  length  of  message header and body lines that Postfix will send via SMTP. This
       limit does not include the <CR><LF> at the end of each line.  Longer lines are broken by  in‐
       serting  "<CR><LF><SPACE>",  to  minimize  the damage to MIME formatted mail. Specify zero to
       disable this limit.

       The Postfix limit of 998 characters not including <CR><LF> is consistent with the SMTP  limit
       of  1000  characters including <CR><LF>.  The Postfix limit was 990 with Postfix 2.8 and ear‐
       lier.

**smtp**___**mail**___**timeout** **(default:** **300s)**
       The Postfix SMTP client time limit for sending the MAIL FROM command, and for  receiving  the
       remote SMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtp**___**mime**___**header**___**checks** **(default:** **empty)**
       Restricted **mime**___**header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown) tables  for  the  Postfix  SMTP  client.  These  tables  are
       searched while mail is being delivered.  Actions that change the delivery time or destination
       are not available.

       This feature is available in Postfix 2.5 and later.

**smtp**___**mx**___**address**___**limit** **(default:** **5)**
       The maximal number of MX (mail exchanger) IP addresses that  can  result  from  Postfix  SMTP
       client  mail  exchanger lookups, or zero (no limit). Prior to Postfix version 2.3, this limit
       was disabled by default.

       This feature is available in Postfix 2.1 and later.

**smtp**___**mx**___**session**___**limit** **(default:** **2)**
       The maximal number of SMTP sessions per delivery request before the Postfix SMTP client gives
       up  or  delivers to a fall-back relay host, or zero (no limit). This restriction ignores ses‐
       sions that fail to complete the SMTP initial handshake (Postfix version 2.2 and  earlier)  or
       that fail to complete the EHLO and TLS handshake (Postfix version 2.3 and later).

       This feature is available in Postfix 2.1 and later.

**smtp**___**nested**___**header**___**checks** **(default:** **empty)**
       Restricted  **nested**___**header**___**[checks**(5)](https://www.chedong.com/phpMan.php/man/checks/5/markdown)  tables  for  the  Postfix  SMTP client. These tables are
       searched while mail is being delivered.  Actions that change the delivery time or destination
       are not available.

       This feature is available in Postfix 2.5 and later.

**smtp**___**never**___**send**___**ehlo** **(default:** **no)**
       Never  send  EHLO at the start of an SMTP session. See also the smtp_always_send_ehlo parame‐
       ter.

**smtp**___**per**___**record**___**deadline** **(default:** **no)**
       Change the behavior of the smtp_*_timeout time limits, from a time limit per  read  or  write
       system call, to a time limit to send or receive a complete record (an SMTP command line, SMTP
       response line, SMTP message content line, or TLS protocol message).  This limits  the  impact
       from hostile peers that trickle data one byte at a time.

       Note: when per-record deadlines are enabled, a short timeout may cause problems with TLS over
       very slow network connections.  The reasons are that a TLS protocol message can be up  to  16
       kbytes  long  (with  TLSv1), and that an entire TLS protocol message must be sent or received
       within the per-record deadline.

       This feature is available in Postfix 2.9 and later. With older Postfix releases, the behavior
       is as if this parameter is set to "no".

**smtp**___**pix**___**workaround**___**delay**___**time** **(default:** **10s)**
       How  long  the  Postfix SMTP client pauses before sending ".<CR><LF>" in order to work around
       the PIX firewall "<CR><LF>.<CR><LF>" bug.

       Choosing a too short time makes this workaround ineffective when sending large messages  over
       slow network connections.

**smtp**___**pix**___**workaround**___**maps** **(default:** **empty)**
       Lookup  tables,  indexed  by the remote SMTP server address, with per-destination workarounds
       for CISCO PIX firewall bugs.  The table is not  indexed  by  hostname  for  consistency  with
       smtp_discard_ehlo_keyword_address_maps.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       This feature is available in Postfix 2.4 and later.

**smtp**___**pix**___**workaround**___**threshold**___**time** **(default:** **500s)**
       How long a message must be queued before the Postfix SMTP client turns on  the  PIX  firewall
       "<CR><LF>.<CR><LF>"  bug  workaround  for  delivery  through firewalls with "smtp fixup" mode
       turned on.

       By default, the workaround is turned off for mail that is queued for less than  500  seconds.
       In other words, the workaround is normally turned off for the first delivery attempt.

       Specify 0 to enable the PIX firewall "<CR><LF>.<CR><LF>" bug workaround upon the first deliv‐
       ery attempt.

**smtp**___**pix**___**workarounds** **(default:** **disable**___**esmtp,** **delay**___**dotcrlf)**
       A list that specifies zero or more workarounds for CISCO PIX firewall bugs. These workarounds
       are implemented by the Postfix SMTP client. Workaround names are separated by comma or space,
       and are case insensitive.  This parameter  setting  can  be  overruled  with  per-destination
       smtp_pix_workaround_maps settings.

       **delay**___**dotcrlf**
              Insert  a  delay before sending ".<CR><LF>" after the end of the message content.  The
              delay is subject to the smtp_pix_workaround_delay_time and smtp_pix_workaround_thresh‐
              old_time parameter settings.

       **disable**___**esmtp**
              Disable all extended SMTP commands: send HELO instead of EHLO.

       This  feature  is available in Postfix 2.4 and later. The default settings are backwards com‐
       patible with earlier Postfix versions.

**smtp**___**quit**___**timeout** **(default:** **300s)**
       The Postfix SMTP client time limit for sending the QUIT command, and for receiving the remote
       SMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtp**___**quote**___**rfc821**___**envelope** **(default:** **yes)**
       Quote addresses in Postfix SMTP client MAIL FROM and RCPT TO  commands  as  required  by  RFC
       5321. This includes putting quotes around an address localpart that ends in ".".

       The  default  is  to  comply with RFC 5321. If you have to send mail to a broken SMTP server,
       configure a special SMTP client in master.cf:

           /etc/postfix/master.cf:
               broken-smtp . . . smtp -o smtp_quote_rfc821_envelope=no

       and route mail for the destination in question to the "broken-smtp" message delivery  with  a
       [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table.

       This feature is available in Postfix 2.1 and later.

**smtp**___**randomize**___**addresses** **(default:** **yes)**
       Randomize  the order of equal-preference MX host addresses.  This is a performance feature of
       the Postfix SMTP client.

**smtp**___**rcpt**___**timeout** **(default:** **300s)**
       The Postfix SMTP client time limit for sending the SMTP RCPT TO command,  and  for  receiving
       the remote SMTP server response.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtp**___**reply**___**filter** **(default:** **empty)**
       A mechanism to transform replies from remote SMTP servers one line at  a  time.   This  is  a
       last-resort  tool  to work around server replies that break interoperability with the Postfix
       SMTP client.  Other uses involve fault injection to test Postfix's handling  of  invalid  re‐
       sponses.

       Notes:

       •      In the case of a multi-line reply, the Postfix SMTP client uses the final reply line's
              numerical SMTP reply code and enhanced status code.

       •      The numerical SMTP reply code (XYZ) takes precedence over  the  enhanced  status  code
              (X.Y.Z).  When the enhanced status code initial digit differs from the SMTP reply code
              initial digit, or when no enhanced status code is present,  the  Postfix  SMTP  client
              uses a generic enhanced status code (X.0.0) instead.

       Specify  the  name  of  a "type:table" lookup table. The search string is a single SMTP reply
       line as received from the remote SMTP server, except that the trailing <CR><LF> are  removed.
       When the lookup succeeds, the result replaces the single SMTP reply line.

       Examples:

       /etc/postfix/main.cf:
           smtp_reply_filter = pcre:/etc/postfix/reply_filter

       /etc/postfix/reply_filter:
           # Transform garbage into "250-filler..." so that it looks like
           # one line from a multi-line reply. It does not matter what we
           # substitute here as long it has the right syntax.  The Postfix
           # SMTP client will use the final line's numerical SMTP reply
           # code and enhanced status code.
           !/^([2-5][0-9][0-9]($|[- ]))/ 250-filler for garbage

       This feature is available in Postfix 2.7.

**smtp**___**rset**___**timeout** **(default:** **20s)**
       The Postfix SMTP client time limit for sending the RSET command, and for receiving the remote
       SMTP server response. The SMTP client sends RSET in  order  to  finish  a  recipient  address
       probe, or to verify that a cached session is still usable.

       This feature is available in Postfix 2.1 and later.

**smtp**___**sasl**___**auth**___**cache**___**name** **(default:** **empty)**
       An  optional table to prevent repeated SASL authentication failures with the same remote SMTP
       server hostname, username and password. Each table (key, value) pair contains a server  name,
       a  username and password, and the full server response. This information is stored when a re‐
       mote SMTP server rejects an authentication attempt with a 535 reply code.   As  long  as  the
       smtp_sasl_password_maps    information    does    no    change,    and   as   long   as   the
       smtp_sasl_auth_cache_name information does not  expire  (see  smtp_sasl_auth_cache_time)  the
       Postfix  SMTP  client  avoids SASL authentication attempts with the same server, username and
       password,   and   instead   bounces   or    defers    mail    as    controlled    with    the
       smtp_sasl_auth_soft_bounce configuration parameter.

       Use  a  per-destination  delivery  concurrency  of  1 (for example, "smtp_destination_concur‐
       rency_limit = 1", "relay_destination_concurrency_limit = 1", etc.), otherwise multiple deliv‐
       ery agents may experience a login failure at the same time.

       The  table  must  be  accessed  via the proxywrite service, i.e. the map name must start with
       "proxy:". The table should be stored under the directory specified  with  the  data_directory
       parameter.

       This  feature  uses  cryptographic hashing to protect plain-text passwords, and requires that
       Postfix is compiled with TLS support.

       Example:

       smtp_sasl_auth_cache_name = proxy:btree:/var/lib/postfix/sasl_auth_cache

       This feature is available in Postfix 2.5 and later.

**smtp**___**sasl**___**auth**___**cache**___**time** **(default:** **90d)**
       The maximal age of an smtp_sasl_auth_cache_name entry before it is removed.

       This feature is available in Postfix 2.5 and later.

**smtp**___**sasl**___**auth**___**enable** **(default:** **no)**
       Enable SASL authentication in the Postfix SMTP client.  By default, the Postfix  SMTP  client
       uses no authentication.

       Example:

       smtp_sasl_auth_enable = yes

**smtp**___**sasl**___**auth**___**soft**___**bounce** **(default:** **yes)**
       When  a remote SMTP server rejects a SASL authentication request with a 535 reply code, defer
       mail delivery instead of returning mail as undeliverable. The latter behavior was  hard-coded
       prior to Postfix version 2.5.

       Note: the setting "yes" overrides the global soft_bounce parameter, but the setting "no" does
       not.

       Example:

       # Default as of Postfix 2.5
       smtp_sasl_auth_soft_bounce = yes
       # The old hard-coded default
       smtp_sasl_auth_soft_bounce = no

       This feature is available in Postfix 2.5 and later.

**smtp**___**sasl**___**mechanism**___**filter** **(default:** **empty)**
       If non-empty, a Postfix SMTP client filter for the remote SMTP server's list of offered  SASL
       mechanisms.   Different  client  and  server  implementations may support different mechanism
       lists;  by  default,  the  Postfix  SMTP  client  will  use  the  intersection  of  the  two.
       smtp_sasl_mechanism_filter specifies an optional third mechanism list to intersect with.

       Specify  mechanism names, "/file/name" patterns or "type:table" lookup tables. The right-hand
       side result from "type:table" lookups is ignored. Specify "!pattern" to exclude  a  mechanism
       name  from  the  list.  The  form  "!/file/name" is supported only in Postfix version 2.4 and
       later.

       This feature is available in Postfix 2.2 and later.

       Examples:

       smtp_sasl_mechanism_filter = plain, login
       smtp_sasl_mechanism_filter = /etc/postfix/smtp_mechs
       smtp_sasl_mechanism_filter = !gssapi, !login, static:rest

**smtp**___**sasl**___**password**___**maps** **(default:** **empty)**
       Optional Postfix SMTP client lookup tables with one username:password entry per  sender,  re‐
       mote  hostname  or  next-hop domain. Per-sender lookup is done only when sender-dependent au‐
       thentication is enabled.  If no username:password entry  is  found,  then  the  Postfix  SMTP
       client will not attempt to authenticate to the remote host.

       The  Postfix SMTP client opens the lookup table before going to chroot jail, so you can leave
       the password file in /etc/postfix.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

**smtp**___**sasl**___**path** **(default:** **empty)**
       Implementation-specific  information  that the Postfix SMTP client passes through to the SASL
       plug-in implementation that is selected with **smtp**___**sasl**___**type**.  Typically  this  specifies  the
       name of a configuration file or rendezvous point.

       This feature is available in Postfix 2.3 and later.

**smtp**___**sasl**___**security**___**options** **(default:** **noplaintext,** **noanonymous)**
       Postfix  SMTP  client SASL security options; as of Postfix 2.3 the list of available features
       depends on the SASL client implementation that is selected with **smtp**___**sasl**___**type**.

       The following security features are defined for the **cyrus** client SASL implementation:

       Specify zero or more of the following:

### noplaintext
              Disallow methods that use plaintext passwords.

### noactive
              Disallow methods subject to active (non-dictionary) attack.

### nodictionary
              Disallow methods subject to passive (dictionary) attack.

### noanonymous
              Disallow methods that allow anonymous authentication.

       **mutual**___**auth**
              Only allow methods that provide mutual authentication (not available with SASL version
              1).

       Example:

       smtp_sasl_security_options = noplaintext

**smtp**___**sasl**___**tls**___**security**___**options** **(default:** **$smtp**___**sasl**___**security**___**options)**
       The  SASL authentication security options that the Postfix SMTP client uses for TLS encrypted
       SMTP sessions.

       This feature is available in Postfix 2.2 and later.

**smtp**___**sasl**___**tls**___**verified**___**security**___**options** **(default:** **$smtp**___**sasl**___**tls**___**security**___**options)**
       The SASL authentication security options that the Postfix SMTP client uses for TLS  encrypted
       SMTP sessions with a verified server certificate.

       When  mail  is sent to the public MX host for the recipient's domain, server certificates are
       by default optional, and delivery proceeds even if certificate verification fails. For deliv‐
       ery via a submission service that requires SASL authentication, it may be appropriate to send
       plaintext passwords only when the connection to the server  is  strongly  encrypted  **and**  the
       server identity is verified.

       The smtp_sasl_tls_verified_security_options parameter makes it possible to only enable plain‐
       text mechanisms when a secure connection to the server is available. Submission servers  sub‐
       ject  to this policy must either have verifiable certificates or offer suitable non-plaintext
       SASL mechanisms.

       This feature is available in Postfix 2.6 and later.

**smtp**___**sasl**___**type** **(default:** **cyrus)**
       The SASL plug-in type that the Postfix SMTP client should use for authentication.  The avail‐
       able types are listed with the "**postconf** **-A**" command.

       This feature is available in Postfix 2.3 and later.

**smtp**___**send**___**dummy**___**mail**___**auth** **(default:** **no)**
       Whether  or not to append the "AUTH=<>" option to the MAIL FROM command in SASL-authenticated
       SMTP sessions. The default is not to send this, to avoid problems  with  broken  remote  SMTP
       servers.  Before Postfix 2.9 the behavior is as if "smtp_send_dummy_mail_auth = yes".

       This feature is available in Postfix 2.9 and later.

**smtp**___**send**___**xforward**___**command** **(default:** **no)**
       Send  the  non-standard XFORWARD command when the Postfix SMTP server EHLO response announces
       XFORWARD support.

       This allows a Postfix SMTP delivery agent, used for injecting mail into a content filter,  to
       forward  the name, address, protocol and HELO name of the original client to the content fil‐
       ter and downstream queuing SMTP server. This can produce  more  useful  logging  than  local‐
       host[127.0.0.1] etc.

       This feature is available in Postfix 2.1 and later.

**smtp**___**sender**___**dependent**___**authentication** **(default:** **no)**
       Enable  sender-dependent  authentication  in  the Postfix SMTP client; this is available only
       with SASL authentication, and disables SMTP connection caching to ensure that mail from  dif‐
       ferent senders will use the appropriate credentials.

       This feature is available in Postfix 2.3 and later.

**smtp**___**skip**___**4xx**___**greeting** **(default:** **yes)**
       Skip SMTP servers that greet with a 4XX status code (go away, try again later).

       By   default,   the   Postfix  SMTP  client  moves  on  the  next  mail  exchanger.   Specify
       "smtp_skip_4xx_greeting = no" if Postfix should defer delivery immediately.

       This feature is available in Postfix 2.0 and earlier.  Later Postfix versions always skip re‐
       mote SMTP servers that greet with a 4XX status code.

**smtp**___**skip**___**5xx**___**greeting** **(default:** **yes)**
       Skip remote SMTP servers that greet with a 5XX status code.

       By   default,   the   Postfix   SMTP  client  moves  on  the  next  mail  exchanger.  Specify
       "smtp_skip_5xx_greeting = no" if Postfix should bounce the  mail  immediately.  Caution:  the
       latter behavior appears to contradict RFC 2821.

**smtp**___**skip**___**quit**___**response** **(default:** **yes)**
       Do not wait for the response to the SMTP QUIT command.

**smtp**___**starttls**___**timeout** **(default:** **300s)**
       Time  limit for Postfix SMTP client write and read operations during TLS startup and shutdown
       handshake procedures.

       This feature is available in Postfix 2.2 and later.

**smtp**___**tcp**___**port** **(default:** **smtp)**
       The default TCP port that the Postfix SMTP client connects to.  Specify a symbolic name  (see
       [**services**(5)](https://www.chedong.com/phpMan.php/man/services/5/markdown)) or a numeric port.

**smtp**___**tls**___**CAfile** **(default:** **empty)**
       A  file containing CA certificates of root CAs trusted to sign either remote SMTP server cer‐
       tificates or intermediate CA certificates.  These are loaded into memory before  the  [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown)
       client  enters  the  chroot  jail.  If  the  number of trusted roots is large, consider using
       smtp_tls_CApath instead, but note that the latter directory must be  present  in  the  chroot
       jail if the [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) client is chrooted. This file may also be used to augment the client cer‐
       tificate trust chain, but it is best to include all the  required  certificates  directly  in
       $smtp_tls_cert_file (or, Postfix >= 3.4 $smtp_tls_chain_files).

       Specify  "smtp_tls_CAfile  = /path/to/system_CA_file" to use ONLY the system-supplied default
       Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from  appending  the  system-supplied
       default CAs and trusting third-party certificates.

       Example:

       smtp_tls_CAfile = /etc/postfix/CAcert.pem

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**CApath** **(default:** **empty)**
       Directory  with  PEM format Certification Authority certificates that the Postfix SMTP client
       uses to verify a remote SMTP server certificate.  Don't forget to create the necessary "hash"
       links with, for example, "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs".

       To use this option in chroot mode, this directory (or a copy) must be inside the chroot jail.

       Specify  "smtp_tls_CApath = /path/to/system_CA_directory" to use ONLY the system-supplied de‐
       fault Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from  appending  the  system-supplied
       default CAs and trusting third-party certificates.

       Example:

       smtp_tls_CApath = /etc/postfix/certs

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**block**___**early**___**mail**___**reply** **(default:** **no)**
       Try  to detect a mail hijacking attack based on a TLS protocol vulnerability (CVE-2009-3555),
       where an attacker prepends malicious HELO, MAIL, RCPT, DATA commands to a Postfix SMTP client
       TLS  session.  The attack would succeed with non-Postfix SMTP servers that reply to the mali‐
       cious HELO, MAIL, RCPT, DATA commands after negotiating the Postfix SMTP client TLS session.

       This feature is available in Postfix 2.7.

**smtp**___**tls**___**cert**___**file** **(default:** **empty)**
       File with the Postfix SMTP client RSA certificate in PEM format.  This file may also  contain
       the Postfix SMTP client private RSA key, and these may be the same as the Postfix SMTP server
       RSA certificate and key file.  With Postfix >= 3.4 the preferred way to configure client keys
       and certificates is via the "smtp_tls_chain_files" parameter.

       Do  not  configure client certificates unless you **must** present client TLS certificates to one
       or more servers. Client certificates are not usually needed, and can cause problems  in  con‐
       figurations  that  work  well  without  them.  The recommended setting is to let the defaults
       stand:

           smtp_tls_cert_file =
           smtp_tls_key_file =
           smtp_tls_eccert_file =
           smtp_tls_eckey_file =
           # Obsolete DSA parameters
           smtp_tls_dcert_file =
           smtp_tls_dkey_file =
           # Postfix >= 3.4 interface
           smtp_tls_chain_files =

       The best way to use the default settings is to comment out the above parameters in main.cf if
       present.

       To  enable  remote SMTP servers to verify the Postfix SMTP client certificate, the issuing CA
       certificates must be made available to the server. You should include the  required  certifi‐
       cates  in  the  client certificate file, the client certificate first, then the issuing CA(s)
       (bottom-up order).

       Example: the certificate for "client.example.com" was issued by "intermediate CA"  which  it‐
       self  has  a certificate issued by "root CA".  As the "root" super-user create the client.pem
       file with:

           # **umask** **077**
           # **cat** **client**___**key.pem** **client**___**cert.pem** **intermediate**___**CA.pem** **>** **chain.pem**

       If you also want to verify remote SMTP server certificates issued by these CAs, you  can  add
       the CA certificates to the smtp_tls_CAfile, in which case it is not necessary to have them in
       the smtp_tls_cert_file, smtp_tls_dcert_file (obsolete) or smtp_tls_eccert_file.

       A certificate supplied here must be usable as an SSL client certificate and  hence  pass  the
       "openssl verify -purpose sslclient ..." test.

       Example:

       smtp_tls_cert_file = /etc/postfix/chain.pem

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**chain**___**files** **(default:** **empty)**
       List  of  one or more PEM files, each holding one or more private keys directly followed by a
       corresponding certificate chain.  The file names are separated by commas  and/or  whitespace.
       This  parameter  obsoletes  the  legacy algorithm-specific key and certificate file settings.
       When this parameter is non-empty, the legacy parameters are ignored, and a warning is  logged
       if any are also non-empty.

       With the proliferation of multiple private key algorithms-which, as of OpenSSL 1.1.1, include
       DSA (obsolete), RSA, ECDSA, Ed25519 and Ed448-it is increasingly impractical to use  separate
       parameters to configure the key and certificate chain for each algorithm.  Therefore, Postfix
       now supports storing multiple keys and corresponding certificate chains in a single  file  or
       in a set of files.

       Each key must appear **immediately** **before** the corresponding certificate, optionally followed by
       additional issuer certificates that complete the certificate chain for that key.  When multi‐
       ple files are specified, they are equivalent to a single file that is concatenated from those
       files in the given order.  Thus, while a key must always precede its certificate  and  issuer
       chain,  it  can  be in a separate file, so long as that file is listed immediately before the
       file that holds the corresponding certificate chain.  Once all the  files  are  concatenated,
       the sequence of PEM objects must be: _key1,_ _cert1,_ _[chain1],_ _key2,_ _cert2,_ _[chain2],_ _...,_ _keyN,_
       _certN,_ _[chainN]._

       Storing the private key in the same file as the corresponding certificate is  more  reliable.
       With  the key and certificate in separate files, there is a chance that during key rollover a
       Postfix process might load a private key and  certificate  from  separate  files  that  don't
       match.   Various  operational  errors may even result in a persistent broken configuration in
       which the certificate does not match the private key.

       The file or files must contain at most one key of each type.  If, for example,  two  or  more
       RSA keys and corresponding chains are listed, depending on the version of OpenSSL either only
       the last one will be used or an  configuration  error  may  be  detected.   Note  that  while
       "Ed25519" and "Ed448" are considered separate algorithms, the various ECDSA curves (typically
       one of prime256v1, secp384r1 or secp521r1) are considered as different parameters of a single
       "ECDSA"  algorithm, so it is not presently possible to configure keys for more than one ECDSA
       curve.

       Example (separate files for each key and corresponding certificate chain):

           /etc/postfix/main.cf:
               smtp_tls_chain_files =
                   ${config_directory}/ed25519.pem,
                   ${config_directory}/ed448.pem,
                   ${config_directory}/rsa.pem

           /etc/postfix/ed25519.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----

           /etc/postfix/ed448.pem:
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----

           /etc/postfix/rsa.pem:
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       Example (all keys and certificates in a single file):

           /etc/postfix/main.cf:
               smtp_tls_chain_files = ${config_directory}/chains.pem

           /etc/postfix/chains.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       This feature is available in Postfix 3.4 and later.

**smtp**___**tls**___**cipherlist** **(default:** **empty)**
       Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS cipher list. As  this  feature
       applies  to all TLS security levels, it is easy to create interoperability problems by choos‐
       ing a non-default cipher list. Do not use a non-default TLS cipher list on hosts that deliver
       email  to  the public Internet: you will be unable to send email to servers that only support
       the ciphers you exclude. Using a restricted cipher list may be more appropriate for an inter‐
       nal  MTA,  where  one  can  exert some control over the TLS software and settings of the peer
       servers.

       **Note:** do not use "" quotes around the parameter value.

       This feature is available in Postfix version 2.2. It is not used with Postfix 2.3 and  later;
       use smtp_tls_mandatory_ciphers instead.

**smtp**___**tls**___**ciphers** **(default:** **medium)**
       The minimum TLS cipher grade that the Postfix SMTP client will use with opportunistic TLS en‐
       cryption. Cipher types listed in smtp_tls_exclude_ciphers are excluded from the base  defini‐
       tion of the selected cipher grade.   The default value is "medium" for Postfix releases after
       the middle of 2015, "export" for older releases.

       When TLS is mandatory the cipher grade is chosen via the smtp_tls_mandatory_ciphers  configu‐
       ration  parameter,  see there for syntax details. See smtp_tls_policy_maps for information on
       how to configure ciphers on a per-destination basis.

       This feature is available in Postfix 2.6 and later. With earlier Postfix  releases  only  the
       smtp_tls_mandatory_ciphers  parameter  is implemented, and opportunistic TLS always uses "ex‐
       port" or better (i.e. all) ciphers.

**smtp**___**tls**___**connection**___**reuse** **(default:** **no)**
       Try to make multiple deliveries per TLS-encrypted connection.  This uses the [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) ser‐
       vice  to  encrypt an SMTP connection, uses the [**scache**(8)](https://www.chedong.com/phpMan.php/man/scache/8/markdown) service to save that connection, and
       relies on hints from the [**qmgr**(8)](https://www.chedong.com/phpMan.php/man/qmgr/8/markdown) daemon.

       See "Client-side TLS connection reuse" for background details.

       This feature is available in Postfix 3.4 and later.

**smtp**___**tls**___**dane**___**insecure**___**mx**___**policy** **(default:** **see** **postconf** **-d** **output)**
       The TLS policy for MX hosts with "secure" TLSA records when the nexthop destination  security
       level is **dane**, but the MX record was found via an "insecure" MX lookup.  The choices are:

       **may**    The  TLSA  records  will be ignored and TLS will be optional.  If the MX host does not
              appear to support STARTTLS, or the STARTTLS handshake fails, mail may be sent  in  the
              clear.

### encrypt
              The  TLSA  records will signal a requirement to use TLS.  While TLS encryption will be
              required, authentication will not be performed.

       **dane**   The TLSA records will be used just as with "secure" MX records.  TLS  encryption  will
              be required, and, if at least one of the TLSA records is "usable", authentication will
              be required.  When authentication succeeds, it will be logged only as  "Trusted",  not
              "Verified", because the MX host name could have been forged.
              The  default  setting  for  Postfix  >=  3.6 is "dane" with "smtp_tls_security_level =
              dane", otherwise "may". This  behavior  was  backported  to  Postfix  versions  3.5.9,
              3.4.19,  3.3.16. 3.2.21.  With earlier Postfix versions the default setting was always
              "dane".

       Though with "insecure" MX records an active attacker can compromise SMTP  transport  security
       by  returning  forged MX records, such attacks are "tamper-evident" since any forged MX host‐
       names will be recorded in the mail logs.  Attackers who place a high value staying hidden may
       be deterred from forging MX records.

       This  feature  is  available in Postfix 3.1 and later. The **may** policy is backwards-compatible
       with earlier Postfix versions.

**smtp**___**tls**___**dcert**___**file** **(default:** **empty)**
       File with the Postfix SMTP client DSA certificate in PEM format.  This file may also  contain
       the  Postfix  SMTP  client  private DSA key.  The DSA algorithm is obsolete and should not be
       used.

       See the discussion under smtp_tls_cert_file for more details.

       Example:

       smtp_tls_dcert_file = /etc/postfix/client-dsa.pem

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**dkey**___**file** **(default:** **$smtp**___**tls**___**dcert**___**file)**
       File with the Postfix SMTP client DSA private key in PEM format.  This file may  be  combined
       with  the  Postfix  SMTP client DSA certificate file specified with $smtp_tls_dcert_file. The
       DSA algorithm is obsolete and should not be used.

       The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
       permissions  should  grant  read-only access to the system superuser account ("root"), and no
       access to anyone else.

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**eccert**___**file** **(default:** **empty)**
       File with the Postfix SMTP client ECDSA certificate in PEM format.  This file may  also  con‐
       tain  the  Postfix  SMTP  client ECDSA private key.  With Postfix >= 3.4 the preferred way to
       configure client keys and certificates is via the "smtp_tls_chain_files" parameter.

       See the discussion under smtp_tls_cert_file for more details.

       Example:

       smtp_tls_eccert_file = /etc/postfix/ecdsa-ccert.pem

       This feature is available in Postfix 2.6 and later, when Postfix is compiled and linked  with
       OpenSSL 1.0.0 or later.

**smtp**___**tls**___**eckey**___**file** **(default:** **$smtp**___**tls**___**eccert**___**file)**
       File with the Postfix SMTP client ECDSA private key in PEM format.  This file may be combined
       with the Postfix SMTP client ECDSA certificate  file  specified  with  $smtp_tls_eccert_file.
       With  Postfix  >=  3.4 the preferred way to configure client keys and certificates is via the
       "smtp_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
       permissions  should  grant  read-only access to the system superuser account ("root"), and no
       access to anyone else.

       This feature is available in Postfix 2.6 and later, when Postfix is compiled and linked  with
       OpenSSL 1.0.0 or later.

**smtp**___**tls**___**enforce**___**peername** **(default:** **yes)**
       With  mandatory  TLS encryption, require that the remote SMTP server hostname matches the in‐
       formation in the remote SMTP server certificate.  As of RFC 2487 the requirements  for  host‐
       name checking for MTA clients are not specified.

       This  option can be set to "no" to disable strict peer name checking. This setting has no ef‐
       fect on sessions that are controlled via the smtp_tls_per_site table.

       Disabling the hostname verification can make sense in closed environment  where  special  CAs
       are  created.   If  not used carefully, this option opens the danger of a "man-in-the-middle"
       attack (the CommonName of this attacker will be logged).

       This feature is available  in  Postfix  2.2  and  later.  With  Postfix  2.3  and  later  use
       smtp_tls_security_level instead.

**smtp**___**tls**___**exclude**___**ciphers** **(default:** **empty)**
       List  of  ciphers  or cipher types to exclude from the Postfix SMTP client cipher list at all
       TLS security levels. This is not an OpenSSL cipherlist, it is  a  simple  list  separated  by
       whitespace  and/or commas. The elements are a single cipher, or one or more "+" separated ci‐
       pher properties, in which case only ciphers matching **all** the properties are excluded.

       Examples (some of these will cause problems):

           smtp_tls_exclude_ciphers = aNULL
           smtp_tls_exclude_ciphers = MD5, DES
           smtp_tls_exclude_ciphers = DES+MD5
           smtp_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
           smtp_tls_exclude_ciphers = kEDH+aRSA

       The first setting, disables anonymous ciphers. The next setting disables ciphers that use the
       MD5  digest algorithm or the (single) DES encryption algorithm. The next setting disables ci‐
       phers that use MD5 and DES together.  The next setting disables the two ciphers  "AES256-SHA"
       and  "DES-CBC3-MD5".  The  last setting disables ciphers that use "EDH" key exchange with RSA
       authentication.

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**fingerprint**___**cert**___**match** **(default:** **empty)**
       List of acceptable remote SMTP server certificate fingerprints for the "fingerprint" TLS  se‐
       curity  level  (**smtp**___**tls**___**security**___**level** = fingerprint). At this security level, Certification
       Authorities are not used, and certificate expiration times are ignored. Instead, server  cer‐
       tificates  are  verified directly via their certificate fingerprint or public key fingerprint
       (Postfix 2.9 and later). The fingerprint is a message digest of the  server  certificate  (or
       public key). The digest algorithm is selected via the **smtp**___**tls**___**fingerprint**___**digest** parameter.

       The  colons  between  each  pair of nibbles in the fingerprint value are optional (Postfix >=
       3.6). These were required in earlier Postfix releases.

       When an **smtp**___**tls**___**policy**___**maps** table entry specifies  the  "fingerprint"  security  level,  any
       "match" attributes in that entry specify the list of valid fingerprints for the corresponding
       destination. Multiple fingerprints can be combined with a "|" delimiter in a single match at‐
       tribute, or multiple match attributes can be employed.

       Example:  Certificate  fingerprint  verification with internal mailhub.  Two matching finger‐
       prints are listed. The relayhost may be multiple physical hosts behind a load-balancer,  each
       with  its  own private/public key and self-signed certificate. Alternatively, a single relay‐
       host may be in the process of switching from one set of private/public keys to  another,  and
       both keys are trusted just prior to the transition.

           relayhost = [mailhub.example.com]
           smtp_tls_security_level = fingerprint
           smtp_tls_fingerprint_digest = sha256
           smtp_tls_fingerprint_cert_match =
               cd:fc:d8:db:f8:c4:82:96:6c:...:28:71:e8:f5:8d:a5:0d:9b:d4:a6
               dd:5c:ef:f5:c3:bc:64:25:36:...:99:36:06:ce:40:ef:de:2e:ad:a4

       Example:  Certificate fingerprint verification with selected destinations.  As in the example
       above, we show two matching fingerprints:

           /etc/postfix/main.cf:
               smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
               smtp_tls_fingerprint_digest = sha256

           /etc/postfix/tls_policy:
               example.com fingerprint
                   match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76
                   match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f

       This feature is available in Postfix 2.5 and later.

**smtp**___**tls**___**fingerprint**___**digest** **(default:** **see** **postconf** **-d** **output)**
       The message digest algorithm used to construct remote SMTP server  certificate  fingerprints.
       At  the  "fingerprint" TLS security level (**smtp**___**tls**___**security**___**level** = fingerprint), the server
       certificate is verified by directly matching its certificate fingerprint or  its  public  key
       fingerprint (Postfix 2.9 and later). The fingerprint is the message digest of the server cer‐
       tificate (or its public key) using the selected algorithm. With a digest algorithm  resistant
       to  "second  pre-image" attacks, it is not feasible to create a new public key and a matching
       certificate (or public/private key-pair) that has the same fingerprint.

       The default algorithm is **sha256** with Postfix >= 3.6 and the **compatibility**___**level** set to 3.6 or
       higher. With Postfix <= 3.5, the default algorithm is **md5**.

       The  best-practice  algorithm  is  now **sha256**. Recent advances in hash function cryptanalysis
       have led to md5 and sha1 being deprecated in favor of sha256.  However, as long as there  are
       no  known "second pre-image" attacks against the older algorithms, their use in this context,
       though not recommended, is still likely safe.

       While additional digest algorithms are often available with OpenSSL's libcrypto,  only  those
       used by libssl in SSL cipher suites are available to Postfix.  You'll likely find support for
       md5, sha1, sha256 and sha512.

       To find the fingerprint of a specific certificate file, with  a  specific  digest  algorithm,
       run:

           $ openssl x509 -noout -fingerprint -_digest_ -in _certfile_.pem

       The text to the right of "=" sign is the desired fingerprint.  For example:

           $ openssl x509 -noout -fingerprint -sha256 -in cert.pem
           SHA256 Fingerprint=D4:6A:AB:19:24:...:BB:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

       To extract the public key fingerprint from an X.509 certificate, you need to extract the pub‐
       lic key from the certificate and compute the appropriate digest of its DER (ASN.1)  encoding.
       With  OpenSSL  the  "-pubkey"  option of the "x509" command extracts the public key always in
       "PEM" format. We pipe the result to another OpenSSL command that converts the key to DER  and
       then to the "dgst" command to compute the fingerprint.

       The actual command to transform the key to DER format depends on the version of OpenSSL used.
       As of OpenSSL 1.0.0, the "pkey" command supports all key types.

           # OpenSSL >= 1.0 with SHA-256 fingerprints.
           $ openssl x509 -in cert.pem -noout -pubkey |
               openssl pkey -pubin -outform DER |
               openssl dgst -sha256 -c
           (stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:...:fc:09:1a:61:98:b5:bc:7c:60:58

       The Postfix SMTP server and client log the peer (leaf) certificate fingerprint and public key
       fingerprint when the TLS loglevel is 2 or higher.

       This feature is available in Postfix 2.5 and later.

**smtp**___**tls**___**force**___**insecure**___**host**___**tlsa**___**lookup** **(default:** **no)**
       Lookup  the  associated  DANE TLSA RRset even when a hostname is not an alias and its address
       records lie in an unsigned zone.  This is unlikely to ever yield  DNSSEC  validated  results,
       since  child  zones of unsigned zones are also unsigned in the absence of DLV or locally con‐
       figured non-root trust-anchors.  We anticipate that such mechanisms will not be used for just
       the "_tcp" subdomain of a host.  Suppressing the TLSA RRset lookup reduces latency and avoids
       potential interoperability problems with nameservers for unsigned zones that are not prepared
       to handle the new TLSA RRset.

       This feature is available in Postfix 2.11.

**smtp**___**tls**___**key**___**file** **(default:** **$smtp**___**tls**___**cert**___**file)**
       File  with  the Postfix SMTP client RSA private key in PEM format.  This file may be combined
       with the Postfix SMTP client RSA certificate file specified with  $smtp_tls_cert_file.   With
       Postfix  >=  3.4  the  preferred  way  to  configure  client keys and certificates is via the
       "smtp_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
       permissions  should  grant  read-only access to the system superuser account ("root"), and no
       access to anyone else.

       Example:

       smtp_tls_key_file = $smtp_tls_cert_file

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**loglevel** **(default:** **0)**
       Enable additional Postfix SMTP client logging of TLS activity.  Each logging level  also  in‐
       cludes the information that is logged at a lower logging level.

              0 Disable logging of TLS activity.

              1  Log  only a summary message on TLS handshake completion - no logging of remote SMTP
              server certificate trust-chain verification errors if server certificate  verification
              is  not  required.  With Postfix 2.8 and earlier, log the summary message and uncondi‐
              tionally log trust-chain verification errors.

              2 Also log levels during TLS negotiation.

              3 Also log hexadecimal and ASCII dump of TLS negotiation process.

              4 Also log hexadecimal and ASCII dump of complete transmission after STARTTLS.

       Do not use "smtp_tls_loglevel = 2" or higher except in case of problems. Use of loglevel 4 is
       strongly discouraged.

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**mandatory**___**ciphers** **(default:** **medium)**
       The minimum TLS cipher grade that the Postfix SMTP client will use with mandatory TLS encryp‐
       tion.  The default value "medium" is suitable for most destinations with which you  may  want
       to  enforce  TLS, and is beyond the reach of today's cryptanalytic methods. See smtp_tls_pol‐
       icy_maps for information on how to configure ciphers on a per-destination basis.

       The following cipher grades are supported:

       **export** Enable "EXPORT" grade or better OpenSSL ciphers.  The underlying cipherlist is  speci‐
              fied via the tls_export_cipherlist configuration parameter, which you are strongly en‐
              couraged to not change.  This choice is insecure and SHOULD NOT be used.

       **low**    Enable "LOW" grade or better OpenSSL ciphers.  The underlying cipherlist is  specified
              via  the tls_low_cipherlist configuration parameter, which you are strongly encouraged
              to not change.  This choice is insecure and SHOULD NOT be used.

       **medium** Enable "MEDIUM" grade or better OpenSSL ciphers.  The underlying cipherlist is  speci‐
              fied via the tls_medium_cipherlist configuration parameter, which you are strongly en‐
              couraged to not change.

       **high**   Enable only "HIGH" grade OpenSSL ciphers.  This setting may be  appropriate  when  all
              mandatory  TLS destinations (e.g. when all mail is routed to a suitably capable relay‐
              host) support at least one "HIGH" grade cipher. The underlying cipherlist is specified
              via the tls_high_cipherlist configuration parameter, which you are strongly encouraged
              to not change.

       **null**   Enable only the "NULL" OpenSSL ciphers, these provide authentication  without  encryp‐
              tion.  This setting is only appropriate in the rare case that all servers are prepared
              to use NULL ciphers (not normally enabled in TLS servers). A plausible use-case is  an
              LMTP server listening on a UNIX-domain socket that is configured to support "NULL" ci‐
              phers. The underlying cipherlist is specified via the  tls_null_cipherlist  configura‐
              tion parameter, which you are strongly encouraged to not change.

       The  underlying cipherlists for grades other than "null" include anonymous ciphers, but these
       are automatically filtered out if the Postfix SMTP client is configured to verify server cer‐
       tificates.   You  are  very  unlikely to need to take any steps to exclude anonymous ciphers,
       they are excluded automatically as necessary.  If you must exclude anonymous ciphers  at  the
       "may"  or  "encrypt"  security levels, when the Postfix SMTP client does not need or use peer
       certificates, set "smtp_tls_exclude_ciphers = aNULL". To exclude anonymous ciphers only  when
       TLS is enforced, set "smtp_tls_mandatory_exclude_ciphers = aNULL".

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**mandatory**___**exclude**___**ciphers** **(default:** **empty)**
       Additional  list  of  ciphers  or cipher types to exclude from the Postfix SMTP client cipher
       list at mandatory TLS security levels. This list works in addition to the  exclusions  listed
       with smtp_tls_exclude_ciphers (see there for syntax details).

       Starting with Postfix 2.6, the mandatory cipher exclusions can be specified on a per-destina‐
       tion basis via the TLS policy "exclude" attribute. See smtp_tls_policy_maps for notes and ex‐
       amples.

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**mandatory**___**protocols** **(default:** **see** **postconf** **-d** **output)**
       TLS  protocols  that  the  Postfix  SMTP  client  will use with mandatory TLS encryption.  In
       main.cf the values are separated by whitespace, commas or colons. In the policy table "proto‐
       cols"  attribute (see smtp_tls_policy_maps) the only valid separator is colon. An empty value
       means allow all protocols.

       The valid protocol names (see **SSL**___**get**___**[version**(3)](https://www.chedong.com/phpMan.php/man/version/3/markdown)) are "SSLv2", "SSLv3",  "TLSv1",  "TLSv1.1",
       "TLSv1.2"  and  "TLSv1.3".   Starting with Postfix 3.6, the default value is ">=TLSv1", which
       sets TLS 1.0 as the lowest supported TLS protocol version (see below).   Older  releases  use
       the "!" exclusion syntax, also described below.

       As  of  Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set a
       lowest acceptable TLS protocol version and/or a highest acceptable TLS protocol version.   To
       set the lower bound include an element of the form: ">=_version_" where _version_ is a either one
       of the TLS protocol names listed above, or a hexadecimal number corresponding to the  desired
       TLS  protocol  version  (0301 for TLS 1.0, 0302 for TLS 1.1, etc.).  For the upper bound, use
       "<=_version_".  There must be no whitespace between the ">=" or "<=" symbols and  the  protocol
       name or number.

       Hexadecimal  protocol  numbers  make  it possible to specify protocol bounds for TLS versions
       that are known to OpenSSL, but might not be known to Postfix.  They cannot be used  with  the
       legacy  exclusion  syntax.   Leading  "0"  or  "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent  to  "TLSv1".   Hexadecimal
       versions  unknown to OpenSSL will fail to set the upper or lower bound, and a warning will be
       logged.  Hexadecimal versions should only be used when Postfix is  linked  with  some  future
       version  of  OpenSSL  that supports TLS 1.4 or later, but Postfix does not yet support a sym‐
       bolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtp_tls_mandatory_protocols = >=TLSv1.2, <=0305
           # Allow only TLS 1.2 and up:
           smtp_tls_mandatory_protocols = >=0x0303

       With Postfix < 3.6 there is no support for a minimum or maximum  version,  and  the  protocol
       range   is   configured   via  protocol  exclusions.   To  require  at  least  TLS  1.0,  set
       "smtp_tls_mandatory_protocols = !SSLv2, !SSLv3". Listing the  protocols  to  include,  rather
       than  protocols to exclude, is supported, but not recommended.  The exclusion syntax more ac‐
       curately matches the underlying OpenSSL interface.

       When using the exclusion syntax, take care to ensure that the range of protocols supported by
       the  Postfix  SMTP  client  is contiguous.  When a protocol version is enabled, disabling any
       higher version implicitly disables all versions above that higher version.  Thus,  for  exam‐
       ple:

           smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1.1

       also disables any protocols version higher than TLSv1.1 leaving only "TLSv1" enabled.

       Support  for  "TLSv1.3"  was  introduced  in  OpenSSL  1.1.1.   Disabling  this  protocol via
       "!TLSv1.3" is supported since Postfix 3.4 (or patch releases >=  3.0.14,  3.1.10,  3.2.7  and
       3.3.2).

       While  the vast majority of SMTP servers with DANE TLSA records now support at least TLS 1.2,
       a few still only support TLS 1.0.  If you use "dane" or "dane-only" it is best to not disable
       TLSv1,  except  perhaps via the policy table for destinations which you are sure will support
       "TLSv1.2".

       See the documentation of the smtp_tls_policy_maps parameter and TLS_README for more  informa‐
       tion about security levels.

       Example:
       # Preferred syntax with Postfix >= 3.6:
       smtp_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
       # Legacy syntax:
       smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**note**___**starttls**___**offer** **(default:** **no)**
       Log  the  hostname  of a remote SMTP server that offers STARTTLS, when TLS is not already en‐
       abled for that server.

       The logfile record looks like:

       postfix/smtp[pid]:  Host offered STARTTLS: [name.of.host]

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**per**___**site** **(default:** **empty)**
       Optional lookup tables with the Postfix SMTP client TLS usage policy by next-hop  destination
       and  by  remote  SMTP server hostname.  When both lookups succeed, the more specific per-site
       policy (NONE, MUST, etc) overrides the less specific one (MAY), and the more secure  per-site
       policy  (MUST,  etc)  overrides  the  less  secure  one  (NONE).   With Postfix 2.3 and later
       smtp_tls_per_site is strongly discouraged: use smtp_tls_policy_maps instead.

       Use of the bare hostname as the per-site table lookup key is discouraged. Always use the full
       destination  nexthop  (enclosed  in [] with a possible ":port" suffix). A recipient domain or
       MX-enabled transport next-hop with no port suffix may look like a bare hostname, but is still
       a suitable _destination_.

       Specify a next-hop destination or server hostname on the left-hand side; no wildcards are al‐
       lowed. The next-hop destination is either the recipient domain, or the destination  specified
       with a [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table, the relayhost parameter, or the relay_transport parameter.  On the
       right hand side specify one of the following keywords:

       NONE   Don't use TLS at all. This overrides a less specific **MAY** lookup result from the alter‐
              nate  host  or  next-hop  lookup  key, and overrides the global smtp_use_tls, smtp_en‐
              force_tls, and smtp_tls_enforce_peername settings.

       MAY    Try to use TLS if the server announces support, otherwise use the unencrypted  connec‐
              tion.  This  has less precedence than a more specific result (including **NONE**) from the
              alternate host or next-hop lookup key, and has less precedence than the more  specific
              global "smtp_enforce_tls = yes" or "smtp_tls_enforce_peername = yes".

       MUST_NOPEERMATCH
              Require  TLS  encryption,  but  do  not  require  that the remote SMTP server hostname
              matches the information in the remote SMTP server certificate, or that the server cer‐
              tificate  was issued by a trusted CA. This overrides a less secure **NONE** or a less spe‐
              cific **MAY** lookup result from the alternate host or next-hop lookup key, and  overrides
              the global smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.

       MUST   Require  TLS  encryption, require that the remote SMTP server hostname matches the in‐
              formation in the remote SMTP server certificate, and  require  that  the  remote  SMTP
              server  certificate  was issued by a trusted CA. This overrides a less secure **NONE** and
              **MUST**___**NOPEERMATCH** or a less specific **MAY** lookup  result  from  the  alternate  host  or
              next-hop  lookup  key,  and  overrides  the  global smtp_use_tls, smtp_enforce_tls and
              smtp_tls_enforce_peername settings.

       The above keywords correspond to the "none", "may", "encrypt" and  "verify"  security  levels
       for  the new smtp_tls_security_level parameter introduced in Postfix 2.3. Starting with Post‐
       fix 2.3, and independently of how the policy is specified, the smtp_tls_mandatory_ciphers and
       smtp_tls_mandatory_protocols  parameters  apply when TLS encryption is mandatory. Connections
       for which encryption is optional typically enable all "export" grade and better ciphers  (see
       smtp_tls_ciphers and smtp_tls_protocols).

       As  long  as  no secure DNS lookup mechanism is available, false hostnames in MX or CNAME re‐
       sponses can change the server hostname that Postfix uses for TLS  policy  lookup  and  server
       certificate  verification.  Even  with  a  perfect  match between the server hostname and the
       server certificate, there is no guarantee that Postfix is connected to the right server.  See
       TLS_README  (Closing  a  DNS  loophole  with  obsolete  per-site TLS policies) for a possible
       work-around.

       This feature is available  in  Postfix  2.2  and  later.  With  Postfix  2.3  and  later  use
       smtp_tls_policy_maps instead.

**smtp**___**tls**___**policy**___**maps** **(default:** **empty)**
       Optional  lookup tables with the Postfix SMTP client TLS security policy by next-hop destina‐
       tion; when a non-empty value is specified, this overrides the obsolete smtp_tls_per_site  pa‐
       rameter.  See TLS_README for a more detailed discussion of TLS security levels.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       The TLS policy table is indexed by the full next-hop destination, which is either the recipi‐
       ent  domain,  or  the  verbatim  next-hop specified in the transport table, $local_transport,
       $virtual_transport, $relay_transport  or  $default_transport.  This  includes  any  enclosing
       square brackets and any non-default destination server port suffix. The LMTP socket type pre‐
       fix (inet: or unix:) is not included in the lookup key.

       Only the next-hop domain, or $myhostname with LMTP over UNIX-domain sockets, is used  as  the
       nexthop  name  for  certificate  verification. The port and any enclosing square brackets are
       used in the table lookup key, but are not used for server name verification.

       When the lookup key is a domain name without enclosing square brackets or  any  _:port_  suffix
       (typically the recipient domain), and the full domain is not found in the table, just as with
       the [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) table, the parent domain starting with a leading "." is matched recursively.
       This allows one to specify a security policy for a recipient domain and all its sub-domains.

       The  lookup  result  is  a  security level, followed by an optional list of whitespace and/or
       comma separated name=value attributes that override related main.cf settings. The  TLS  secu‐
       rity levels in order of increasing security are:

       **none**   No TLS. No additional attributes are supported at this level.

       **may**    Opportunistic  TLS.  Since sending in the clear is acceptable, demanding stronger than
              default TLS security merely reduces interoperability.  The  optional  "ciphers",  "ex‐
              clude",  and  "protocols"  attributes (available for opportunistic TLS with Postfix >=
              2.6) and "connection_reuse" attribute (Postfix  >=  3.4)  override  the  "smtp_tls_ci‐
              phers", "smtp_tls_exclude_ciphers", "smtp_tls_protocols", and "smtp_tls_connection_re‐
              use" configuration parameters. When opportunistic TLS handshakes fail, Postfix retries
              the  connection with TLS disabled.  This allows mail delivery to sites with non-inter‐
              operable TLS implementations.

### encrypt
              Mandatory TLS encryption. At this level and higher, the optional "protocols" attribute
              overrides  the  main.cf smtp_tls_mandatory_protocols parameter, the optional "ciphers"
              attribute overrides the main.cf  smtp_tls_mandatory_ciphers  parameter,  the  optional
              "exclude"  attribute  (Postfix  >=  2.6)  overrides the main.cf smtp_tls_mandatory_ex‐
              clude_ciphers parameter, and the optional  "connection_reuse"  attribute  (Postfix  >=
              3.4)  overrides  the main.cf smtp_tls_connection_reuse parameter. In the policy table,
              multiple protocols or excluded ciphers must be separated by colons, as attribute  val‐
              ues may not contain whitespace or commas.

       **dane**   Opportunistic  DANE  TLS.   The  TLS  policy  for the destination is obtained via TLSA
              records in DNSSEC.  If no TLSA records are found, the effective security level used is
              may.   If TLSA records are found, but none are usable, the effective security level is
              encrypt.  When usable TLSA records are obtained for the remote SMTP server, the server
              certificate  must  match  the  TLSA  records.   RFC 7672 (DANE) TLS authentication and
              DNSSEC support is available with Postfix 2.11 and later. The optional  "connection_re‐
              use" attribute (Postfix >= 3.4) overrides the main.cf smtp_tls_connection_reuse param‐
              eter.

### dane-only
              Mandatory DANE TLS.  The TLS policy for the destination is obtained via  TLSA  records
              in DNSSEC.  If no TLSA records are found, or none are usable, no connection is made to
              the server.  When usable TLSA records are obtained for the  remote  SMTP  server,  the
              server  certificate  must  match the TLSA records.  RFC 7672 (DANE) TLS authentication
              and DNSSEC support is available with Postfix 2.11 and  later.  The  optional  "connec‐
              tion_reuse" attribute (Postfix >= 3.4) overrides the main.cf smtp_tls_connection_reuse
              parameter.

### fingerprint
              Certificate fingerprint verification. Available with Postfix 2.5 and  later.  At  this
              security  level, there are no trusted Certification Authorities. The certificate trust
              chain, expiration date, ... are not checked. Instead, the optional **match** attribute, or
              else the main.cf **smtp**___**tls**___**fingerprint**___**cert**___**match** parameter, lists the certificate fin‐
              gerprints or the public key fingerprint (Postfix 2.9 and later) of  the  valid  server
              certificate. The digest algorithm used to calculate the fingerprint is selected by the
              **smtp**___**tls**___**fingerprint**___**digest** parameter. Multiple fingerprints can be  combined  with  a
              "|"  delimiter  in  a  single match attribute, or multiple match attributes can be em‐
              ployed. The ":" character is not used as a delimiter as it occurs between each pair of
              fingerprint  (hexadecimal)  digits. The optional "connection_reuse" attribute (Postfix
              >= 3.4) overrides the main.cf smtp_tls_connection_reuse parameter.

       **verify** Mandatory TLS verification.  At this security level, DNS MX lookups are trusted to  be
              secure enough, and the name verified in the server certificate is usually obtained in‐
              directly via unauthenticated DNS MX lookups.  The optional "match" attribute overrides
              the  main.cf smtp_tls_verify_cert_match parameter. In the policy table, multiple match
              patterns and strategies must be separated by colons.   In  practice  explicit  control
              over  matching  is more common with the "secure" policy, described below. The optional
              "connection_reuse" attribute (Postfix >= 3.4) overrides the  main.cf  smtp_tls_connec‐
              tion_reuse parameter.

       **secure** Secure-channel TLS. At this security level, DNS MX lookups, though potentially used to
              determine the candidate next-hop gateway IP addresses, are **not** trusted  to  be  secure
              enough for TLS peername verification. Instead, the default name verified in the server
              certificate is obtained directly from the next-hop, or is explicitly specified via the
              optional **match** attribute which overrides the main.cf smtp_tls_secure_cert_match param‐
              eter. In the policy table, multiple match patterns and strategies must be separated by
              colons.   The  match  attribute  is most useful when multiple domains are supported by
              common server, the policy entries for additional domains specify  matching  rules  for
              the  primary domain certificate. While transport table overrides routing the secondary
              domains to the primary nexthop also allow secure verification, they risk  delivery  to
              the  wrong  destination  when domains change hands or are re-assigned to new gateways.
              With the "match" attribute approach, routing is not perturbed, and mail is deferred if
              verification  of a new MX host fails. The optional "connection_reuse" attribute (Post‐
              fix >= 3.4) overrides the main.cf smtp_tls_connection_reuse parameter.

       Example:

       /etc/postfix/main.cf:
           smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
           # Postfix 2.5 and later.
           #
           # The default digest is sha256 with Postfix >= 3.6 and
           # compatibility level >= 3.
           #
           smtp_tls_fingerprint_digest = sha256

       /etc/postfix/tls_policy:
           example.edu                 none
           example.mil                 may
           example.gov                 encrypt protocols=TLSv1
           example.com                 verify ciphers=high
           example.net                 secure
           .example.net                secure match=.example.net:example.net
           [mail.example.org]:587      secure match=nexthop
           # Postfix 2.5 and later
           [thumb.example.org]          fingerprint
               match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f
               match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76

       **Note:** The **hostname** strategy if listed in a non-default setting of  smtp_tls_secure_cert_match
       or  in  the **match** attribute in the policy table can render the **secure** level vulnerable to DNS
       forgery. Do not use the **hostname** strategy for secure-channel configurations  in  environments
       where DNS security is not assured.

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**protocols** **(default:** **see** **postconf** **-d** **output)**
       TLS  protocols  that  the Postfix SMTP client will use with opportunistic TLS encryption.  In
       main.cf the values are separated by whitespace, commas or colons. In the policy table "proto‐
       cols" attribute (see smtp_tls_policy_maps) the only valid separator is colon.  An empty value
       means allow all protocols.

       The valid protocol names (see **SSL**___**get**___**[version**(3)](https://www.chedong.com/phpMan.php/man/version/3/markdown)) are "SSLv2", "SSLv3",  "TLSv1",  "TLSv1.1",
       "TLSv1.2"  and  "TLSv1.3".   Starting with Postfix 3.6, the default value is ">=TLSv1", which
       sets TLS 1.0 as the lowest supported TLS protocol version (see below).   Older  releases  use
       the "!" exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set the
       lowest acceptable TLS protocol version and/or the highest acceptable  TLS  protocol  version.
       To  set the lower bound include an element of the form: ">=_version_" where _version_ is a either
       one of the TLS protocol names listed above, or a hexadecimal number corresponding to the  de‐
       sired  TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.).  For the upper bound,
       use "<=_version_".  There must be no whitespace between the ">=" or "<=" symbols and the proto‐
       col name or number.

       Hexadecimal  protocol  numbers  make  it possible to specify protocol bounds for TLS versions
       that are known to OpenSSL, but might not be known to Postfix.  They cannot be used  with  the
       legacy  exclusion  syntax.   Leading  "0"  or  "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent  to  "TLSv1".   Hexadecimal
       versions  unknown to OpenSSL will fail to set the upper or lower bound, and a warning will be
       logged.  Hexadecimal versions should only be used when Postfix is  linked  with  some  future
       version  of  OpenSSL  that supports TLS 1.4 or later, but Postfix does not yet support a sym‐
       bolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtp_tls_protocols = >=TLSv1, <=0305
           # Allow only TLS 1.0 and up:
           smtp_tls_protocols = >=0x0301

       With Postfix < 3.6 there is no support for a minimum or maximum  version,  and  the  protocol
       range is configured via protocol exclusions.  To require at least TLS 1.0, set "smtp_tls_pro‐
       tocols = !SSLv2, !SSLv3".  Listing the protocols to include, rather  than  protocols  to  ex‐
       clude, is supported, but not recommended.  The exclusion form more accurately matches the un‐
       derlying OpenSSL interface.

       When using the exclusion syntax, take care to ensure that the range of  protocols  advertised
       by an SSL/TLS client is contiguous.  When a protocol version is enabled, disabling any higher
       version implicitly disables all versions above that higher version.  Thus, for example:

           smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1.1
       also disables any protocols version higher than TLSv1.1 leaving only "TLSv1" enabled.

       Support for  "TLSv1.3"  was  introduced  in  OpenSSL  1.1.1.   Disabling  this  protocol  via
       "!TLSv1.3"  is  supported  since  Postfix 3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and
       3.3.2).

       Example:
       # Preferred syntax with Postfix >= 3.6:
       smtp_tls_protocols = >=TLSv1, <=TLSv1.3
       # Legacy syntax:
       smtp_tls_protocols = !SSLv2, !SSLv3

       This feature is available in Postfix 2.6 and later.

**smtp**___**tls**___**scert**___**verifydepth** **(default:** **9)**
       The verification depth for remote SMTP server certificates. A depth of 1 is sufficient if the
       issuing CA is listed in a local CA file.

       The  default  verification  depth  is  9 (the OpenSSL default) for compatibility with earlier
       Postfix behavior. Prior to Postfix 2.5, the default value was 5, but the limit was not  actu‐
       ally  enforced.  If  you have set this to a lower non-default value, certificates with longer
       trust chains may now fail to verify. Certificate chains with 1 or 2 CAs  are  common,  deeper
       chains  are  more  rare  and  any  number between 5 and 9 should suffice in practice. You can
       choose a lower number if, for example, you trust certificates directly signed by  an  issuing
       CA but not any CAs it delegates to.

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**secure**___**cert**___**match** **(default:** **nexthop,** **dot-nexthop)**
       How the Postfix SMTP client verifies the server certificate peername for the "secure" TLS se‐
       curity level. In a "secure" TLS  policy  table  ($smtp_tls_policy_maps)  entry  the  optional
       "match" attribute overrides this main.cf setting.

       This  parameter  specifies one or more patterns or strategies separated by commas, whitespace
       or colons.  In the policy table the only valid separator is the colon character.

       For a description of the pattern and strategy syntax see the  smtp_tls_verify_cert_match  pa‐
       rameter.  The  "hostname"  strategy should be avoided in this context, as in the absence of a
       secure global DNS, using the results of MX lookups in certificate verification is not  immune
       to active (man-in-the-middle) attacks on DNS.

       Sample main.cf setting:

           smtp_tls_secure_cert_match = nexthop

       Sample policy table override:

           example.net     secure match=example.com:.example.com
           .example.net    secure match=example.com:.example.com

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**security**___**level** **(default:** **empty)**
       The  default  SMTP  TLS security level for the Postfix SMTP client; when a non-empty value is
       specified,  this  overrides  the  obsolete  parameters  smtp_use_tls,  smtp_enforce_tls,  and
       smtp_tls_enforce_peername.

       Specify one of the following security levels:

       **none**   No   TLS.  TLS  will  not  be  used  unless  enabled  for  specific  destinations  via
              smtp_tls_policy_maps.

       **may**    Opportunistic TLS. Use TLS if this is supported by the remote SMTP  server,  otherwise
              use  plaintext.  Since sending in the clear is acceptable, demanding stronger than de‐
              fault TLS  security  merely  reduces  interoperability.   The  "smtp_tls_ciphers"  and
              "smtp_tls_protocols"  (Postfix  >=  2.6) configuration parameters provide control over
              the protocols and cipher grade used with opportunistic TLS.  With earlier releases the
              opportunistic TLS cipher grade is always "export" and no protocols are disabled.  When
              TLS handshakes fail, the connection is retried with TLS disabled.   This  allows  mail
              delivery to sites with non-interoperable TLS implementations.

### encrypt
              Mandatory TLS encryption. Since a minimum level of security is intended, it is reason‐
              able to be specific about sufficiently secure protocol versions and ciphers.  At  this
              security  level  and  higher,  the main.cf parameters smtp_tls_mandatory_protocols and
              smtp_tls_mandatory_ciphers specify the TLS protocols and minimum  cipher  grade  which
              the administrator considers secure enough for mandatory encrypted sessions. This secu‐
              rity level is not an appropriate default for systems delivering mail to the Internet.

       **dane**   Opportunistic DANE TLS.  At this security level, the TLS policy for the destination is
              obtained  via  DNSSEC.  For TLSA policy to be in effect, the destination domain's con‐
              taining DNS zone must be signed and the Postfix SMTP client's operating system must be
              configured to send its DNS queries to a recursive DNS nameserver that is able to vali‐
              date the signed records.  Each MX host's DNS zone should also be  signed,  and  should
              publish  DANE  TLSA (RFC 7672) records that specify how that MX host's TLS certificate
              is to be verified.  TLSA records do not preempt the normal SMTP MX host selection  al‐
              gorithm,  if some MX hosts support TLSA and others do not, TLS security will vary from
              delivery to delivery.  It is up to the domain owner to configure their  MX  hosts  and
              their  DNS  sensibly.  To configure the Postfix SMTP client for DNSSEC lookups see the
              documentation for the smtp_dns_support_level main.cf parameter.  When DNSSEC-validated
              TLSA  records  are  not  found  the  effective tls security level is "may".  When TLSA
              records are found, but are all unusable the effective  security  level  is  "encrypt".
              For  purposes  of  protocol and cipher selection, the "dane" security level is treated
              like a "mandatory" TLS security level, and weak ciphers and  protocols  are  disabled.
              Since  DANE  authenticates server certificates the "aNULL" cipher-suites are transpar‐
              ently excluded at this level, no need to configure this manually.  RFC 7672 (DANE) TLS
              authentication is available with Postfix 2.11 and later.

### dane-only
              Mandatory  DANE  TLS.  This is just like "dane" above, but DANE TLSA authentication is
              required.  There is no fallback to "may" or "encrypt" when TLSA records are missing or
              unusable.   RFC  7672  (DANE)  TLS  authentication  is available with Postfix 2.11 and
              later.

### fingerprint
              Certificate fingerprint verification.  At this security level, there  are  no  trusted
              Certification  Authorities.   The  certificate trust chain, expiration date, etc., are
              not checked. Instead, the **smtp**___**tls**___**fingerprint**___**cert**___**match** parameter lists the certifi‐
              cate fingerprint or public key fingerprint (Postfix 2.9 and later) of the valid server
              certificate. The digest algorithm used to calculate the fingerprint is selected by the
              **smtp**___**tls**___**fingerprint**___**digest** parameter. Available with Postfix 2.5 and later.

       **verify** Mandatory  TLS  verification. At this security level, DNS MX lookups are trusted to be
              secure enough, and the name verified in the server certificate is usually obtained in‐
              directly  via unauthenticated DNS MX lookups. The smtp_tls_verify_cert_match parameter
              controls how the server name is verified. In practice explicit control  over  matching
              is  more  common at the "secure" level, described below. This security level is not an
              appropriate default for systems delivering mail to the Internet.

       **secure** Secure-channel TLS.  At this security level, DNS MX lookups, though  potentially  used
              to determine the candidate next-hop gateway IP addresses, are **not** trusted to be secure
              enough for TLS peername verification. Instead, the default name verified in the server
              certificate  is  obtained  from  the  next-hop domain as specified in the smtp_tls_se‐
              cure_cert_match configuration parameter. The default matching rule is  that  a  server
              certificate  matches  when  its name is equal to or is a sub-domain of the nexthop do‐
              main. This security level is not an appropriate default for systems delivering mail to
              the Internet.

       Examples:

       # No TLS. Formerly: smtp_use_tls=no and smtp_enforce_tls=no.
       smtp_tls_security_level = none

       # Opportunistic TLS.
       smtp_tls_security_level = may
       # Do not tweak opportunistic ciphers or protocol unless it is essential
       # to do so (if a security vulnerability is found in the SSL library that
       # can be mitigated by disabling a particular protocol or raising the
       # cipher grade).
       smtp_tls_ciphers = medium
       smtp_tls_protocols = >=TLSv1
       # Legacy (Postfix < 3.6) syntax:
       smtp_tls_protocols = !SSLv2, !SSLv3

       # Mandatory (high-grade) TLS encryption.
       smtp_tls_security_level = encrypt
       smtp_tls_mandatory_ciphers = high

       # Authenticated TLS 1.2 or better matching the nexthop domain or a
       # subdomain.
       smtp_tls_security_level = secure
       smtp_tls_mandatory_ciphers = high
       smtp_tls_mandatory_protocols = >=TLSv1.2
       smtp_tls_secure_cert_match = nexthop, dot-nexthop

       # Certificate fingerprint verification (Postfix >= 2.5).
       # The CA-less "fingerprint" security level only scales to a limited
       # number of destinations. As a global default rather than a per-site
       # setting, this is practical only when mail for all recipients is sent
       # to a central mail hub.
       relayhost = [mailhub.example.com]
       smtp_tls_security_level = fingerprint
       smtp_tls_mandatory_protocols = >=TLSv1.2
       smtp_tls_mandatory_ciphers = high
       smtp_tls_fingerprint_cert_match =
           3D:95:34:51:...:40:99:C0:C1
           EC:3B:2D:B0:...:A3:9D:72:F6

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**servername** **(default:** **empty)**
       Optional  name  to send to the remote SMTP server in the TLS Server Name Indication (SNI) ex‐
       tension.  The SNI extension is always on when DANE is used to authenticate the server, and in
       that case the SNI name sent is the one required by RFC7672 and this parameter is ignored.

       Some  SMTP  servers  use  the received SNI name to select an appropriate certificate chain to
       present to the client.  While this may improve interoperability with such servers, it may re‐
       duce  interoperability with other servers that choose to abort the connection when they don't
       have a certificate chain configured for the requested name.  Such servers should select a de‐
       fault  certificate  chain  and  continue  the handshake, but some may not.  Therefore, absent
       DANE, no SNI name is sent by default.

       The SNI name must be either a valid DNS hostname, or else one of the special values  **hostname**
       or  **nexthop**, which select either the remote hostname or the nexthop domain respectively.  DNS
       names for SNI must be in A-label (punycode) form.  Invalid DNS names log a configuration  er‐
       ror warning and mail delivery is deferred.

       Except  when using a relayhost to forward all email, the only sensible non-empty main.cf set‐
       ting for this parameter is **hostname**.  Other non-empty values are only practical on a per-des‐
       tination  basis via the **servername** attribute of the Postfix TLS policy table.  When in doubt,
       leave this parameter empty, and configure per-destination SNI as needed.

       This feature is available in Postfix 3.4 and later.

**smtp**___**tls**___**session**___**cache**___**database** **(default:** **empty)**
       Name of the file containing the optional Postfix SMTP client TLS  session  cache.  Specify  a
       database  type  that supports enumeration, such as **btree** or **sdbm**; there is no need to support
       concurrent access.  The file is created if it does not exist. The [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) daemon does not use
       this  parameter directly, rather the cache is implemented indirectly in the [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) daemon.
       This means that per-smtp-instance master.cf overrides of this parameter  are  not  effective.
       Note,  that  each  of  the  cache  databases  supported  by [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) daemon: $smtpd_tls_ses‐
       sion_cache_database,  $smtp_tls_session_cache_database  (and  with  Postfix  2.3  and   later
       $lmtp_tls_session_cache_database), needs to be stored separately. It is not at this time pos‐
       sible to store multiple caches in a single database.

       Note: **dbm** databases are not suitable. TLS session objects are too large.

       As of version 2.5, Postfix no longer uses root privileges when opening this  file.  The  file
       should  now  be stored under the Postfix-owned data_directory. As a migration aid, an attempt
       to open the file under a non-Postfix directory is redirected to the Postfix-owned data_direc‐
       tory, and a warning is logged.

       Example:

       smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_scache

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**session**___**cache**___**timeout** **(default:** **3600s)**
       The expiration time of Postfix SMTP client TLS session cache information.  A cache cleanup is
       performed periodically every $smtp_tls_session_cache_timeout seconds. As with  $smtp_tls_ses‐
       sion_cache_database,  this  parameter  is  implemented  in the [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) daemon and therefore
       per-smtp-instance master.cf overrides are not possible.

       As of Postfix 2.11 this setting cannot exceed 100 days.  If set <= 0, session caching is dis‐
       abled.   If  set  to  a positive value less than 2 minutes, the minimum value of 2 minutes is
       used instead.

       This feature is available in Postfix 2.2 and later.

**smtp**___**tls**___**trust**___**anchor**___**file** **(default:** **empty)**
       Zero or more PEM-format files with trust-anchor certificates and/or public keys.  If the  pa‐
       rameter  is  not  empty the root CAs in CAfile and CApath are no longer trusted.  Rather, the
       Postfix SMTP client will only trust certificate-chains signed by  one  of  the  trust-anchors
       contained  in  the chosen files.  The specified trust-anchor certificates and public keys are
       not subject to expiration, and need not be (self-signed) root CAs.  They may, if desired,  be
       intermediate certificates. Therefore, these certificates also may be found "in the middle" of
       the trust chain presented by the remote SMTP server, and any untrusted  issuing  parent  cer‐
       tificates will be ignored.  Specify a list of pathnames separated by comma or whitespace.

       Whether  specified  in main.cf, or on a per-destination basis, the trust-anchor PEM file must
       be accessible to the Postfix SMTP client in the chroot jail if applicable.  The  trust-anchor
       file  should  contain only certificates and public keys, no private key material, and must be
       readable by the non-privileged $mail_owner user.  This allows destinations to be bound  to  a
       set of specific CAs or public keys without trusting the same CAs for all destinations.

       The main.cf parameter supports single-purpose Postfix installations that send mail to a fixed
       set of SMTP peers.  At most sites, if trust-anchor files are used at all, they will be speci‐
       fied  on a per-destination basis via the "tafile" attribute of the "verify" and "secure" lev‐
       els in smtp_tls_policy_maps.

       The underlying mechanism is in support of RFC 7672 (DANE TLSA), which defines mechanisms  for
       an SMTP client MTA to securely determine server TLS certificates via DNS.

       If  you  want your trust anchors to be public keys, with OpenSSL you can extract a single PEM
       public key from a PEM X.509 file containing a single certificate, as follows:

           $ openssl x509 -in cert.pem -out ta-key.pem -noout -pubkey

       This feature is available in Postfix 2.11 and later.

**smtp**___**tls**___**verify**___**cert**___**match** **(default:** **hostname)**
       How the Postfix SMTP client verifies the server certificate peername for the "verify" TLS se‐
       curity  level.  In  a  "verify"  TLS  policy table ($smtp_tls_policy_maps) entry the optional
       "match" attribute overrides this main.cf setting.

       This parameter specifies one or more patterns or strategies separated by  commas,  whitespace
       or colons.  In the policy table the only valid separator is the colon character.

       Patterns specify domain names, or domain name suffixes:

       _example.com_
              Match  the _example.com_ domain, i.e. one of the names in the server certificate must be
              _example.com_.  Upper and lower case distinctions are ignored.

       _.example.com_
              Match subdomains of the _example.com_ domain, i.e. match a name in the  server  certifi‐
              cate  that  consists of a non-zero number of labels followed by a _.example.com_ suffix.
              Case distinctions are ignored.

       Strategies specify a transformation from the next-hop domain to  the  expected  name  in  the
       server certificate:

       nexthop
              Match against the next-hop domain, which is either the recipient domain, or the trans‐
              port next-hop configured for the domain stripped of any optional socket  type  prefix,
              enclosing  square brackets and trailing port. When MX lookups are not suppressed, this
              is the original nexthop domain prior to the MX  lookup,  not  the  result  of  the  MX
              lookup.  For LMTP delivery via UNIX-domain sockets, the verified next-hop name is $my‐
              hostname.  This strategy is suitable for use with the "secure"  policy.  Case  is  ig‐
              nored.

       dot-nexthop
              As  above,  but match server certificate names that are subdomains of the next-hop do‐
              main. Case is ignored.

       hostname
              Match against the hostname of the server, often obtained via an unauthenticated DNS MX
              lookup.  For  LMTP delivery via UNIX-domain sockets, the verified name is $myhostname.
              This matches  the  verification  strategy  of  the  "MUST"  keyword  in  the  obsolete
              smtp_tls_per_site  table,  and  is  suitable for use with the "verify" security level.
              When the next-hop name is enclosed in square brackets  to  suppress  MX  lookups,  the
              "hostname" strategy is the same as the "nexthop" strategy. Case is ignored.

       Sample main.cf setting:

       smtp_tls_verify_cert_match = hostname, nexthop, dot-nexthop

       Sample policy table override:

       example.com     verify  match=hostname:nexthop
       .example.com    verify  match=example.com:.example.com:hostname

       This feature is available in Postfix 2.3 and later.

**smtp**___**tls**___**wrappermode** **(default:** **no)**
       Request  that the Postfix SMTP client connects using the legacy SMTPS protocol instead of us‐
       ing the STARTTLS command.

       This mode requires "smtp_tls_security_level = encrypt" or stronger.

       Example: deliver all remote mail via a provider's server "mail.example.com".

       /etc/postfix/main.cf:
           # Client-side SMTPS requires "encrypt" or stronger.
           smtp_tls_security_level = encrypt
           smtp_tls_wrappermode = yes
           # The [] suppress MX lookups.
           relayhost = [mail.example.com]:465

       More examples are in TLS_README, including examples for older Postfix versions.

       This feature is available in Postfix 3.0 and later.

**smtp**___**use**___**tls** **(default:** **no)**
       Opportunistic mode: use TLS when a remote SMTP server announces STARTTLS  support,  otherwise
       send  the  mail in the clear. Beware: some SMTP servers offer STARTTLS even if it is not con‐
       figured.  With Postfix < 2.3, if the TLS handshake fails, and no other server  is  available,
       delivery  is  deferred  and  mail  stays  in the queue. If this is a concern for you, use the
       smtp_tls_per_site feature instead.

       This feature is available  in  Postfix  2.2  and  later.  With  Postfix  2.3  and  later  use
       smtp_tls_security_level instead.

**smtp**___**xforward**___**timeout** **(default:** **300s)**
       The  Postfix  SMTP  client time limit for sending the XFORWARD command, and for receiving the
       remote SMTP server response.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

       This feature is available in Postfix 2.1 and later.

**smtpd**___**authorized**___**verp**___**clients** **(default:** **$authorized**___**verp**___**clients)**
       What  remote  SMTP  clients  are allowed to specify the XVERP command.  This command requests
       that mail be delivered one recipient at a time with a per recipient return address.

       By default, no clients are allowed to specify XVERP.

       This parameter was renamed with Postfix version 2.1. The default value is backwards  compati‐
       ble with Postfix version 2.0.

       Specify  a  list of network/netmask patterns, separated by commas and/or whitespace. The mask
       specifies the number of bits in the network part of a host  address.  You  can  also  specify
       hostnames  or  .domain  names (the initial dot causes the domain to match any name below it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is replaced by its contents; a
       "type:table"  lookup  table is matched when a table entry matches a lookup string (the lookup
       result is ignored).  Continue long lines by starting the next line with  whitespace.  Specify
       "!pattern"  to  exclude  an address or network block from the list. The form "!/file/name" is
       supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must  be  specified  inside  []  in  the  smtpd_autho‐
       rized_verp_clients  value,  and in files specified with "/file/name".  IP version 6 addresses
       contain the ":" character, and would otherwise be confused with a "type:table" pattern.

**smtpd**___**authorized**___**xclient**___**hosts** **(default:** **empty)**
       What remote SMTP clients are allowed to use the XCLIENT feature.  This command overrides  re‐
       mote  SMTP  client information that is used for access control. Typical use is for SMTP-based
       content filters, fetchmail-like programs,  or  SMTP  server  access  rule  testing.  See  the
       XCLIENT_README document for details.

       This feature is available in Postfix 2.1 and later.

       By default, no clients are allowed to specify XCLIENT.

       Specify  a  list of network/netmask patterns, separated by commas and/or whitespace. The mask
       specifies the number of bits in the network part of a host  address.  You  can  also  specify
       hostnames  or  .domain  names (the initial dot causes the domain to match any name below it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is replaced by its contents; a
       "type:table"  lookup  table is matched when a table entry matches a lookup string (the lookup
       result is ignored).  Continue long lines by starting the next line with  whitespace.  Specify
       "!pattern"  to  exclude  an address or network block from the list. The form "!/file/name" is
       supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must  be  specified  inside  []  in  the  smtpd_autho‐
       rized_xclient_hosts  value, and in files specified with "/file/name".  IP version 6 addresses
       contain the ":" character, and would otherwise be confused with a "type:table" pattern.

**smtpd**___**authorized**___**xforward**___**hosts** **(default:** **empty)**
       What remote SMTP clients are allowed to use the XFORWARD feature.  This command forwards  in‐
       formation  that  is  used  to improve logging after SMTP-based content filters. See the XFOR‐
       WARD_README document for details.

       This feature is available in Postfix 2.1 and later.

       By default, no clients are allowed to specify XFORWARD.

       Specify a list of network/netmask patterns, separated by commas and/or whitespace.  The  mask
       specifies  the  number  of  bits  in the network part of a host address. You can also specify
       hostnames or .domain names (the initial dot causes the domain to match any  name  below  it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is replaced by its contents; a
       "type:table" lookup table is matched when a table entry matches a lookup string  (the  lookup
       result  is  ignored).  Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude an address or network block from the list. The  form  "!/file/name"  is
       supported only in Postfix version 2.4 and later.

       Note:  IP  version  6  address  information  must  be specified inside [] in the smtpd_autho‐
       rized_xforward_hosts value, and in files specified with "/file/name".  IP version 6 addresses
       contain the ":" character, and would otherwise be confused with a "type:table" pattern.

**smtpd**___**banner** **(default:** **$myhostname** **ESMTP** **$mail**___**name)**
       The  text  that  follows the 220 status code in the SMTP greeting banner. Some people like to
       see the mail version advertised. By default, Postfix shows no version.

       You MUST specify $myhostname at the start of the text. This is required by the SMTP protocol.

       Example:

       smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)

**smtpd**___**client**___**auth**___**rate**___**limit** **(default:** **0)**
       The maximal number of AUTH commands that any client is allowed to send to  this  service  per
       time  unit,  regardless  of whether or not Postfix actually accepts those commands.  The time
       unit is specified with the anvil_rate_time_unit configuration parameter.

       By default, there is no limit on the number AUTH commands that a client may send.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be used to  regulate  le‐
       gitimate mail traffic.

       This feature is available in Postfix 3.1 and later.

**smtpd**___**client**___**connection**___**count**___**limit** **(default:** **50)**
       How many simultaneous connections any client is allowed to make to this service.  By default,
       the limit is set to half the default process limit value.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be used to  regulate  le‐
       gitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**client**___**connection**___**rate**___**limit** **(default:** **0)**
       The  maximal  number of connection attempts any client is allowed to make to this service per
       time unit.  The time unit is specified with the anvil_rate_time_unit configuration parameter.

       By default, a client can make as many connections per time unit as Postfix can accept.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be used to  regulate  le‐
       gitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

       Example:

       smtpd_client_connection_rate_limit = 1000

**smtpd**___**client**___**event**___**limit**___**exceptions** **(default:** **$mynetworks)**
       Clients  that  are excluded from smtpd_client_*_count/rate_limit restrictions. See the mynet‐
       works parameter description for the parameter value syntax.

       By default, clients in trusted networks are excluded. Specify a list of network blocks, host‐
       names or .domain names (the initial dot causes the domain to match any name below it).

       Note:   IP   version   6   address   information   must   be   specified  inside  []  in  the
       smtpd_client_event_limit_exceptions value, and in files specified with "/file/name".  IP ver‐
       sion  6 addresses contain the ":" character, and would otherwise be confused with a "type:ta‐
       ble" pattern.

       Pattern  matching  of  domain  names  is  controlled  by   the   presence   or   absence   of
       "smtpd_client_event_limit_exceptions" in the parent_domain_matches_subdomains parameter value
       (postfix 3.0 and later).

       This feature is available in Postfix 2.2 and later.

**smtpd**___**client**___**message**___**rate**___**limit** **(default:** **0)**
       The maximal number of message delivery requests that any client is allowed to  make  to  this
       service  per time unit, regardless of whether or not Postfix actually accepts those messages.
       The time unit is specified with the anvil_rate_time_unit configuration parameter.

       By default, a client can send as many message delivery requests per time unit as Postfix  can
       accept.

       To disable this feature, specify a limit of 0.

       WARNING:  The  purpose of this feature is to limit abuse. It must not be used to regulate le‐
       gitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

       Example:

       smtpd_client_message_rate_limit = 1000

**smtpd**___**client**___**new**___**tls**___**session**___**rate**___**limit** **(default:** **0)**
       The maximal number of new (i.e., uncached) TLS sessions that a remote SMTP client is  allowed
       to  negotiate  with  this  service  per  time  unit.   The  time  unit  is specified with the
       anvil_rate_time_unit configuration parameter.

       By default, a remote SMTP client can negotiate as many new TLS  sessions  per  time  unit  as
       Postfix can accept.

       To  disable  this  feature, specify a limit of 0. Otherwise, specify a limit that is at least
       the per-client concurrent session limit, or else legitimate client sessions may be rejected.

       WARNING: The purpose of this feature is to limit abuse. It must not be used to  regulate  le‐
       gitimate mail traffic.

       This feature is available in Postfix 2.3 and later.

       Example:

       smtpd_client_new_tls_session_rate_limit = 100

**smtpd**___**client**___**port**___**logging** **(default:** **no)**
       Enable logging of the remote SMTP client port in addition to the hostname and IP address. The
       logging format is "host[address]:port".

       This feature is available in Postfix 2.5 and later.

**smtpd**___**client**___**recipient**___**rate**___**limit** **(default:** **0)**
       The maximal number of recipient addresses that any client is allowed to send to this  service
       per  time  unit, regardless of whether or not Postfix actually accepts those recipients.  The
       time unit is specified with the anvil_rate_time_unit configuration parameter.

       By default, a client can send as many recipient addresses per time unit as  Postfix  can  ac‐
       cept.

       To disable this feature, specify a limit of 0.

       WARNING:  The  purpose of this feature is to limit abuse. It must not be used to regulate le‐
       gitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

       Example:

       smtpd_client_recipient_rate_limit = 1000

**smtpd**___**client**___**restrictions** **(default:** **empty)**
       Optional restrictions that the Postfix SMTP server applies in the context of a client connec‐
       tion  request.   See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restric‐
       tion lists" for a discussion of evaluation context and time.

       The default is to allow all connection requests.

       Specify a list of restrictions, separated by commas and/or whitespace.  Continue  long  lines
       by  starting  the next line with whitespace.  Restrictions are applied in the order as speci‐
       fied; the first restriction that matches wins.

       The following restrictions are specific to client hostname or client network address informa‐
       tion.

       **check**___**ccert**___**access** _type:table_
              By  default  use the remote SMTP client certificate fingerprint or the public key fin‐
              gerprint (Postfix 2.9 and later) as lookup key for the specified  [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown)  database;
              with Postfix version 2.2, also require that the remote SMTP client certificate is ver‐
              ified  successfully.   The  fingerprint  digest  algorithm  is  configurable  via  the
              smtpd_tls_fingerprint_digest  parameter  (hard-coded  as  md5 prior to Postfix version
              2.5).  This feature requires "smtpd_tls_ask_ccert = yes" and is available with Postfix
              version 2.2 and later.
              The default algorithm is **sha256** with Postfix >= 3.6 and the **compatibility**___**level** set to
              3.6 or higher. With Postfix <= 3.5, the default algorithm is **md5**.   The  best-practice
              algorithm  is  now  **sha256**. Recent advances in hash function cryptanalysis have led to
              md5 and sha1 being deprecated in favor of sha256.  However, as long as  there  are  no
              known  "second pre-image" attacks against the older algorithms, their use in this con‐
              text, though not recommended, is still likely safe.
              Alternatively, check_ccert_access accepts an explicit search order  (Postfix  3.5  and
              later). The default search order as described above corresponds with:
              check_ccert_access { type:table, { search_order = cert_fingerprint, pubkey_fingerprint
              } }
              The commas are optional.

       **check**___**client**___**access** _type:table_
              Search the specified access database for the client hostname, parent  domains,  client
              IP  address,  or  networks obtained by stripping least significant octets. See the **ac**‐‐
              [**cess**(5)](https://www.chedong.com/phpMan.php/man/cess/5/markdown) manual page for details.

       **check**___**client**___**a**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the IP addresses for the client  hostname,
              and  execute  the  corresponding  action.   Note:  a result of "OK" is not allowed for
              safety reasons. Instead, use DUNNO in order to exclude specific hosts from  denylists.
              This feature is available in Postfix 3.0 and later.

       **check**___**client**___**mx**___**access** _type:table_
              Search  the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the MX hosts for the client hostname, and
              execute the corresponding action.  If no MX  record  is  found,  look  up  A  or  AAAA
              records,  just  like  the Postfix SMTP client would. Note: a result of "OK" is not al‐
              lowed for safety reasons. Instead, use DUNNO in order to exclude specific  hosts  from
              denylists.  This feature is available in Postfix 2.7 and later.

       **check**___**client**___**ns**___**access** _type:table_
              Search  the  specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the DNS servers for the client hostname,
              and execute the corresponding action.  Note: a result  of  "OK"  is  not  allowed  for
              safety  reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
              This feature is available in Postfix 2.7 and later.

       **check**___**reverse**___**client**___**hostname**___**access** _type:table_
              Search the specified access database for the unverified reverse client hostname,  par‐
              ent  domains,  client  IP address, or networks obtained by stripping least significant
              octets. See the [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) manual page for details.  Note: a result of "OK" is not  al‐
              lowed  for safety reasons.  Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 2.6 and later.

       **check**___**reverse**___**client**___**hostname**___**a**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the IP addresses for  the  unverified  re‐
              verse  client  hostname, and execute the corresponding action.  Note: a result of "OK"
              is not allowed for safety reasons.  Instead, use DUNNO in order  to  exclude  specific
              hosts from denylists.  This feature is available in Postfix 3.0 and later.

       **check**___**reverse**___**client**___**hostname**___**mx**___**access** _type:table_
              Search  the  specified  [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the MX hosts for the unverified reverse
              client hostname, and execute the corresponding action.  If no MX record is found, look
              up A or AAAA records, just like the Postfix SMTP client would.  Note: a result of "OK"
              is not allowed for safety reasons.  Instead, use DUNNO in order  to  exclude  specific
              hosts from denylists.  This feature is available in Postfix 2.7 and later.

       **check**___**reverse**___**client**___**hostname**___**ns**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the DNS servers for the unverified reverse
              client hostname, and execute the corresponding action.  Note: a result of "OK" is  not
              allowed  for  safety  reasons.   Instead, use DUNNO in order to exclude specific hosts
              from denylists.  This feature is available in Postfix 2.7 and later.

       **check**___**sasl**___**access** _type:table_
              Use the remote SMTP client SASL user name as lookup key for  the  specified  [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown)
              database.  The  lookup  key has the form "username@domainname" when the smtpd_sasl_lo‐
              cal_domain parameter value is  non-empty.   Unlike  the  check_client_access  feature,
              check_sasl_access  does  not  perform  matches  of parent domains or IP subnet ranges.
              This feature is available with Postfix version 2.11 and later.

       **permit**___**inet**___**interfaces**
              Permit the request when the client IP address matches $inet_interfaces.

       **permit**___**mynetworks**
              Permit the request when the client IP address matches any network or  network  address
              listed in  $mynetworks.

       **permit**___**sasl**___**authenticated**
              Permit  the  request  when  the  client is successfully authenticated via the RFC 4954
              (AUTH) protocol.

       **permit**___**tls**___**all**___**clientcerts**
              Permit the request when the remote SMTP client certificate is  verified  successfully.
              This option must be used only if a special CA issues the certificates and only this CA
              is listed as trusted CA. Otherwise, clients with a third-party certificate would  also
              be  allowed  to  relay.   Specify  "tls_append_default_CA = no" when the trusted CA is
              specified with smtpd_tls_CAfile or smtpd_tls_CApath, to prevent Postfix from appending
              the  system-supplied  default  CAs.  This feature requires "smtpd_tls_ask_ccert = yes"
              and is available with Postfix version 2.2 and later.

       **permit**___**tls**___**clientcerts**
              Permit the request when the remote SMTP client certificate fingerprint or  public  key
              fingerprint  (Postfix 2.9 and later) is listed in $relay_clientcerts.  The fingerprint
              digest  algorithm  is  configurable  via  the  smtpd_tls_fingerprint_digest  parameter
              (hard-coded   as   md5   prior   to  Postfix  version  2.5).   This  feature  requires
              "smtpd_tls_ask_ccert = yes" and is available with Postfix version 2.2 and later.
              The default algorithm is **sha256** with Postfix >= 3.6 and the **compatibility**___**level** set to
              3.6  or  higher. With Postfix <= 3.5, the default algorithm is **md5**.  The best-practice
              algorithm is now **sha256**. Recent advances in hash function cryptanalysis  have  led  to
              md5  and  sha1  being deprecated in favor of sha256.  However, as long as there are no
              known "second pre-image" attacks against the older algorithms, their use in this  con‐
              text, though not recommended, is still likely safe.

       **reject**___**rbl**___**client** _rbl_domain=d.d.d.d_
              Reject  the  request  when  the  reversed  client network address is listed with the A
              record "_d.d.d.d_" under _rbl_domain_ (Postfix version 2.1 and later only).  Each "_d_" is a
              number,  or  a  pattern inside "[]" that contains one or more ";"-separated numbers or
              number..number ranges (Postfix version 2.8 and later).  If no "_=d.d.d.d_" is specified,
              reject  the  request  when  the  reversed  client network address is listed with any A
              record under _rbl_domain_.
              The maps_rbl_reject_code parameter specifies the response code for  rejected  requests
              (default:   554), the default_rbl_reply  parameter specifies the default server reply,
              and the rbl_reply_maps  parameter specifies tables  with  server  replies  indexed  by
              _rbl_domain_.  This feature is available in Postfix 2.0 and later.

       **permit**___**dnswl**___**client** _dnswl_domain=d.d.d.d_
              Accept  the  request  when  the  reversed  client network address is listed with the A
              record "_d.d.d.d_" under _dnswl_domain_.  Each "_d_" is a number, or a pattern  inside  "[]"
              that  contains  one  or  more  ";"-separated  numbers or number..number ranges.  If no
              "_=d.d.d.d_" is specified, accept the request when the reversed client  network  address
              is listed with any A record under _dnswl_domain_.
              For  safety,  permit_dnswl_client  is  silently  ignored  when  it  would override re‐
              ject_unauth_destination.  The result is DEFER_IF_REJECT when allowlist  lookup  fails.
              This feature is available in Postfix 2.8 and later.

       **reject**___**rhsbl**___**client** _rbl_domain=d.d.d.d_
              Reject  the request when the client hostname is listed with the A record "_d.d.d.d_" un‐
              der _rbl_domain_ (Postfix version 2.1 and later only).  Each "_d_" is a number, or a  pat‐
              tern  inside  "[]"  that  contains one or more ";"-separated numbers or number..number
              ranges (Postfix version 2.8 and later).  If no "_=d.d.d.d_" is specified, reject the re‐
              quest  when  the client hostname is listed with any A record under _rbl_domain_. See the
              reject_rbl_client description above for additional RBL related  configuration  parame‐
              ters.   This  feature  is available in Postfix 2.0 and later; with Postfix version 2.8
              and later, reject_rhsbl_reverse_client will usually produce better results.

       **permit**___**rhswl**___**client** _rhswl_domain=d.d.d.d_
              Accept the request when the client hostname is listed with the A record "_d.d.d.d_"  un‐
              der _rhswl_domain_.  Each "_d_" is a number, or a pattern inside "[]" that contains one or
              more ";"-separated numbers or number..number ranges. If no  "_=d.d.d.d_"  is  specified,
              accept  the  request  when  the  client  hostname  is  listed  with any A record under
              _rhswl_domain_.
              Caution: client name allowlisting is fragile, since the client name  lookup  can  fail
              due  to  temporary  outages.   Client  name allowlisting should be used only to reduce
              false positives in e.g.  DNS-based blocklists, and not for making access  rule  excep‐
              tions.
              For  safety,  permit_rhswl_client  is  silently  ignored  when  it  would override re‐
              ject_unauth_destination.  The result is DEFER_IF_REJECT when allowlist  lookup  fails.
              This feature is available in Postfix 2.8 and later.

       **reject**___**rhsbl**___**reverse**___**client** _rbl_domain=d.d.d.d_
              Reject  the  request  when the unverified reverse client hostname is listed with the A
              record "_d.d.d.d_" under _rbl_domain_.  Each "_d_" is a number, or  a  pattern  inside  "[]"
              that  contains  one  or  more  ";"-separated  numbers or number..number ranges.  If no
              "_=d.d.d.d_" is specified, reject the request when the unverified reverse  client  host‐
              name  is listed with any A record under _rbl_domain_. See the reject_rbl_client descrip‐
              tion above for additional RBL  related  configuration  parameters.   This  feature  is
              available in Postfix 2.8 and later.

       **reject**___**unknown**___**client**___**hostname** (with Postfix < 2.3: reject_unknown_client)
              Reject  the  request  when  1)  the  client  IP address->name mapping fails, or 2) the
              name->address mapping fails, or 3) the name->address mapping does not match the client
              IP address.
              This  is  a  stronger restriction than the reject_unknown_reverse_client_hostname fea‐
              ture, which triggers only under condition 1) above.
              The unknown_client_reject_code parameter specifies the response code for rejected  re‐
              quests  (default: 450). The reply is always 450 in case the address->name or name->ad‐
              dress lookup failed due to a temporary problem.

       **reject**___**unknown**___**reverse**___**client**___**hostname**
              Reject the request when the client IP address has no address->name mapping.
              This is a weaker restriction than the  reject_unknown_client_hostname  feature,  which
              requires  not  only  that the address->name and name->address mappings exist, but also
              that the two mappings reproduce the client IP address.
              The unknown_client_reject_code parameter specifies the response code for rejected  re‐
              quests  (default:  450).   The  reply  is  always 450 in case the address->name lookup
              failed due to a temporary problem.
              This feature is available in Postfix 2.3 and later.

       In addition, you can use any of the following generic restrictions.  These  restrictions  are
       applicable in any SMTP command context.

       **check**___**policy**___**service** _servername_
              Query  the  specified policy server. See the SMTPD_POLICY_README document for details.
              This feature is available in Postfix 2.1 and later.

       **defer**  Defer the request. The client is told to try again later. This restriction  is  useful
              at the end of a restriction list, to make the default policy explicit.
              The defer_code parameter specifies the SMTP server reply code (default: 450).

       **defer**___**if**___**permit**
              Defer  the  request  if some later restriction would result in an explicit or implicit
              PERMIT action.  This is useful when a denylisting feature fails  due  to  a  temporary
              problem.  This feature is available in Postfix version 2.1 and later.

       **defer**___**if**___**reject**
              Defer  the request if some later restriction would result in a REJECT action.  This is
              useful when an allowlisting feature fails due to a temporary problem.  This feature is
              available in Postfix version 2.1 and later.

       **permit** Permit  the  request.  This restriction is useful at the end of a restriction list, to
              make the default policy explicit.

       **reject**___**multi**___**recipient**___**bounce**
              Reject the request when the envelope sender is the null address, and the  message  has
              multiple  envelope  recipients. This usage has rare but legitimate applications: under
              certain conditions, multi-recipient mail that was  posted  with  the  DSN  option  NO‐
              TIFY=NEVER may be forwarded with the null sender address.
              Note:  this restriction can only work reliably when used in smtpd_data_restrictions or
              smtpd_end_of_data_restrictions, because the total number of recipients is not known at
              an earlier stage of the SMTP conversation.  Use at the RCPT stage will only reject the
              second etc.  recipient.
              The multi_recipient_bounce_reject_code parameter specifies the response code  for  re‐
              jected requests (default:  550).  This feature is available in Postfix 2.1 and later.

       **reject**___**plaintext**___**session**
              Reject  the  request when the connection is not encrypted. This restriction should not
              be used before the client has had a chance to negotiate encryption with  the  AUTH  or
              STARTTLS commands.
              The  plaintext_reject_code parameter specifies the response code for rejected requests
              (default:  450).  This feature is available in Postfix 2.3 and later.

       **reject**___**unauth**___**pipelining**
              Reject the request when the client sends SMTP commands ahead of time where it  is  not
              allowed,  or  when  the  client sends SMTP commands ahead of time without knowing that
              Postfix actually supports ESMTP command pipelining. This stops  mail  from  bulk  mail
              software  that  improperly uses ESMTP command pipelining in order to speed up deliver‐
              ies.
              With Postfix 2.6 and later, the SMTP server sets a per-session flag  whenever  it  de‐
              tects  illegal  pipelining,  including  pipelined  HELO  or  EHLO  commands.  The  re‐
              ject_unauth_pipelining feature simply tests whether the flag was set at any  point  in
              time during the session.
              With older Postfix versions, reject_unauth_pipelining checks the current status of the
              input  read  queue,  and  its  usage  is  not  recommended  in  contexts  other   than
              smtpd_data_restrictions.

       **reject** Reject  the  request.  This restriction is useful at the end of a restriction list, to
              make the default policy explicit.  The reject_code configuration  parameter  specifies
              the response code for rejected requests (default: 554).

       **sleep** _seconds_
              Pause for the specified number of seconds and proceed with the next restriction in the
              list, if any. This may stop zombie mail when used as:
              /etc/postfix/main.cf:
                  smtpd_client_restrictions =
                      sleep 1, reject_unauth_pipelining
                  smtpd_delay_reject = no
              This feature is available in Postfix 2.3.

       **warn**___**if**___**reject**
              A safety net for testing. When "warn_if_reject" is placed  before  a  reject-type  re‐
              striction,  access  table  query,  or  check_policy_service  query,  this  logs a "re‐
              ject_warning" message instead of rejecting a request (when a  reject-type  restriction
              fails  due to a temporary error, this logs a "reject_warning" message for any implicit
              "defer_if_permit" actions that would normally prevent mail from being accepted by some
              later access restriction). This feature has no effect on defer_if_reject restrictions.

       Other restrictions that are valid in this context:

       •      SMTP  command  specific  restrictions that are described under the smtpd_helo_restric‐
              tions,  smtpd_sender_restrictions  or  smtpd_recipient_restrictions  parameters.  When
              helo,  sender  or  recipient  restrictions are listed under smtpd_client_restrictions,
              they have effect only with "smtpd_delay_reject = yes", so that  $smtpd_client_restric‐
              tions is evaluated at the time of the RCPT TO command.

       Example:

       smtpd_client_restrictions = permit_mynetworks, reject_unknown_client_hostname

**smtpd**___**command**___**filter** **(default:** **empty)**
       A  mechanism  to  transform commands from remote SMTP clients.  This is a last-resort tool to
       work around client commands that break interoperability with the Postfix SMTP server.   Other
       uses involve fault injection to test Postfix's handling of invalid commands.

       Specify the name of a "type:table" lookup table. The search string is the SMTP command as re‐
       ceived from the remote SMTP client, except that initial whitespace and the trailing  <CR><LF>
       are removed.  The result value is executed by the Postfix SMTP server.

       There is no need to use smtpd_command_filter for the following cases:

       •      Use "resolve_numeric_domain = yes" to accept "_user@ipaddress_".

       •      Postfix already accepts the correct form "_user@[ipaddress]_". Use virtual_alias_maps or
              canonical_maps to translate these into domain names if necessary.

       •      Use "strict_rfc821_envelopes = no" to accept "RCPT TO:<_User_ _Name_ _<<user@example.com>>>_".
              Postfix  will  ignore  the  "_User_ _Name_" part and deliver to the _<<user@example.com>>_ ad‐
              dress.

       Examples of problems that can be solved with the smtpd_command_filter feature:

       /etc/postfix/main.cf:
           smtpd_command_filter = pcre:/etc/postfix/command_filter

       /etc/postfix/command_filter:
           # Work around clients that send malformed HELO commands.
           /^HELO\s*$/ HELO domain.invalid

           # Work around clients that send empty lines.
           /^\s*$/     NOOP

           # Work around clients that send RCPT TO:<'user@domain'>.
           # WARNING: do not lose the parameters that follow the address.
           /^(RCPT\s+TO:\s*<)'([^[:space:]]+)'(>.*)/     $1$2$3

           # Append XVERP to MAIL FROM commands to request VERP-style delivery.
           # See VERP_README for more information on how to use Postfix VERP.
           /^(MAIL FROM:\s*<listname@example\.com>.*)/   $1 XVERP

           # Bounce-never mail sink. Use notify_classes=bounce,resource,software
           # to send bounced mail to the postmaster (with message body removed).
           /^(RCPT\s+TO:\s*<.*>.*)\s+NOTIFY=\S+(.*)/     $1 NOTIFY=NEVER$2
           /^(RCPT\s+TO:.*)/                             $1 NOTIFY=NEVER

       This feature is available in Postfix 2.7.

**smtpd**___**data**___**restrictions** **(default:** **empty)**
       Optional access restrictions that the Postfix SMTP server applies in the context of the  SMTP
       DATA  command.   See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restric‐
       tion lists" for a discussion of evaluation context and time.

       This feature is available in Postfix 2.0 and later.

       Specify a list of restrictions, separated by commas and/or whitespace.  Continue  long  lines
       by  starting  the next line with whitespace.  Restrictions are applied in the order as speci‐
       fied; the first restriction that matches wins.

       The following restrictions are valid in this context:

       •      Generic restrictions that can be used in any SMTP  command  context,  described  under
              smtpd_client_restrictions.

       •      SMTP   command   specific   restrictions  described  under  smtpd_client_restrictions,
              smtpd_helo_restrictions, smtpd_sender_restrictions or smtpd_recipient_restrictions.

       •      However, no recipient information is available in the case  of  multi-recipient  mail.
              Acting on only one recipient would be misleading, because any decision will affect all
              recipients equally. Acting on all recipients  would  require  a  possibly  very  large
              amount of memory, and would also be misleading for the reasons mentioned before.

       Examples:

       smtpd_data_restrictions = reject_unauth_pipelining
       smtpd_data_restrictions = reject_multi_recipient_bounce

**smtpd**___**delay**___**open**___**until**___**valid**___**rcpt** **(default:** **yes)**
       Postpone  the  start  of  an SMTP mail transaction until a valid RCPT TO command is received.
       Specify "no" to create a mail transaction as soon as the Postfix SMTP server receives a valid
       MAIL FROM command.

       With  sites  that  reject  lots of mail, the default setting reduces the use of disk, CPU and
       memory resources. The downside is that rejected recipients are logged with NOQUEUE instead of
       a mail transaction ID. This complicates the logfile analysis of multi-recipient mail.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**delay**___**reject** **(default:** **yes)**
       Wait  until the RCPT TO command before evaluating $smtpd_client_restrictions, $smtpd_helo_re‐
       strictions and $smtpd_sender_restrictions, or wait until the ETRN command  before  evaluating
       $smtpd_client_restrictions and $smtpd_helo_restrictions.

       This  feature  is  turned  on  by default because some clients apparently mis-behave when the
       Postfix SMTP server rejects commands before RCPT TO.

       The default setting has one major benefit: it allows Postfix to log recipient address  infor‐
       mation when rejecting a client name/address or sender address, so that it is possible to find
       out whose mail is being rejected.

**smtpd**___**discard**___**ehlo**___**keyword**___**address**___**maps** **(default:** **empty)**
       Lookup tables, indexed by the remote SMTP client address, with case insensitive lists of EHLO
       keywords (pipelining, starttls, auth, etc.) that the Postfix SMTP server will not send in the
       EHLO response to a remote SMTP client. See smtpd_discard_ehlo_keywords for details.  The  ta‐
       bles are not searched by hostname for robustness reasons.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**discard**___**ehlo**___**keywords** **(default:** **empty)**
       A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that the  Postfix
       SMTP server will not send in the EHLO response to a remote SMTP client.

       This feature is available in Postfix 2.2 and later.

       Notes:

       •      Specify the **silent-discard** pseudo keyword to prevent this action from being logged.

       •      Use  the  smtpd_discard_ehlo_keyword_address_maps feature to discard EHLO keywords se‐
              lectively.

**smtpd**___**dns**___**reply**___**filter** **(default:** **empty)**
       Optional filter for Postfix SMTP server DNS lookup results.   See  smtp_dns_reply_filter  for
       details including an example.

       This feature is available in Postfix 3.0 and later.

**smtpd**___**end**___**of**___**data**___**restrictions** **(default:** **empty)**
       Optional  access restrictions that the Postfix SMTP server applies in the context of the SMTP
       END-OF-DATA command.  See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access re‐
       striction lists" for a discussion of evaluation context and time.

       This feature is available in Postfix 2.2 and later.

       See smtpd_data_restrictions for details and limitations.

**smtpd**___**enforce**___**tls** **(default:** **no)**
       Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require that clients use
       TLS encryption.  According to RFC 2487 this MUST NOT be applied in case of a  publicly-refer‐
       enced SMTP server.  This option is therefore off by default.

       Note 1: "smtpd_enforce_tls = yes" implies "smtpd_tls_auth_only = yes".

       Note  2:  when  invoked via "**sendmail** **-bs**", Postfix will never offer STARTTLS due to insuffi‐
       cient privileges to access the server private key. This is intended behavior.

       This feature is available  in  Postfix  2.2  and  later.  With  Postfix  2.3  and  later  use
       smtpd_tls_security_level instead.

**smtpd**___**error**___**sleep**___**time** **(default:** **1s)**
       With  Postfix  version  2.1 and later: the SMTP server response delay after a client has made
       more than $smtpd_soft_error_limit errors,  and  fewer  than  $smtpd_hard_error_limit  errors,
       without delivering mail.

       With  Postfix  version 2.0 and earlier: the SMTP server delay before sending a reject (4xx or
       5xx) response, when the client has made fewer than $smtpd_soft_error_limit errors without de‐
       livering mail.

**smtpd**___**etrn**___**restrictions** **(default:** **empty)**
       Optional  restrictions  that  the Postfix SMTP server applies in the context of a client ETRN
       command.  See SMTPD_ACCESS_README, section "Delayed evaluation  of  SMTP  access  restriction
       lists" for a discussion of evaluation context and time.

       The  Postfix  ETRN implementation accepts only destinations that are eligible for the Postfix
       "fast flush" service. See the ETRN_README file for details.

       Specify a list of restrictions, separated by commas and/or whitespace.  Continue  long  lines
       by  starting  the next line with whitespace.  Restrictions are applied in the order as speci‐
       fied; the first restriction that matches wins.

       The following restrictions are specific to the domain name information received with the ETRN
       command.

       **check**___**etrn**___**access** _type:table_
              Search  the  specified access database for the ETRN domain name or its parent domains.
              See the [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) manual page for details.

       Other restrictions that are valid in this context:

       •      Generic restrictions that can be used in any SMTP  command  context,  described  under
              smtpd_client_restrictions.

       •      SMTP  command  specific  restrictions  described  under  smtpd_client_restrictions and
              smtpd_helo_restrictions.

       Example:

       smtpd_etrn_restrictions = permit_mynetworks, reject

**smtpd**___**expansion**___**filter** **(default:** **see** **postconf** **-d** **output)**
       What characters are allowed in $name expansions of RBL reply templates. Characters not in the
       allowed  set  are  replaced by "_".  Use C like escapes to specify special characters such as
       whitespace.

       The smtpd_expansion_filter value is not subject to Postfix configuration parameter $name  ex‐
       pansion.

       This feature is available in Postfix 2.0 and later.

**smtpd**___**forbid**___**bare**___**newline** **(default:** **Postfix** **<** **3.9:** **no)**
       Reject  or  restrict input lines from an SMTP client that end in <LF> instead of the standard
       <CR><LF>. Such line endings are commonly allowed with UNIX-based SMTP servers, but they  vio‐
       late RFC 5321, and allowing such line endings can make a server vulnerable to SMTP smuggling.

       Specify one of the following values (case does not matter):

### normalize
              Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>.  Otherwise, allow command
              or message content lines ending in the non-standard <LF>, and process them as  if  the
              client sent the standard <CR><LF>.
              This maintains compatibility with many legitimate SMTP client applications that send a
              mix of standard and non-standard line endings, but will fail  to  receive  email  from
              client   implementations  that  do  not  terminate  DATA  content  with  the  standard
              End-of-DATA sequence <CR><LF>.<CR><LF>.
              Such clients can be excluded with smtpd_forbid_bare_newline_exclusions.

       **yes**    Compatibility alias for **normalize**.

       **reject** Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>. Reject a command or  mes‐
              sage content when a line contains bare <LF>, log a "bare <LF> received" error, and re‐
              ply with the SMTP status code in $smtpd_forbid_bare_newline_reject_code.
              This will reject email from SMTP clients that send any non-standard line endings  such
              as web applications, netcat, or load balancer health checks.
              This will also reject email from services that use BDAT to send MIME text containing a
              bare newline (RFC 3030 Section 3 requires  canonical  MIME  format  for  text  message
              types, defined in RFC 2045 Sections 2.7 and 2.8).
              Such  clients  can  be  excluded with smtpd_forbid_bare_newline_exclusions (or, in the
              case of BDAT violations, BDAT can be selectively disabled with smtpd_discard_ehlo_key‐
              word_address_maps, or globally disabled with smtpd_discard_ehlo_keywords).

### no (default)
              Do  not  require the standard End-of-DATA sequence <CR><LF>.<CR><LF>. Always process a
              bare <LF> as if the client sent <CR><LF>. This option is fully  backwards  compatible,
              but is not recommended for an Internet-facing SMTP server, because it is vulnerable to
              SMTP smuggling.

       Recommended settings:

           # Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>.
           # Otherwise, allow bare <LF> and process it as if the client sent
           # <CR><LF>.
           #
           # This maintains compatibility with many legitimate SMTP client
           # applications that send a mix of standard and non-standard line
           # endings, but will fail to receive email from client implementations
           # that do not terminate DATA content with the standard End-of-DATA
           # sequence <CR><LF>.<CR><LF>.
           #
           # Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
           # The example below allowlists SMTP clients in trusted networks.
           #
           smtpd_forbid_bare_newline = normalize
           smtpd_forbid_bare_newline_exclusions = $mynetworks

       Alternative:

           # Reject input lines that contain <LF> and log a "bare <LF> received"
           # error. Require that input lines end in <CR><LF>, and require the
           # standard End-of-DATA sequence <CR><LF>.<CR><LF>.
           #
           # This will reject email from SMTP clients that send any non-standard
           # line endings such as web applications, netcat, or load balancer
           # health checks.
           #
           # This will also reject email from services that use BDAT to send
           # MIME text containing a bare newline (RFC 3030 Section 3 requires
           # canonical MIME format for text message types, defined in RFC 2045
           # Sections 2.7 and 2.8).
           #
           # Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
           # The example below allowlists SMTP clients in trusted networks.
           #
           smtpd_forbid_bare_newline = reject
           smtpd_forbid_bare_newline_exclusions = $mynetworks
           #
           # Alternatively, in the case of BDAT violations, BDAT can be selectively
           # disabled with smtpd_discard_ehlo_keyword_address_maps, or globally
           # disabled with smtpd_discard_ehlo_keywords.
           #
           # smtpd_discard_ehlo_keyword_address_maps = cidr:/path/to/file
           # /path/to/file:
           #     10.0.0.0/24 chunking, silent-discard
           # smtpd_discard_ehlo_keywords = chunking, silent-discard

       This feature with settings **yes** and **no** is available  in  Postfix  3.8.4,  3.7.9,  3.6.13,  and
       3.5.23.  Additionally,  the settings **reject**, and **normalize** are available with Postfix >= 3.9,
       3.8.5, 3.7.10, 3.6.14, and 3.5.24.

**smtpd**___**forbid**___**bare**___**newline**___**exclusions** **(default:** **$mynetworks)**
       Exclude the specified clients from smtpd_forbid_bare_newline enforcement. This  setting  uses
       the same syntax and parent-domain matching behavior as mynetworks.

       This feature is available in Postfix >= 3.9, 3.8.4, 3.7.9, 3.6.13, and 3.5.23.

**smtpd**___**forbid**___**bare**___**newline**___**reject**___**code** **(default:** **550)**
       The  numerical  Postfix  SMTP  server response code when rejecting a request with "smtpd_for‐
       bid_bare_newline = reject".  Specify a 5XX status code (521 to disconnect).

       This feature is available in Postfix >= 3.9, 3.8.5, 3.7.10, 3.6.14, and 3.5.24.

**smtpd**___**forbidden**___**commands** **(default:** **CONNECT,** **GET,** **POST)**
       List of commands that cause the Postfix SMTP server to immediately terminate the session with
       a  221  code. This can be used to disconnect clients that obviously attempt to abuse the sys‐
       tem. In addition to the commands listed in this parameter, commands that follow the  "Label:"
       format of message headers will also cause a disconnect.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**hard**___**error**___**limit** **(default:** **normal:** **20,** **overload:** **1)**
       The maximal number of errors a remote SMTP client is allowed to make without delivering mail.
       The Postfix SMTP server disconnects when the limit is exceeded. Normally the default limit is
       20,  but  it  changes under overload to just 1. With Postfix 2.5 and earlier, the SMTP server
       always allows up to 20 errors by default.

**smtpd**___**helo**___**required** **(default:** **no)**
       Require that a remote SMTP client introduces itself with the  HELO  or  EHLO  command  before
       sending the MAIL command or other commands that require EHLO negotiation.

       Example:

       smtpd_helo_required = yes

**smtpd**___**helo**___**restrictions** **(default:** **empty)**
       Optional  restrictions  that  the Postfix SMTP server applies in the context of a client HELO
       command.  See SMTPD_ACCESS_README, section "Delayed evaluation  of  SMTP  access  restriction
       lists" for a discussion of evaluation context and time.

       The default is to permit everything.

       Note:  specify  "smtpd_helo_required  =  yes"  to  fully  enforce  this  restriction (without
       "smtpd_helo_required = yes", a client can simply skip smtpd_helo_restrictions by not  sending
       HELO or EHLO).

       Specify  a  list of restrictions, separated by commas and/or whitespace.  Continue long lines
       by starting the next line with whitespace.  Restrictions are applied in the order  as  speci‐
       fied; the first restriction that matches wins.

       The following restrictions are specific to the hostname information received with the HELO or
       EHLO command.

       **check**___**helo**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the HELO or EHLO hostname  or  parent  do‐
              mains,  and  execute  the  corresponding action.  Note: specify "smtpd_helo_required =
              yes" to fully enforce this restriction (without "smtpd_helo_required = yes", a  client
              can simply skip check_helo_access by not sending HELO or EHLO).

       **check**___**helo**___**a**___**access** _type:table_
              Search  the  specified  [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown)  database  for the IP addresses for the HELO or EHLO
              hostname, and execute the corresponding action.  Note 1: a result of "OK" is  not  al‐
              lowed  for  safety reasons. Instead, use DUNNO in order to exclude specific hosts from
              denylists.  Note 2: specify "smtpd_helo_required = yes" to fully enforce this restric‐
              tion  (without  "smtpd_helo_required = yes", a client can simply skip check_helo_a_ac‐
              cess by not sending HELO or EHLO).  This feature  is  available  in  Postfix  3.0  and
              later.

       **check**___**helo**___**mx**___**access** _type:table_
              Search  the  specified  [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the MX hosts for the HELO or EHLO host‐
              name, and execute the corresponding action.  If no MX record is found, look  up  A  or
              AAAA  records,  just  like the Postfix SMTP client would.  Note 1: a result of "OK" is
              not allowed for safety reasons. Instead, use DUNNO in order to exclude specific  hosts
              from denylists.  Note 2: specify "smtpd_helo_required = yes" to fully enforce this re‐
              striction  (without  "smtpd_helo_required  =  yes",   a   client   can   simply   skip
              check_helo_mx_access by not sending HELO or EHLO).  This feature is available in Post‐
              fix 2.1 and later.

       **check**___**helo**___**ns**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the DNS servers for the HELO or EHLO host‐
              name,  and  execute the corresponding action.  Note 1: a result of "OK" is not allowed
              for safety reasons. Instead, use  DUNNO  in  order  to  exclude  specific  hosts  from
              denylists.  Note 2: specify "smtpd_helo_required = yes" to fully enforce this restric‐
              tion (without "smtpd_helo_required = yes", a client can simply skip  check_helo_ns_ac‐
              cess by not sending HELO or EHLO). This feature is available in Postfix 2.1 and later.

       **reject**___**invalid**___**helo**___**hostname** (with Postfix < 2.3: reject_invalid_hostname)
              Reject  the  request  when  the  HELO  or  EHLO  hostname is malformed.  Note: specify
              "smtpd_helo_required = yes" to fully enforce this restriction (without "smtpd_helo_re‐
              quired  =  yes",  a client can simply skip reject_invalid_helo_hostname by not sending
              HELO or EHLO).
              The invalid_hostname_reject_code specifies the response  code  for  rejected  requests
              (default: 501).

       **reject**___**non**___**fqdn**___**helo**___**hostname** (with Postfix < 2.3: reject_non_fqdn_hostname)
              Reject  the request when the HELO or EHLO hostname is not in fully-qualified domain or
              address literal form, as required by the RFC.  Note:  specify  "smtpd_helo_required  =
              yes"  to fully enforce this restriction (without "smtpd_helo_required = yes", a client
              can simply skip reject_non_fqdn_helo_hostname by not sending HELO or EHLO).
              The non_fqdn_reject_code parameter specifies the response code for  rejected  requests
              (default: 504).

       **reject**___**rhsbl**___**helo** _rbl_domain=d.d.d.d_
              Reject  the  request  when  the  HELO  or  EHLO  hostname  is listed with the A record
              "_d.d.d.d_" under _rbl_domain_ (Postfix version 2.1 and later only).  Each "_d_" is  a  num‐
              ber,  or a pattern inside "[]" that contains one or more ";"-separated numbers or num‐
              ber..number ranges (Postfix version 2.8 and later).  If no  "_=d.d.d.d_"  is  specified,
              reject  the  request  when the HELO or EHLO hostname is listed with any A record under
              _rbl_domain_. See the reject_rbl_client description for additional RBL related  configu‐
              ration  parameters.   Note:  specify "smtpd_helo_required = yes" to fully enforce this
              restriction (without "smtpd_helo_required  =  yes",  a  client  can  simply  skip  re‐
              ject_rhsbl_helo by not sending HELO or EHLO). This feature is available in Postfix 2.0
              and later.

       **reject**___**unknown**___**helo**___**hostname** (with Postfix < 2.3: reject_unknown_hostname)
              Reject the request when the HELO or EHLO hostname has no DNS A or MX record.
              The reply is specified with the unknown_hostname_reject_code parameter (default:  450)
              or  unknown_helo_hostname_tempfail_action (default: defer_if_permit).  See the respec‐
              tive parameter descriptions for details.
              Note: specify "smtpd_helo_required = yes" to fully enforce this  restriction  (without
              "smtpd_helo_required  = yes", a client can simply skip reject_unknown_helo_hostname by
              not sending HELO or EHLO).

       Other restrictions that are valid in this context:

       •      Generic restrictions that can be used in any SMTP  command  context,  described  under
              smtpd_client_restrictions.

       •      Client   hostname   or   network   address   specific   restrictions  described  under
              smtpd_client_restrictions.

       •      SMTP  command  specific  restrictions  described  under  smtpd_sender_restrictions  or
              smtpd_recipient_restrictions.   When sender or recipient restrictions are listed under
              smtpd_helo_restrictions, they have effect only with  "smtpd_delay_reject  =  yes",  so
              that $smtpd_helo_restrictions is evaluated at the time of the RCPT TO command.

       Examples:

       smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname
       smtpd_helo_restrictions = permit_mynetworks, reject_unknown_helo_hostname

**smtpd**___**history**___**flush**___**threshold** **(default:** **100)**
       The  maximal  number of lines in the Postfix SMTP server command history before it is flushed
       upon receipt of EHLO, RSET, or end of DATA.

**smtpd**___**junk**___**command**___**limit** **(default:** **normal:** **100,** **overload:** **1)**
       The number of junk commands (NOOP, VRFY, ETRN or RSET) that a remote SMTP client can send be‐
       fore  the  Postfix  SMTP server starts to increment the error counter with each junk command.
       The junk command count is reset after mail is delivered.  See also the smtpd_error_sleep_time
       and  smtpd_soft_error_limit configuration parameters.  Normally the default limit is 100, but
       it changes under overload to just 1. With Postfix 2.5 and earlier, the SMTP server always al‐
       lows up to 100 junk commands by default.

**smtpd**___**log**___**access**___**permit**___**actions** **(default:** **empty)**
       Enable  logging  of  the  named "permit" actions in SMTP server access lists (by default, the
       SMTP server logs "reject" actions but not "permit" actions).  This feature  does  not  affect
       conditional actions such as "defer_if_permit".

       Specify  a list of "permit" action names, "/file/name" or "type:table" patterns, separated by
       commas and/or whitespace. The list is matched left to right, and  the  search  stops  on  the
       first  match. A "/file/name" pattern is replaced by its contents; a "type:table" lookup table
       is matched when a name matches a lookup key (the lookup result is  ignored).   Continue  long
       lines  by  starting  the next line with whitespace. Specify "!pattern" to exclude a name from
       the list.

       Examples:

       /etc/postfix/main.cf:
           # Log all "permit" actions.
           smtpd_log_access_permit_actions = static:all

       /etc/postfix/main.cf:
           # Log "permit_dnswl_client" only.
           smtpd_log_access_permit_actions = permit_dnswl_client

       This feature is available in Postfix 2.10 and later.

**smtpd**___**milter**___**maps** **(default:** **empty)**
       Lookup tables with Milter settings per remote SMTP client  IP  address.   The  lookup  result
       overrides the smtpd_milters setting, and has the same syntax.

       Note:  lookup  tables cannot return empty responses. Specify a lookup result of DISABLE (case
       does not matter) to indicate that Milter support should be disabled.

       Example to disable Milters for local clients:

       /etc/postfix/main.cf:
           smtpd_milter_maps = cidr:/etc/postfix/smtpd_milter_map
           smtpd_milters = inet:host:port, { inet:host:port, ... }, ...

       /etc/postfix/smtpd_milter_map:
           # Disable Milters for local clients.
           127.0.0.0/8    DISABLE
           192.168.0.0/16 DISABLE
           ::/64          DISABLE
           2001:db8::/32  DISABLE

       This feature is available in Postfix 3.2 and later.

**smtpd**___**milters** **(default:** **empty)**
       A list of Milter (mail filter) applications  for  new  mail  that  arrives  via  the  Postfix
       [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown)  server. Specify space or comma as separator. See the MILTER_README document for de‐
       tails.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**noop**___**commands** **(default:** **empty)**
       List of commands that the Postfix SMTP server replies to with "250  Ok",  without  doing  any
       syntax  checks  and  without changing state.  This list overrides any commands built into the
       Postfix SMTP server.

**smtpd**___**null**___**access**___**lookup**___**key** **(default:** **<>)**
       The lookup key to be used in SMTP [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) tables instead of the null sender address.

**smtpd**___**peername**___**lookup** **(default:** **yes)**
       Attempt to look up the remote SMTP client hostname, and verify  that  the  name  matches  the
       client IP address. A client name is set to "unknown" when it cannot be looked up or verified,
       or when name lookup is disabled.  Turning off name lookup reduces delays due  to  DNS  lookup
       and increases the maximal inbound delivery rate.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**per**___**record**___**deadline** **(default:** **normal:** **no,** **overload:** **yes)**
       Change  the behavior of the smtpd_timeout and smtpd_starttls_timeout time limits, from a time
       limit per read or write system call, to a time limit to send or receive a complete record (an
       SMTP  command  line, SMTP response line, SMTP message content line, or TLS protocol message).
       This limits the impact from hostile peers that trickle data one byte at a time.

       Note: when per-record deadlines are enabled, a short timeout may cause problems with TLS over
       very  slow  network connections.  The reasons are that a TLS protocol message can be up to 16
       kbytes long (with TLSv1), and that an entire TLS protocol message must be  sent  or  received
       within the per-record deadline.

       This feature is available in Postfix 2.9 and later. With older Postfix releases, the behavior
       is as if this parameter is set to "no".

**smtpd**___**policy**___**service**___**default**___**action** **(default:** **451** **4.3.5** **Server** **configuration** **problem)**
       The default action when an SMTPD policy service request fails.  Specify "DUNNO" to behave  as
       if  the  failed   SMTPD policy service request was not sent, and to continue processing other
       access restrictions, if any.

       Limitations:

       •      This parameter may specify any value that would be a valid  SMTPD  policy  server  re‐
              sponse  (or  [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown)  map lookup result).  An [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) map or policy server in this
              parameter value may need to be declared in advance with a restriction_class setting.

       •      If the specified action invokes another  check_policy_service  request,  that  request
              will have the built-in default action.

       This feature is available in Postfix 3.0 and later.

**smtpd**___**policy**___**service**___**max**___**idle** **(default:** **300s)**
       The time after which an idle SMTPD policy service connection is closed.

       This feature is available in Postfix 2.1 and later.

**smtpd**___**policy**___**service**___**max**___**ttl** **(default:** **1000s)**
       The time after which an active SMTPD policy service connection is closed.

       This feature is available in Postfix 2.1 and later.

**smtpd**___**policy**___**service**___**policy**___**context** **(default:** **empty)**
       Optional information that the Postfix SMTP server specifies in the "policy_context" attribute
       of a policy service request (originally, to share the same service  endpoint  among  multiple
       check_policy_service clients).

       This feature is available in Postfix 3.1 and later.

**smtpd**___**policy**___**service**___**request**___**limit** **(default:** **0)**
       The  maximal number of requests per SMTPD policy service connection, or zero (no limit). Once
       a connection reaches this limit, the connection is closed and the next request will  be  sent
       over  a  new  connection.  This  is  a  workaround to avoid error-recovery delays with policy
       servers that cannot maintain a persistent connection.

       This feature is available in Postfix 3.0 and later.

**smtpd**___**policy**___**service**___**retry**___**delay** **(default:** **1s)**
       The delay between attempts to resend a failed SMTPD policy service request. Specify  a  value
       greater than zero.

       This feature is available in Postfix 3.0 and later.

**smtpd**___**policy**___**service**___**timeout** **(default:** **100s)**
       The  time  limit  for  connecting  to, writing to, or receiving from a delegated SMTPD policy
       server.

       This feature is available in Postfix 2.1 and later.

**smtpd**___**policy**___**service**___**try**___**limit** **(default:** **2)**
       The maximal number of attempts to send an SMTPD policy  service  request  before  giving  up.
       Specify a value greater than zero.

       This feature is available in Postfix 3.0 and later.

**smtpd**___**proxy**___**ehlo** **(default:** **$myhostname)**
       How  the  Postfix  SMTP server announces itself to the proxy filter.  By default, the Postfix
       hostname is used.

       This feature is available in Postfix 2.1 and later.

**smtpd**___**proxy**___**filter** **(default:** **empty)**
       The hostname and TCP port of the mail filtering proxy server.  The proxy  receives  all  mail
       from  the  Postfix  SMTP  server,  and is supposed to give the result to another Postfix SMTP
       server process.

       Specify "host:port" or "inet:host:port" for a TCP endpoint, or "unix:pathname" for a UNIX-do‐
       main  endpoint.  The  host  can  be  specified  as an IP address or as a symbolic name; no MX
       lookups are done.  When no "host" or "host:"  are specified, the local  machine  is  assumed.
       Pathname interpretation is relative to the Postfix queue directory.

       This feature is available in Postfix 2.1 and later.

       The "inet:" and "unix:" prefixes are available in Postfix 2.3 and later.

**smtpd**___**proxy**___**options** **(default:** **empty)**
       List  of  options  that  control how the Postfix SMTP server communicates with a before-queue
       content filter. Specify zero or more of the following, separated by comma or whitespace.

       **speed**___**adjust**
              Do not connect to a before-queue content filter until an entire message has  been  re‐
              ceived. This reduces the number of simultaneous before-queue content filter processes.

       NOTE  1:  A  filter must not _selectively_ reject recipients of a multi-recipient message.  Re‐
       jecting all recipients is OK, as is accepting all recipients.

       NOTE 2: This feature increases the minimum amount of free queue space by $message_size_limit.
       The extra space is needed to save the message to a temporary file.

       This feature is available in Postfix 2.7 and later.

**smtpd**___**proxy**___**timeout** **(default:** **100s)**
       The  time  limit  for  connecting to a proxy filter and for sending or receiving information.
       When a connection fails the client gets a generic error message while more detailed  informa‐
       tion is logged to the maillog file.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

       This feature is available in Postfix 2.1 and later.

**smtpd**___**recipient**___**limit** **(default:** **1000)**
       The maximal number of recipients that the Postfix SMTP server accepts  per  message  delivery
       request.

**smtpd**___**recipient**___**overshoot**___**limit** **(default:** **1000)**
       The  number of recipients that a remote SMTP client can send in excess of the limit specified
       with $smtpd_recipient_limit, before the Postfix SMTP server increments the per-session  error
       count for each excess recipient.

**smtpd**___**recipient**___**restrictions** **(default:** **see** **postconf** **-d** **output)**
       Optional restrictions that the Postfix SMTP server applies in the context of a client RCPT TO
       command, after smtpd_relay_restrictions.  See SMTPD_ACCESS_README, section  "Delayed  evalua‐
       tion of SMTP access restriction lists" for a discussion of evaluation context and time.

       With Postfix versions before 2.10, the rules for relay permission and spam blocking were com‐
       bined under smtpd_recipient_restrictions, resulting  in  error-prone  configuration.   As  of
       Postfix  2.10,  relay  permission  rules are preferably implemented with smtpd_relay_restric‐
       tions, so that a permissive spam blocking policy under smtpd_recipient_restrictions  will  no
       longer result in a permissive mail relay policy.

       For  backwards  compatibility,  sites  that migrate from Postfix versions before 2.10 can set
       smtpd_relay_restrictions to the empty value, and use smtpd_recipient_restrictions exactly  as
       before.

       IMPORTANT:  Either the smtpd_relay_restrictions or the smtpd_recipient_restrictions parameter
       must specify at least one of the following restrictions. Otherwise Postfix will refuse to re‐
       ceive mail:

           reject, reject_unauth_destination

           defer, defer_if_permit, defer_unauth_destination

       Specify  a  list of restrictions, separated by commas and/or whitespace.  Continue long lines
       by starting the next line with whitespace.  Restrictions are applied in the order  as  speci‐
       fied; the first restriction that matches wins.

       The  following  restrictions  are specific to the recipient address that is received with the
       RCPT TO command.

       **check**___**recipient**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the resolved RCPT TO address, domain, par‐
              ent domains, or localpart@, and execute the corresponding action.

       **check**___**recipient**___**a**___**access** _type:table_
              Search  the  specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the IP addresses for the RCPT TO domain,
              and execute the corresponding action.  Note: a result  of  "OK"  is  not  allowed  for
              safety  reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
              This feature is available in Postfix 3.0 and later.

       **check**___**recipient**___**mx**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the MX hosts for the RCPT TO  domain,  and
              execute  the  corresponding  action.   If  no  MX  record  is found, look up A or AAAA
              records, just like the Postfix SMTP client would. Note: a result of "OK"  is  not  al‐
              lowed  for  safety reasons. Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 2.1 and later.

       **check**___**recipient**___**ns**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the DNS servers for the  RCPT  TO  domain,
              and  execute  the  corresponding  action.   Note:  a result of "OK" is not allowed for
              safety reasons. Instead, use DUNNO in order to exclude specific hosts from  denylists.
              This feature is available in Postfix 2.1 and later.

       **permit**___**auth**___**destination**
              Permit the request when one of the following is true:

       •      Postfix  is  mail  forwarder:  the resolved RCPT TO domain matches $relay_domains or a
              subdomain thereof, and the address contains no  sender-specified  routing  (user@else‐
              where@domain),

       •      Postfix  is the final destination: the resolved RCPT TO domain matches $mydestination,
              $inet_interfaces, $proxy_interfaces, $virtual_alias_domains,  or  $virtual_mailbox_do‐
              mains, and the address contains no sender-specified routing (user@elsewhere@domain).

       **permit**___**mx**___**backup**
              Permit  the request when the local mail system is backup MX for the RCPT TO domain, or
              when the domain is an authorized destination (see permit_auth_destination for  defini‐
              tion).

       •      Safety:  permit_mx_backup does not accept addresses that have sender-specified routing
              information (example: user@elsewhere@domain).

       •      Safety: permit_mx_backup can be vulnerable to mis-use when access  is  not  restricted
              with permit_mx_backup_networks.

       •      Safety: as of Postfix version 2.3, permit_mx_backup no longer accepts the address when
              the local mail system is  primary  MX  for  the  recipient  domain.   Exception:  per‐
              mit_mx_backup  accepts  the  address  when it specifies an authorized destination (see
              permit_auth_destination for definition).

       •      Limitation: mail may be rejected in case of a temporary DNS lookup problem with  Post‐
              fix prior to version 2.0.

       **reject**___**non**___**fqdn**___**recipient**
              Reject  the  request  when  the  RCPT  TO  address  specifies  a domain that is not in
              fully-qualified domain form, as required by the RFC.
              The non_fqdn_reject_code parameter specifies the response code for  rejected  requests
              (default: 504).

       **reject**___**rhsbl**___**recipient** _rbl_domain=d.d.d.d_
              Reject the request when the RCPT TO domain is listed with the A record "_d.d.d.d_" under
              _rbl_domain_ (Postfix version 2.1 and later only).  Each "_d_" is a number, or  a  pattern
              inside  "[]"  that contains one or more ";"-separated numbers or number..number ranges
              (Postfix version 2.8 and later). If no "_=d.d.d.d_" is  specified,  reject  the  request
              when the RCPT TO domain is listed with any A record under _rbl_domain_.
              The  maps_rbl_reject_code  parameter specifies the response code for rejected requests
              (default: 554); the default_rbl_reply parameter specifies the  default  server  reply;
              and  the  rbl_reply_maps  parameter  specifies  tables  with server replies indexed by
              _rbl_domain_.  This feature is available in Postfix version 2.0 and later.

       **reject**___**unauth**___**destination**
              Reject the request unless one of the following is true:

       •      Postfix is mail forwarder: the resolved RCPT TO domain  matches  $relay_domains  or  a
              subdomain thereof, and contains no sender-specified routing (user@elsewhere@domain),

       •      Postfix  is the final destination: the resolved RCPT TO domain matches $mydestination,
              $inet_interfaces, $proxy_interfaces, $virtual_alias_domains,  or  $virtual_mailbox_do‐
              mains, and contains no sender-specified routing (user@elsewhere@domain).
              The  relay_domains_reject_code  parameter specifies the response code for rejected re‐
              quests (default: 554).

       **defer**___**unauth**___**destination**
              Reject the same requests as  reject_unauth_destination,  with  a  non-permanent  error
              code.  This feature is available in Postfix 2.10 and later.

       **reject**___**unknown**___**recipient**___**domain**
              Reject the request when Postfix is not final destination for the recipient domain, and
              the RCPT TO domain has 1) no DNS MX and no DNS A record or 2) a  malformed  MX  record
              such as a record with a zero-length MX hostname (Postfix version 2.3 and later).
              The  reply is specified with the unknown_address_reject_code parameter (default: 450),
              unknown_address_tempfail_action (default: defer_if_permit), or  556  (nullmx,  Postfix
              3.0 and later). See the respective parameter descriptions for details.

       **reject**___**unlisted**___**recipient** (with Postfix version 2.0: check_recipient_maps)
              Reject the request when the RCPT TO address is not listed in the list of valid recipi‐
              ents for its domain class. See the smtpd_reject_unlisted_recipient parameter  descrip‐
              tion for details.  This feature is available in Postfix 2.1 and later.

       **reject**___**unverified**___**recipient**
              Reject  the  request  when mail to the RCPT TO address is known to bounce, or when the
              recipient address destination is not reachable.  Address verification  information  is
              managed by the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) server; see the ADDRESS_VERIFICATION_README file for details.
              The  unverified_recipient_reject_code  parameter specifies the numerical response code
              when an address is known to bounce (default: 450, change into 550 when you are  confi‐
              dent that it is safe to do so).
              The  unverified_recipient_defer_code  parameter  specifies the numerical response code
              when an address probe failed due to a temporary problem (default: 450).
              The unverified_recipient_tempfail_action parameter specifies the action after  address
              probe failure due to a temporary problem (default: defer_if_permit).
              This feature breaks for aliased addresses with "enable_original_recipient = no" (Post‐
              fix <= 3.2).
              This feature is available in Postfix 2.1 and later.

       Other restrictions that are valid in this context:

       •      Generic restrictions that can be used in any SMTP  command  context,  described  under
              smtpd_client_restrictions.

       •      SMTP   command   specific   restrictions  described  under  smtpd_client_restrictions,
              smtpd_helo_restrictions and smtpd_sender_restrictions.

       Example:

       # The Postfix before 2.10 default mail relay policy. Later Postfix
       # versions implement this preferably with smtpd_relay_restrictions.
       smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

**smtpd**___**reject**___**footer** **(default:** **empty)**
       Optional information that is appended after each Postfix SMTP server 4XX or 5XX response.

       The following example uses "\c" at the start of the template (supported in Postfix  2.10  and
       later)  to  suppress  the line break between the reply text and the footer text. With earlier
       Postfix versions, the footer text always begins on a new line, and the "\c" is output  liter‐
       ally.

       /etc/postfix/main.cf:
           smtpd_reject_footer = \c. For assistance, call 800-555-0101.
            Please provide the following information in your problem report:
            time ($localtime), client ($client_address) and server
            ($server_name).

       Server response:

           550-5.5.1 <user@example> Recipient address rejected: User
           unknown. For assistance, call 800-555-0101. Please provide the
           following information in your problem report: time (Jan 4 15:42:00),
           client (192.168.1.248) and server (mail1.example.com).

       Note:  the  above  text  is meant to make it easier to find the Postfix logfile records for a
       failed SMTP session. The text itself is not logged to the Postfix SMTP server's maillog file.

       Be sure to keep the text as short as possible. Long text may be truncated before it is logged
       to  the remote SMTP client's maillog file, or before it is returned to the sender in a deliv‐
       ery status notification.

       The template text is not subject to Postfix configuration parameter $name expansion. Instead,
       this  feature  supports  a  limited  number of $name attributes in the footer text. These at‐
       tributes are replaced with their current value for the SMTP session.

       Note: specify $$name in footer text that is looked up from regexp: or  pcre:-based  smtpd_re‐
       ject_footer_maps,  otherwise  the  Postfix server will not use the footer text and will log a
       warning instead.

       **client**___**address**
              The Client IP address that is logged in the maillog file.

       **client**___**port**
              The client TCP port that is logged in the maillog file.

### localtime
              The server local time (Mmm dd hh:mm:ss) that is logged in the maillog file.

       **server**___**name**
              The server's myhostname value.  This attribute is made available for sites with multi‐
              ple  MTAs  (perhaps behind a load-balancer), where the server name can help the server
              support team to quickly find the right log files.

       Notes:

       •      NOT SUPPORTED are other attributes such as sender, recipient, or main.cf parameters.

       •      For safety reasons, text that does not match $smtpd_expansion_filter is censored.

       This feature supports the two-character sequence \n as a request for  a  line  break  in  the
       footer  text.  Postfix automatically inserts after each line break the three-digit SMTP reply
       code (and optional enhanced status code) from the original Postfix reject message.

       To work around mail software that mis-handles multi-line replies, specify  the  two-character
       sequence  \c  at the start of the template.  This suppresses the line break between the reply
       text and the footer text (Postfix 2.10 and later).

       This feature is available in Postfix 2.8 and later.

**smtpd**___**reject**___**footer**___**maps** **(default:** **empty)**
       Lookup tables, indexed by the complete Postfix SMTP server 4xx or 5xx response,  with  reject
       footer templates. See smtpd_reject_footer for details.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       This feature is available in Postfix 3.4 and later.

**smtpd**___**reject**___**unlisted**___**recipient** **(default:** **yes)**
       Request that the Postfix SMTP server rejects mail for unknown recipient addresses, even  when
       no  explicit  reject_unlisted_recipient  access  restriction  is specified. This prevents the
       Postfix queue from filling up with undeliverable MAILER-DAEMON messages.

       An address is always considered "known" when it matches a [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown) alias or a  [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown)
       mapping.

       •      The  recipient  domain  matches $mydestination, $inet_interfaces or $proxy_interfaces,
              but the recipient is not listed in $local_recipient_maps, and $local_recipient_maps is
              not null.

       •      The recipient domain matches $virtual_alias_domains but the recipient is not listed in
              $virtual_alias_maps.

       •      The recipient domain matches $virtual_mailbox_domains but the recipient is not  listed
              in $virtual_mailbox_maps, and $virtual_mailbox_maps is not null.

       •      The  recipient  domain  matches $relay_domains but the recipient is not listed in $re‐
              lay_recipient_maps, and $relay_recipient_maps is not null.

       This feature is available in Postfix 2.1 and later.

**smtpd**___**reject**___**unlisted**___**sender** **(default:** **no)**
       Request that the Postfix SMTP server rejects mail from unknown sender addresses, even when no
       explicit reject_unlisted_sender access restriction is specified. This can slow down an explo‐
       sion of forged mail from worms or viruses.

       An address is always considered "known" when it matches a [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown) alias or a  [**canonical**(5)](https://www.chedong.com/phpMan.php/man/canonical/5/markdown)
       mapping.

       •      The  sender  domain matches $mydestination, $inet_interfaces or $proxy_interfaces, but
              the sender is not listed in $local_recipient_maps, and  $local_recipient_maps  is  not
              null.

       •      The sender domain matches $virtual_alias_domains but the sender is not listed in $vir‐
              tual_alias_maps.

       •      The sender domain matches $virtual_mailbox_domains but the sender  is  not  listed  in
              $virtual_mailbox_maps, and $virtual_mailbox_maps is not null.

       •      The sender domain matches $relay_domains but the sender is not listed in $relay_recip‐
              ient_maps, and $relay_recipient_maps is not null.

       This feature is available in Postfix 2.1 and later.

**smtpd**___**relay**___**before**___**recipient**___**restrictions** **(default:** **see** **postconf** **-d** **output)**
       Evaluate   smtpd_relay_restrictions   before   smtpd_recipient_restrictions.    Historically,
       smtpd_relay_restrictions was evaluated after smtpd_recipient_restrictions, contradicting doc‐
       umented behavior.

       Background: the smtpd_relay_restrictions feature is primarily designed to enforce a mail  re‐
       laying  policy,  while  smtpd_recipient_restrictions  is  primarily  designed to enforce spam
       blocking policy. Both are evaluated while replying to the RCPT TO command, and  both  support
       the same features.

       This feature is available in Postfix 3.6 and later.

**smtpd**___**relay**___**restrictions** **(default:** **permit**___**mynetworks,** **permit**___**sasl**___**authenticated,** **defer**___**unauth**___**desti**‐‐
### nation)
       Access restrictions for mail relay control that the Postfix SMTP server applies in  the  con‐
       text  of  the RCPT TO command, before smtpd_recipient_restrictions.  See SMTPD_ACCESS_README,
       section "Delayed evaluation of SMTP access restriction lists" for a discussion of  evaluation
       context and time.

       With Postfix versions before 2.10, the rules for relay permission and spam blocking were com‐
       bined under smtpd_recipient_restrictions, resulting  in  error-prone  configuration.   As  of
       Postfix  2.10,  relay  permission  rules are preferably implemented with smtpd_relay_restric‐
       tions, so that a permissive spam blocking policy under smtpd_recipient_restrictions  will  no
       longer result in a permissive mail relay policy.

       For  backwards  compatibility,  sites  that migrate from Postfix versions before 2.10 can set
       smtpd_relay_restrictions to the empty value, and use smtpd_recipient_restrictions exactly  as
       before.

       By default, the Postfix SMTP server accepts:

       •      Mail from clients whose IP address matches $mynetworks, or:

       •      Mail  to remote destinations that match $relay_domains, except for addresses that con‐
              tain sender-specified routing (user@elsewhere@domain), or:

       •      Mail to local destinations that match $inet_interfaces or $proxy_interfaces, $mydesti‐
              nation, $virtual_alias_domains, or $virtual_mailbox_domains.

       IMPORTANT:  Either the smtpd_relay_restrictions or the smtpd_recipient_restrictions parameter
       must specify at least one of the following restrictions. Otherwise Postfix will refuse to re‐
       ceive mail:

           reject, reject_unauth_destination

           defer, defer_if_permit, defer_unauth_destination

       Specify  a  list of restrictions, separated by commas and/or whitespace.  Continue long lines
       by starting the next line with whitespace.  The same restrictions are available as documented
       under smtpd_recipient_restrictions.

       This feature is available in Postix 2.10 and later.

**smtpd**___**restriction**___**classes** **(default:** **empty)**
       User-defined  aliases  for  groups  of  access  restrictions. The aliases can be specified in
       smtpd_recipient_restrictions etc., and on the right-hand side of a Postfix [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) table.

       One major application is for  implementing  per-recipient  UCE  control.   See  the  RESTRIC‐
       TION_CLASS_README document for other examples.

**smtpd**___**sasl**___**application**___**name** **(default:** **smtpd)**
       The  application  name that the Postfix SMTP server uses for SASL server initialization. This
       controls the name of the SASL configuration file. The default value is  **smtpd**,  corresponding
       to a SASL configuration file named **smtpd.conf**.

       This  feature  is  available  in  Postfix  2.1  and  2.2.  With Postfix 2.3 it was renamed to
       smtpd_sasl_path.

**smtpd**___**sasl**___**auth**___**enable** **(default:** **no)**
       Enable SASL authentication in the Postfix SMTP server. By default, the  Postfix  SMTP  server
       does not use authentication.

       If  a  remote  SMTP client is authenticated, the permit_sasl_authenticated access restriction
       can be used to permit relay access, like this:

           # With Postfix 2.10 and later, the mail relay policy is
           # preferably specified under smtpd_relay_restrictions.
           smtpd_relay_restrictions =
               permit_mynetworks, permit_sasl_authenticated, ...

       # With Postfix before 2.10, the relay policy can be
       # specified only under smtpd_recipient_restrictions.
       smtpd_recipient_restrictions =
           permit_mynetworks, permit_sasl_authenticated, ...

       To reject all SMTP connections from unauthenticated clients,  specify  "smtpd_delay_reject  =
       yes" (which is the default) and use:

           smtpd_client_restrictions = permit_sasl_authenticated, reject

       See the SASL_README file for SASL configuration and operation details.

**smtpd**___**sasl**___**authenticated**___**header** **(default:** **no)**
       Report the SASL authenticated user name in the [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) Received message header.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**sasl**___**exceptions**___**networks** **(default:** **empty)**
       What remote SMTP clients the Postfix SMTP server will not offer AUTH support to.

       Some  clients  (Netscape 4 at least) have a bug that causes them to require a login and pass‐
       word whenever AUTH is offered, whether it's necessary or not. To work around  this,  specify,
       for example, $mynetworks to prevent Postfix from offering AUTH to local clients.

       Specify  a  list of network/netmask patterns, separated by commas and/or whitespace. The mask
       specifies the number of bits in the network part of a host address. You can also "/file/name"
       or "type:table" patterns.  A "/file/name" pattern is replaced by its contents; a "type:table"
       lookup table is matched when a table entry matches a lookup string (the lookup result is  ig‐
       nored).  Continue long lines by starting the next line with whitespace. Specify "!pattern" to
       exclude an address or network block from the list.  The form "!/file/name" is supported  only
       in Postfix version 2.4 and later.

       Note:  IP  version 6 address information must be specified inside [] in the smtpd_sasl_excep‐
       tions_networks value, and in files specified with "/file/name".  IP version 6 addresses  con‐
       tain the ":" character, and would otherwise be confused with a "type:table" pattern.

       Example:

       smtpd_sasl_exceptions_networks = $mynetworks

       This feature is available in Postfix 2.1 and later.

**smtpd**___**sasl**___**local**___**domain** **(default:** **empty)**
       The name of the Postfix SMTP server's local SASL authentication realm.

       By default, the local authentication realm name is the null string.

       Examples:

       smtpd_sasl_local_domain = $mydomain
       smtpd_sasl_local_domain = $myhostname

**smtpd**___**sasl**___**mechanism**___**filter** **(default:** **!external,** **static:rest)**
       If  non-empty,  a  filter  for the SASL mechanism names that the Postfix SMTP server will an‐
       nounce in the EHLO response. By default, the Postfix SMTP server will not announce the EXTER‐
       NAL mechanism, because Postfix support for that is not implemented.

       Specify  mechanism  names, "/file/name" patterns, or "type:table" lookup tables, separated by
       comma or whitespace. The right-hand side result from "type:table" lookups is ignored. Specify
       "!pattern" to exclude a mechanism name from the list.

       Examples:

       smtpd_sasl_mechanism_filter = !external, !gssapi, static:rest
       smtpd_sasl_mechanism_filter = login, plain
       smtpd_sasl_mechanism_filter = /etc/postfix/smtpd_mechs

       This feature is available in Postfix 3.6 and later.

**smtpd**___**sasl**___**path** **(default:** **smtpd)**
       Implementation-specific  information  that the Postfix SMTP server passes through to the SASL
       plug-in implementation that is selected with **smtpd**___**sasl**___**type**.  Typically this  specifies  the
       name of a configuration file or rendezvous point.

       This  feature  is  available  in  Postfix  2.3  and  later. In earlier releases it was called
       **smtpd**___**sasl**___**application**___**name**.

**smtpd**___**sasl**___**response**___**limit** **(default:** **12288)**
       The maximum length of a SASL client's response to a  server  challenge.   When  the  client's
       "initial  response"  is  longer than the normal limit for SMTP commands, the client must omit
       its initial response, and wait for an empty server challenge; it can  then  send  what  would
       have  been  its  "initial response" as a response to the empty server challenge.  RFC4954 re‐
       quires the server to accept client responses up to at least 12288  octets  of  base64-encoded
       text.  The default value is therefore also the minimum value accepted for this parameter.

       This  feature  is available in Postfix 3.4 and later. Prior versions use "line_length_limit",
       which may need to be raised to accommodate larger client responses, as  may  be  needed  with
       GSSAPI authentication of Windows AD users who are members of many groups.

**smtpd**___**sasl**___**security**___**options** **(default:** **noanonymous)**
       Postfix  SMTP  server SASL security options; as of Postfix 2.3 the list of available features
       depends on the SASL server implementation that is selected with **smtpd**___**sasl**___**type**.

       The following security features are defined for the **cyrus** server SASL implementation:

       Restrict what authentication mechanisms the Postfix SMTP server will  offer  to  the  client.
       The list of available authentication mechanisms is system dependent.

       Specify zero or more of the following:

### noplaintext
              Disallow methods that use plaintext passwords.

### noactive
              Disallow methods subject to active (non-dictionary) attack.

### nodictionary
              Disallow methods subject to passive (dictionary) attack.

### noanonymous
              Disallow methods that allow anonymous authentication.

       **forward**___**secrecy**
              Only allow methods that support forward secrecy (Dovecot only).

       **mutual**___**auth**
              Only  allow  methods that provide mutual authentication (not available with Cyrus SASL
              version 1).

       By default, the Postfix SMTP server accepts plaintext passwords but not anonymous logins.

       Warning: it appears that clients try authentication methods in the order as advertised by the
       server  (e.g., PLAIN ANONYMOUS CRAM-MD5) which means that if you disable plaintext passwords,
       clients will log in anonymously, even when they should be able to use CRAM-MD5.  So,  if  you
       disable plaintext logins, disable anonymous logins too.  Postfix treats anonymous login as no
       authentication.

       Example:

       smtpd_sasl_security_options = noanonymous, noplaintext

**smtpd**___**sasl**___**service** **(default:** **smtp)**
       The service name that is passed to the SASL plug-in that is selected with **smtpd**___**sasl**___**type** and
       **smtpd**___**sasl**___**path**.

       This  feature  is  available in Postfix 2.11 and later. Prior versions behave as if "**smtp**" is
       specified.

**smtpd**___**sasl**___**tls**___**security**___**options** **(default:** **$smtpd**___**sasl**___**security**___**options)**
       The SASL authentication security options that the Postfix SMTP server uses for TLS  encrypted
       SMTP sessions.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**sasl**___**type** **(default:** **cyrus)**
       The  SASL plug-in type that the Postfix SMTP server should use for authentication. The avail‐
       able types are listed with the "**postconf** **-a**" command.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**sender**___**login**___**maps** **(default:** **empty)**
       Optional lookup table with the SASL login names that own the sender (MAIL FROM) addresses.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be  searched  in the specified order until a match is found.  With lookups from indexed files
       such as DB or DBM, or from networked tables such as NIS, LDAP or SQL,  the  following  search
       operations are done with a sender address of _user@domain_:

       1) _user@domain_
              This table lookup is always done and has the highest precedence.

       2) _user_
              This table lookup is done only when the _domain_ part of the sender address matches $my‐
              origin, $mydestination, $inet_interfaces or $proxy_interfaces.

       3) _@domain_
              This table lookup is done last and has the lowest precedence.

       In all cases the result of table lookup must be either "not found" or a list  of  SASL  login
       names separated by comma and/or whitespace.

**smtpd**___**sender**___**restrictions** **(default:** **empty)**
       Optional  restrictions  that  the Postfix SMTP server applies in the context of a client MAIL
       FROM command.  See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP  access  restric‐
       tion lists" for a discussion of evaluation context and time.

       The default is to permit everything.

       Specify  a  list of restrictions, separated by commas and/or whitespace.  Continue long lines
       by starting the next line with whitespace.  Restrictions are applied in the order  as  speci‐
       fied; the first restriction that matches wins.

       The  following  restrictions  are  specific to the sender address received with the MAIL FROM
       command.

       **check**___**sender**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the MAIL FROM address, domain, parent  do‐
              mains, or localpart@, and execute the corresponding action.

       **check**___**sender**___**a**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the IP addresses for the MAIL FROM domain,
              and execute the corresponding action.  Note: a result  of  "OK"  is  not  allowed  for
              safety  reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
              This feature is available in Postfix 3.0 and later.

       **check**___**sender**___**mx**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the MX hosts for the MAIL FROM domain, and
              execute  the  corresponding  action.   If  no  MX  record  is found, look up A or AAAA
              records, just like the Postfix SMTP client would. Note: a result of "OK"  is  not  al‐
              lowed  for  safety reasons. Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 2.1 and later.

       **check**___**sender**___**ns**___**access** _type:table_
              Search the specified [**access**(5)](https://www.chedong.com/phpMan.php/man/access/5/markdown) database for the DNS servers for the MAIL FROM  domain,
              and  execute  the  corresponding  action.   Note:  a result of "OK" is not allowed for
              safety reasons. Instead, use DUNNO in order to exclude specific hosts from  denylists.
              This feature is available in Postfix 2.1 and later.

       **reject**___**authenticated**___**sender**___**login**___**mismatch**
              Enforces  the reject_sender_login_mismatch restriction for authenticated clients only.
              This feature is available in Postfix version 2.1 and later.

       **reject**___**known**___**sender**___**login**___**mismatch**
              Apply the reject_sender_login_mismatch restriction only to MAIL  FROM  addresses  that
              are  known  in $smtpd_sender_login_maps.  This feature is available in Postfix version
              2.11 and later.

       **reject**___**non**___**fqdn**___**sender**
              Reject the request when the MAIL FROM address  specifies  a  domain  that  is  not  in
              fully-qualified domain form as required by the RFC.
              The  non_fqdn_reject_code  parameter specifies the response code for rejected requests
              (default: 504).

       **reject**___**rhsbl**___**sender** _rbl_domain=d.d.d.d_
              Reject the request when the MAIL FROM domain is listed with the A record "_d.d.d.d_" un‐
              der  _rbl_domain_ (Postfix version 2.1 and later only).  Each "_d_" is a number, or a pat‐
              tern inside "[]" that contains one or more  ";"-separated  numbers  or  number..number
              ranges  (Postfix version 2.8 and later). If no "_=d.d.d.d_" is specified, reject the re‐
              quest when the MAIL FROM domain is listed with any A record under _rbl_domain_.
              The maps_rbl_reject_code parameter specifies the response code for  rejected  requests
              (default:   554);  the default_rbl_reply parameter specifies the default server reply;
              and the rbl_reply_maps parameter specifies  tables  with  server  replies  indexed  by
              _rbl_domain_.  This feature is available in Postfix 2.0 and later.

       **reject**___**sender**___**login**___**mismatch**
              Reject  the request when $smtpd_sender_login_maps specifies an owner for the MAIL FROM
              address, but the client is not (SASL) logged in as that MAIL FROM  address  owner;  or
              when  the  client  is (SASL) logged in, but the client login name doesn't own the MAIL
              FROM address according to $smtpd_sender_login_maps.

       **reject**___**unauthenticated**___**sender**___**login**___**mismatch**
              Enforces the  reject_sender_login_mismatch  restriction  for  unauthenticated  clients
              only. This feature is available in Postfix version 2.1 and later.

       **reject**___**unknown**___**sender**___**domain**
              Reject  the  request when Postfix is not final destination for the sender address, and
              the MAIL FROM domain has 1) no DNS MX and no DNS A record, or 2) a malformed MX record
              such as a record with a zero-length MX hostname (Postfix version 2.3 and later).
              The  reply is specified with the unknown_address_reject_code parameter (default: 450),
              unknown_address_tempfail_action (default: defer_if_permit), or  550  (nullmx,  Postfix
              3.0 and later). See the respective parameter descriptions for details.

       **reject**___**unlisted**___**sender**
              Reject  the  request when the MAIL FROM address is not listed in the list of valid re‐
              cipients for its domain class. See the smtpd_reject_unlisted_sender parameter descrip‐
              tion for details.  This feature is available in Postfix 2.1 and later.

       **reject**___**unverified**___**sender**
              Reject  the request when mail to the MAIL FROM address is known to bounce, or when the
              sender address destination is not reachable.  Address verification information is man‐
              aged by the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) server; see the ADDRESS_VERIFICATION_README file for details.
              The unverified_sender_reject_code parameter specifies the numerical response code when
              an address is known to bounce (default: 450, change into 550 when  you  are  confident
              that it is safe to do so).
              The unverified_sender_defer_code specifies the numerical response code when an address
              probe failed due to a temporary problem (default: 450).
              The unverified_sender_tempfail_action parameter specifies  the  action  after  address
              probe failure due to a temporary problem (default: defer_if_permit).
              This feature breaks for aliased addresses with "enable_original_recipient = no" (Post‐
              fix <= 3.2).
              This feature is available in Postfix 2.1 and later.

       Other restrictions that are valid in this context:

       •      Generic restrictions that can be used in any SMTP  command  context,  described  under
              smtpd_client_restrictions.

       •      SMTP  command  specific  restrictions  described  under  smtpd_client_restrictions and
              smtpd_helo_restrictions.

       •      SMTP command specific restrictions described under smtpd_recipient_restrictions.  When
              recipient  restrictions  are  listed under smtpd_sender_restrictions, they have effect
              only with "smtpd_delay_reject = yes", so that $smtpd_sender_restrictions is  evaluated
              at the time of the RCPT TO command.

       Examples:

       smtpd_sender_restrictions = reject_unknown_sender_domain
       smtpd_sender_restrictions = reject_unknown_sender_domain,
           check_sender_access hash:/etc/postfix/access

**smtpd**___**service**___**name** **(default:** **smtpd)**
       The internal service that [**postscreen**(8)](https://www.chedong.com/phpMan.php/man/postscreen/8/markdown) hands off allowed connections to. In a future version
       there may be different classes of SMTP service.

       This feature is available in Postfix 2.8.

**smtpd**___**soft**___**error**___**limit** **(default:** **10)**
       The number of errors a remote SMTP client is allowed to make without delivering  mail  before
       the Postfix SMTP server slows down all its responses.

       •      With  Postfix  version  2.1 and later, the Postfix SMTP server delays all responses by
              $smtpd_error_sleep_time seconds.

       •      With Postfix versions 2.0 and earlier, the Postfix SMTP server delays all responses by
              (number of errors) seconds.

**smtpd**___**starttls**___**timeout** **(default:** **see** **postconf** **-d** **output)**
       The time limit for Postfix SMTP server write and read operations during TLS startup and shut‐
       down handshake procedures. The current default value is stress-dependent. Before Postfix ver‐
       sion 2.8, it was fixed at 300s.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**timeout** **(default:** **normal:** **300s,** **overload:** **10s)**
       The  time  limit  for  sending a Postfix SMTP server response and for receiving a remote SMTP
       client request. Normally the default limit is 300s, but it changes  under  overload  to  just
       10s.  With  Postfix  2.5 and earlier, the SMTP server always uses a time limit of 300s by de‐
       fault.

       Note: if you set SMTP time limits to very large values you may  have  to  update  the  global
       ipc_timeout parameter.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

**smtpd**___**tls**___**CAfile** **(default:** **empty)**
       A file containing (PEM format) CA certificates of root CAs trusted to sign either remote SMTP
       client certificates or intermediate CA certificates.  These are loaded into memory before the
       [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) server enters the chroot jail. If the number of trusted roots is large, consider us‐
       ing  smtpd_tls_CApath  instead, but note that the latter directory must be present in the ch‐
       root jail if the [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) server is chrooted. This file may  also  be  used  to  augment  the
       server  certificate  trust chain, but it is best to include all the required certificates di‐
       rectly in the server certificate file.

       Specify "smtpd_tls_CAfile = /path/to/system_CA_file" to use ONLY the system-supplied  default
       Certification Authority certificates.

       Specify  "tls_append_default_CA  =  no" to prevent Postfix from appending the system-supplied
       default CAs and trusting third-party certificates.

       By  default  (see  smtpd_tls_ask_ccert),  client  certificates   are   not   requested,   and
       smtpd_tls_CAfile  should remain empty. If you do make use of client certificates, the distin‐
       guished names (DNs) of the Certification Authorities listed in smtpd_tls_CAfile are  sent  to
       the  remote  SMTP client in the client certificate request message. MUAs with multiple client
       certificates may use the list of preferred Certification Authorities to  select  the  correct
       client certificate.  You may want to put your "preferred" CA or CAs in this file, and install
       other trusted CAs in $smtpd_tls_CApath.

       Example:

       smtpd_tls_CAfile = /etc/postfix/CAcert.pem

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**CApath** **(default:** **empty)**
       A directory containing (PEM format) CA certificates of root CAs trusted to sign either remote
       SMTP  client certificates or intermediate CA certificates. Do not forget to create the neces‐
       sary "hash" links with, for example, "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs". To  use
       smtpd_tls_CApath in chroot mode, this directory (or a copy) must be inside the chroot jail.

       Specify "smtpd_tls_CApath = /path/to/system_CA_directory" to use ONLY the system-supplied de‐
       fault Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from  appending  the  system-supplied
       default CAs and trusting third-party certificates.

       By   default   (see   smtpd_tls_ask_ccert),   client  certificates  are  not  requested,  and
       smtpd_tls_CApath should remain empty. In contrast to smtpd_tls_CAfile, DNs  of  Certification
       Authorities installed in $smtpd_tls_CApath are not included in the client certificate request
       message. MUAs with multiple client certificates may use the list of  preferred  Certification
       Authorities  to  select the correct client certificate.  You may want to put your "preferred"
       CA or CAs in $smtpd_tls_CAfile, and install the remaining trusted CAs in $smtpd_tls_CApath.

       Example:

       smtpd_tls_CApath = /etc/postfix/certs

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**always**___**issue**___**session**___**ids** **(default:** **yes)**
       Force the Postfix SMTP server to issue a TLS session id, even when  TLS  session  caching  is
       turned  off  (smtpd_tls_session_cache_database  is  empty).  This behavior is compatible with
       Postfix < 2.3.

       With Postfix 2.3 and later the Postfix SMTP server can disable session id generation when TLS
       session  caching is turned off. This keeps remote SMTP clients from caching sessions that al‐
       most certainly cannot be re-used.

       By default, the Postfix SMTP server always generates TLS session ids.  This  works  around  a
       known  defect in mail client applications such as MS Outlook, and may also prevent interoper‐
       ability issues with other MTAs.

       Example:

       smtpd_tls_always_issue_session_ids = no

       This feature is available in Postfix 2.3 and later.

**smtpd**___**tls**___**ask**___**ccert** **(default:** **no)**
       Ask a remote SMTP client for a client certificate. This information is needed for certificate
       based mail relaying with, for example, the permit_tls_clientcerts feature.

       Some  clients  such  as Netscape will either complain if no certificate is available (for the
       list of CAs in $smtpd_tls_CAfile) or will offer multiple client certificates to choose  from.
       This may be annoying, so this option is "off" by default.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**auth**___**only** **(default:** **no)**
       When  TLS  encryption  is optional in the Postfix SMTP server, do not announce or accept SASL
       authentication over unencrypted connections.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**ccert**___**verifydepth** **(default:** **9)**
       The verification depth for remote SMTP client certificates. A depth of 1 is sufficient if the
       issuing CA is listed in a local CA file.

       The  default  verification  depth  is  9 (the OpenSSL default) for compatibility with earlier
       Postfix behavior. Prior to Postfix 2.5, the default value was 5, but the limit was not  actu‐
       ally  enforced.  If  you have set this to a lower non-default value, certificates with longer
       trust chains may now fail to verify. Certificate chains with 1 or 2 CAs  are  common,  deeper
       chains  are  more  rare  and  any  number between 5 and 9 should suffice in practice. You can
       choose a lower number if, for example, you trust certificates directly signed by  an  issuing
       CA but not any CAs it delegates to.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**cert**___**file** **(default:** **empty)**
       File  with the Postfix SMTP server RSA certificate in PEM format.  This file may also contain
       the Postfix SMTP server private RSA key.  With Postfix >= 3.4 the preferred way to  configure
       server keys and certificates is via the "smtpd_tls_chain_files" parameter.

       Public  Internet  MX hosts without certificates signed by a "reputable" CA must generate, and
       be prepared to present to most clients, a self-signed or private-CA signed  certificate.  The
       client  will  not be able to authenticate the server, but unless it is running Postfix 2.3 or
       similar software, it will still insist on a server certificate.

       For servers that are **not** public Internet MX hosts, Postfix supports  configurations  with  no
       certificates. This entails the use of just the anonymous TLS ciphers, which are not supported
       by typical SMTP clients. Since some clients may not fall back to plain text after a TLS hand‐
       shake  failure,  a  certificate-less Postfix SMTP server will be unable to receive email from
       some TLS-enabled clients. To avoid accidental configurations with  no  certificates,  Postfix
       enables   certificate-less   operation   only   when   the   administrator   explicitly  sets
       "smtpd_tls_cert_file = none". This ensures that new Postfix SMTP server  configurations  will
       not accidentally enable TLS without certificates.

       Note  that server certificates are not optional in TLS 1.3. To run without certificates you'd
       have to disable the TLS 1.3 protocol by including  '!TLSv1.3'  in  "smtpd_tls_protocols"  and
       perhaps also "smtpd_tls_mandatory_protocols".  It is simpler instead to just configure a cer‐
       tificate chain.  Certificate-less operation is not recommended.

       Both RSA and DSA certificates are supported.  When both types are present,  the  cipher  used
       determines  which  certificate  will  be  presented  to the client.  For Netscape and OpenSSL
       clients without special cipher choices the RSA certificate is preferred.

       To enable a remote SMTP client to verify the Postfix SMTP server certificate, the issuing  CA
       certificates  must  be made available to the client. You should include the required certifi‐
       cates in the server certificate file, the server certificate first, then  the  issuing  CA(s)
       (bottom-up order).

       Example:  the  certificate for "server.example.com" was issued by "intermediate CA" which it‐
       self has a certificate of "root CA".  Create the server.pem file  with  "cat  server_cert.pem
       intermediate_CA.pem root_CA.pem > server.pem".

       If  you  also want to verify client certificates issued by these CAs, you can add the CA cer‐
       tificates to the smtpd_tls_CAfile, in which case it is not necessary  to  have  them  in  the
       smtpd_tls_cert_file, smtpd_tls_dcert_file (obsolete) or smtpd_tls_eccert_file.

       A  certificate  supplied  here must be usable as an SSL server certificate and hence pass the
       "openssl verify -purpose sslserver ..." test.

       Example:

       smtpd_tls_cert_file = /etc/postfix/server.pem

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**chain**___**files** **(default:** **empty)**
       List of one or more PEM files, each holding one or more private keys directly followed  by  a
       corresponding  certificate  chain.  The file names are separated by commas and/or whitespace.
       This parameter obsoletes the legacy algorithm-specific key  and  certificate  file  settings.
       When  this parameter is non-empty, the legacy parameters are ignored, and a warning is logged
       if any are also non-empty.

       With the proliferation of multiple private key algorithms-which, as of OpenSSL 1.1.1, include
       DSA  (obsolete), RSA, ECDSA, Ed25519 and Ed448-it is increasingly impractical to use separate
       parameters to configure the key and certificate chain for each algorithm.  Therefore, Postfix
       now  supports  storing multiple keys and corresponding certificate chains in a single file or
       in a set of files.

       Each key must appear **immediately** **before** the corresponding certificate, optionally followed by
       additional issuer certificates that complete the certificate chain for that key.  When multi‐
       ple files are specified, they are equivalent to a single file that is concatenated from those
       files  in  the given order.  Thus, while a key must always precede its certificate and issuer
       chain, it can be in a separate file, so long as that file is listed  immediately  before  the
       file  that  holds  the corresponding certificate chain.  Once all the files are concatenated,
       the sequence of PEM objects must be: _key1,_ _cert1,_ _[chain1],_ _key2,_ _cert2,_ _[chain2],_ _...,_ _keyN,_
       _certN,_ _[chainN]._

       Storing  the  private key in the same file as the corresponding certificate is more reliable.
       With the key and certificate in separate files, there is a chance that during key rollover  a
       Postfix  process  might  load  a  private  key and certificate from separate files that don't
       match.  Various operational errors may even result in a persistent  broken  configuration  in
       which the certificate does not match the private key.

       The  file  or  files must contain at most one key of each type.  If, for example, two or more
       RSA keys and corresponding chains are listed, depending on the version of OpenSSL either only
       the  last  one  will  be  used  or  an  configuration error may be detected.  Note that while
       "Ed25519" and "Ed448" are considered separate algorithms, the various ECDSA curves (typically
       one of prime256v1, secp384r1 or secp521r1) are considered as different parameters of a single
       "ECDSA" algorithm, so it is not presently possible to configure keys for more than one  ECDSA
       curve.

       RSA  is  still  the most widely supported algorithm.  Presently (late 2018), ECDSA support is
       common, but not yet universal, and Ed25519 and Ed448 support is mostly absent.  Therefore, an
       RSA  key  should  generally be configured, along with any additional keys for the other algo‐
       rithms when desired.

       Example (separate files for each key and corresponding certificate chain):

           /etc/postfix/main.cf:
               smtpd_tls_chain_files =
                   ${config_directory}/ed25519.pem,
                   ${config_directory}/ed448.pem,
                   ${config_directory}/rsa.pem

           /etc/postfix/ed25519.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----

           /etc/postfix/ed448.pem:
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----

           /etc/postfix/rsa.pem:
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       Example (all keys and certificates in a single file):

           /etc/postfix/main.cf:
               smtpd_tls_chain_files = ${config_directory}/chains.pem

           /etc/postfix/chains.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       This feature is available in Postfix 3.4 and later.

**smtpd**___**tls**___**cipherlist** **(default:** **empty)**
       Obsolete Postfix < 2.3 control for the Postfix SMTP server TLS cipher list.  It  is  easy  to
       create  interoperability problems by choosing a non-default cipher list. Do not use a non-de‐
       fault TLS cipherlist for MX hosts on the public Internet. Clients that begin  the  TLS  hand‐
       shake,  but  are unable to agree on a common cipher, may not be able to send any email to the
       SMTP server. Using a restricted cipher list may be more appropriate for a dedicated MSA or an
       internal  mailhub, where one can exert some control over the TLS software and settings of the
       connecting clients.

       **Note:** do not use "" quotes around the parameter value.

       This feature is available with Postfix version 2.2. It is  not  used  with  Postfix  2.3  and
       later; use smtpd_tls_mandatory_ciphers instead.

**smtpd**___**tls**___**ciphers** **(default:** **medium)**
       The minimum TLS cipher grade that the Postfix SMTP server will use with opportunistic TLS en‐
       cryption. Cipher types listed in smtpd_tls_exclude_ciphers are excluded from the base defini‐
       tion  of the selected cipher grade.  The default value is "medium" for Postfix releases after
       the middle of 2015, "export" for older releases.

       When TLS is mandatory the cipher grade is chosen via the smtpd_tls_mandatory_ciphers configu‐
       ration parameter, see there for syntax details.

       This  feature  is  available in Postfix 2.6 and later. With earlier Postfix releases only the
       smtpd_tls_mandatory_ciphers parameter is implemented, and opportunistic TLS always uses  "ex‐
       port" or better (i.e. all) ciphers.

**smtpd**___**tls**___**dcert**___**file** **(default:** **empty)**
       File  with the Postfix SMTP server DSA certificate in PEM format.  This file may also contain
       the Postfix SMTP server private DSA key.  The DSA algorithm is obsolete  and  should  not  be
       used.

       See the discussion under smtpd_tls_cert_file for more details.

       Example:

       smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**dh1024**___**param**___**file** **(default:** **empty)**
       File with DH parameters that the Postfix SMTP server should use with non-export EDH ciphers.

       The best-practice choice of parameters uses a 2048-bit prime.  This is fine, despite the his‐
       torical "1024" in the parameter name.  Do not be tempted to use much larger  values,  perfor‐
       mance  degrades  quickly,  and  you  may also cease to interoperate with some mainstream SMTP
       clients.  As of Postfix 3.1, the compiled-in default  prime  is  2048-bits,  and  it  is  not
       strictly necessary, though perhaps somewhat beneficial to generate custom DH parameters.

       Instead  of using the exact same parameter sets as distributed with other TLS packages, it is
       more secure to generate your own set of parameters with something  like  the  following  com‐
       mands:

           openssl dhparam -out /etc/postfix/dh2048.pem 2048
           openssl dhparam -out /etc/postfix/dh1024.pem 1024
           # As of Postfix 3.6, export-grade 512-bit DH parameters are no longer
           # supported or needed.
           openssl dhparam -out /etc/postfix/dh512.pem 512

       It  is  safe to share the same DH parameters between multiple Postfix instances.  If you pre‐
       fer, you can generate separate parameters for each instance.

       If you want to take maximal advantage of ciphers that offer forward secrecy see  the  Getting
       started  section  of FORWARD_SECRECY_README.  The full document conveniently presents all in‐
       formation about Postfix "perfect" forward secrecy support in one place: what forward  secrecy
       is, how to tweak settings, and what you can expect to see when Postfix uses ciphers with for‐
       ward secrecy.

       Example:

       smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem

       This feature is available with Postfix version 2.2.

**smtpd**___**tls**___**dh512**___**param**___**file** **(default:** **empty)**
       File with DH parameters that the Postfix SMTP server should use  with  export-grade  EDH  ci‐
       phers.  The default SMTP server cipher grade is "medium" with Postfix releases after the mid‐
       dle of 2015, and as a result export-grade cipher suites are by default not used.

       With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no longer supported, and this
       parameter is silently ignored.

       See also the discussion under the smtpd_tls_dh1024_param_file configuration parameter.

       Example:

       smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem

       This feature is available with Postfix version 2.2.

**smtpd**___**tls**___**dkey**___**file** **(default:** **$smtpd**___**tls**___**dcert**___**file)**
       File  with  the Postfix SMTP server DSA private key in PEM format.  This file may be combined
       with the Postfix SMTP server DSA certificate file specified with  $smtpd_tls_dcert_file.  The
       DSA algorithm is obsolete and should not be used.

       The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
       permissions should grant read-only access to the system superuser account  ("root"),  and  no
       access to anyone else.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**eccert**___**file** **(default:** **empty)**
       File  with  the Postfix SMTP server ECDSA certificate in PEM format.  This file may also con‐
       tain the Postfix SMTP server private ECDSA key.  With Postfix >= 3.4  the  preferred  way  to
       configure server keys and certificates is via the "smtpd_tls_chain_files" parameter.

       See the discussion under smtpd_tls_cert_file for more details.

       Example:

       smtpd_tls_eccert_file = /etc/postfix/ecdsa-scert.pem

       This  feature is available in Postfix 2.6 and later, when Postfix is compiled and linked with
       OpenSSL 1.0.0 or later.

**smtpd**___**tls**___**eckey**___**file** **(default:** **$smtpd**___**tls**___**eccert**___**file)**
       File with the Postfix SMTP server ECDSA private key in PEM format.  This file may be combined
       with  the  Postfix  SMTP server ECDSA certificate file specified with $smtpd_tls_eccert_file.
       With Postfix >= 3.4 the preferred way to configure server keys and certificates  is  via  the
       "smtpd_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
       permissions should grant read-only access to the system superuser account  ("root"),  and  no
       access to anyone else.

       This  feature is available in Postfix 2.6 and later, when Postfix is compiled and linked with
       OpenSSL 1.0.0 or later.

**smtpd**___**tls**___**eecdh**___**grade** **(default:** **see** **postconf** **-d** **output)**
       The Postfix SMTP server security grade for ephemeral  elliptic-curve  Diffie-Hellman  (EECDH)
       key exchange.   As of Postfix 3.6, the value of this parameter is always ignored, and Postfix
       behaves as though the **auto** value (described below) was chosen.

       The available choices are:

       **auto**   Use the most preferred curve that is supported by both  the  client  and  the  server.
              This  setting requires Postfix >= 3.2 compiled and linked with OpenSSL >= 1.0.2.  This
              is the default setting under the above conditions (and  the  only  setting  used  with
              Postfix >= 3.6).

       **none**   Don't use EECDH. Ciphers based on EECDH key exchange will be disabled. This is the de‐
              fault in Postfix versions 2.6 and 2.7.

       **strong** Use EECDH with approximately 128 bits of security at a reasonable computational  cost.
              This is the default in Postfix versions 2.8-3.5.

       **ultra**  Use  EECDH  with  approximately 192 bits of security at computational cost that is ap‐
              proximately twice as high as 128 bit strength ECC.

       If you want to take maximal advantage of ciphers that offer forward secrecy see  the  Getting
       started  section  of FORWARD_SECRECY_README.  The full document conveniently presents all in‐
       formation about Postfix "perfect" forward secrecy support in one place: what forward  secrecy
       is, how to tweak settings, and what you can expect to see when Postfix uses ciphers with for‐
       ward secrecy.

       This feature is available in Postfix 2.6 and later, when  it  is  compiled  and  linked  with
       OpenSSL 1.0.0 or later on platforms where EC algorithms have not been disabled by the vendor.

**smtpd**___**tls**___**exclude**___**ciphers** **(default:** **empty)**
       List  of ciphers or cipher types to exclude from the SMTP server cipher list at all TLS secu‐
       rity levels. Excluding valid ciphers can create interoperability problems. DO NOT exclude ci‐
       phers unless it is essential to do so. This is not an OpenSSL cipherlist; it is a simple list
       separated by whitespace and/or commas. The elements are a single cipher, or one or  more  "+"
       separated  cipher  properties, in which case only ciphers matching **all** the properties are ex‐
       cluded.

       Examples (some of these will cause problems):

           smtpd_tls_exclude_ciphers = aNULL
           smtpd_tls_exclude_ciphers = MD5, DES
           smtpd_tls_exclude_ciphers = DES+MD5
           smtpd_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
           smtpd_tls_exclude_ciphers = kEDH+aRSA

       The first setting disables anonymous ciphers. The next setting disables ciphers that use  the
       MD5  digest algorithm or the (single) DES encryption algorithm. The next setting disables ci‐
       phers that use MD5 and DES together.  The next setting disables the two ciphers  "AES256-SHA"
       and  "DES-CBC3-MD5".  The  last setting disables ciphers that use "EDH" key exchange with RSA
       authentication.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**tls**___**fingerprint**___**digest** **(default:** **see** **postconf** **-d** **output)**
       The message digest algorithm to construct remote SMTP client-certificate fingerprints or pub‐
       lic   key   fingerprints   (Postfix   2.9   and   later)   for  **check**___**ccert**___**access**  and  **per**‐‐
       **mit**___**tls**___**clientcerts**.

       The default algorithm is **sha256** with Postfix >= 3.6 and the **compatibility**___**level** set to 3.6 or
       higher. With Postfix <= 3.5, the default algorithm is **md5**.

       The  best-practice  algorithm  is  now **sha256**. Recent advances in hash function cryptanalysis
       have led to md5 and sha1 being deprecated in favor of sha256.  However, as long as there  are
       no  known "second pre-image" attacks against the older algorithms, their use in this context,
       though not recommended, is still likely safe.

       While additional digest algorithms are often available with OpenSSL's libcrypto,  only  those
       used by libssl in SSL cipher suites are available to Postfix.  You'll likely find support for
       md5, sha1, sha256 and sha512.

       To find the fingerprint of a specific certificate file, with  a  specific  digest  algorithm,
       run:

           $ openssl x509 -noout -fingerprint -_digest_ -in _certfile_.pem

       The text to the right of "=" sign is the desired fingerprint.  For example:

           $ openssl x509 -noout -fingerprint -sha256 -in cert.pem
           SHA256 Fingerprint=D4:6A:AB:19:24:...:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

       To extract the public key fingerprint from an X.509 certificate, you need to extract the pub‐
       lic key from the certificate and compute the appropriate digest of its DER (ASN.1)  encoding.
       With  OpenSSL  the  "-pubkey"  option of the "x509" command extracts the public key always in
       "PEM" format. We pipe the result to another OpenSSL command that converts the key to DER  and
       then to the "dgst" command to compute the fingerprint.

       Example:

           $ openssl x509 -in cert.pem -noout -pubkey |
               openssl pkey -pubin -outform DER |
               openssl dgst -sha256 -c
           (stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58

       The Postfix SMTP server and client log the peer (leaf) certificate fingerprint and public key
       fingerprint when the TLS loglevel is 2 or higher.

       Example: client-certificate access table, with sha256 fingerprints:

           /etc/postfix/main.cf:
               smtpd_tls_fingerprint_digest = sha256
               smtpd_client_restrictions =
                   check_ccert_access hash:/etc/postfix/access,
                   reject
           /etc/postfix/access:
               # Action folded to next line...
               AF:88:7C:AD:51:95:6F:36:96:...:01:FB:2E:48:CD:AB:49:25:A2:3B
                   OK
               85:16:78:FD:73:6E:CE:70:E0:...:5F:0D:3C:C8:6D:C4:2C:24:59:E1
                   permit_auth_destination

       This feature is available in Postfix 2.5 and later.

**smtpd**___**tls**___**key**___**file** **(default:** **$smtpd**___**tls**___**cert**___**file)**
       File with the Postfix SMTP server RSA private key in PEM format.  This file may  be  combined
       with  the Postfix SMTP server RSA certificate file specified with $smtpd_tls_cert_file.  With
       Postfix >= 3.4 the preferred way to  configure  server  keys  and  certificates  is  via  the
       "smtpd_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
       permissions should grant read-only access to the system superuser account  ("root"),  and  no
       access to anyone else.

**smtpd**___**tls**___**loglevel** **(default:** **0)**
       Enable  additional  Postfix SMTP server logging of TLS activity.  Each logging level also in‐
       cludes the information that is logged at a lower logging level.

              0 Disable logging of TLS activity.

              1 Log only a summary message on TLS handshake completion - no logging of  client  cer‐
              tificate trust-chain verification errors if client certificate verification is not re‐
              quired.  With Postfix 2.8 and earlier, log the summary message, peer certificate  sum‐
              mary information and unconditionally log trust-chain verification errors.

              2 Also log levels during TLS negotiation.

              3 Also log hexadecimal and ASCII dump of TLS negotiation process.

              4 Also log hexadecimal and ASCII dump of complete transmission after STARTTLS.

       Do  not  use "smtpd_tls_loglevel = 2" or higher except in case of problems. Use of loglevel 4
       is strongly discouraged.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**mandatory**___**ciphers** **(default:** **medium)**
       The minimum TLS cipher grade that the Postfix SMTP server will use with mandatory TLS encryp‐
       tion.  The default grade ("medium") is sufficiently strong that any benefit from globally re‐
       stricting TLS sessions to a more stringent grade is likely negligible, especially  given  the
       fact  that many implementations still do not offer any stronger ("high" grade) ciphers, while
       those that do, will always use "high" grade ciphers. So insisting on "high" grade ciphers  is
       generally  counter-productive.  Allowing  "export"  or  "low" ciphers is typically not a good
       idea, as systems limited to just these are  limited  to  obsolete  browsers.  No  known  SMTP
       clients fail to support at least one "medium" or "high" grade cipher.

       The following cipher grades are supported:

       **export** Enable "EXPORT" grade or stronger OpenSSL ciphers.  The underlying cipherlist is spec‐
              ified via the tls_export_cipherlist configuration parameter, which  you  are  strongly
              encouraged to not change.  This choice is insecure and SHOULD NOT be used.

       **low**    Enable "LOW" grade or stronger OpenSSL ciphers. The underlying cipherlist is specified
              via the tls_low_cipherlist configuration parameter, which you are strongly  encouraged
              to not change.  This choice is insecure and SHOULD NOT be used.

       **medium** Enable "MEDIUM" grade or stronger OpenSSL ciphers. These use 128-bit or longer symmet‐
              ric bulk-encryption keys. This is the default minimum strength for mandatory  TLS  en‐
              cryption. The underlying cipherlist is specified via the tls_medium_cipherlist config‐
              uration parameter, which you are strongly encouraged to not change.

       **high**   Enable only "HIGH" grade OpenSSL ciphers. The underlying cipherlist is  specified  via
              the  tls_high_cipherlist configuration parameter, which you are strongly encouraged to
              not change.

       **null**   Enable only the "NULL" OpenSSL ciphers, these provide authentication  without  encryp‐
              tion.  This setting is only appropriate in the rare case that all clients are prepared
              to use NULL ciphers (not normally enabled in TLS clients). The  underlying  cipherlist
              is  specified  via  the  tls_null_cipherlist  configuration  parameter,  which you are
              strongly encouraged to not change.

       Cipher types listed in smtpd_tls_mandatory_exclude_ciphers or  smtpd_tls_exclude_ciphers  are
       excluded from the base definition of the selected cipher grade. See smtpd_tls_ciphers for ci‐
       pher controls that apply to opportunistic TLS.

       The underlying cipherlists for grades other than "null" include anonymous ciphers, but  these
       are automatically filtered out if the server is configured to ask for remote SMTP client cer‐
       tificates.  You are very unlikely to need to take any steps  to  exclude  anonymous  ciphers,
       they are excluded automatically as required.  If you must exclude anonymous ciphers even when
       Postfix does not need or use peer certificates, set "smtpd_tls_exclude_ciphers =  aNULL".  To
       exclude anonymous ciphers only when TLS is enforced, set "smtpd_tls_mandatory_exclude_ciphers
       = aNULL".

       This feature is available in Postfix 2.3 and later.

**smtpd**___**tls**___**mandatory**___**exclude**___**ciphers** **(default:** **empty)**
       Additional list of ciphers or cipher types to exclude from the  Postfix  SMTP  server  cipher
       list  at mandatory TLS security levels.  This list works in addition to the exclusions listed
       with smtpd_tls_exclude_ciphers (see there for syntax details).

       This feature is available in Postfix 2.3 and later.

**smtpd**___**tls**___**mandatory**___**protocols** **(default:** **see** **postconf** **-d** **output)**
       TLS protocols accepted by the Postfix SMTP server with mandatory TLS encryption.  If the list
       is  empty,  the  server supports all available TLS protocol versions.  A non-empty value is a
       list of protocol names to include or exclude, separated by whitespace, commas or colons.

       The valid protocol names (see **SSL**___**get**___**[version**(3)](https://www.chedong.com/phpMan.php/man/version/3/markdown)) are "SSLv2", "SSLv3",  "TLSv1",  "TLSv1.1",
       "TLSv1.2"  and  "TLSv1.3".   Starting with Postfix 3.6, the default value is ">=TLSv1", which
       sets TLS 1.0 as the lowest supported TLS protocol version (see below).   Older  releases  use
       the "!" exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set the
       lowest acceptable TLS protocol version and/or the highest acceptable  TLS  protocol  version.
       To  set the lower bound include an element of the form: ">=_version_" where _version_ is a either
       one of the TLS protocol names listed above, or a hexadecimal number corresponding to the  de‐
       sired  TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.).  For the upper bound,
       use "<=_version_".  There must be no whitespace between the ">=" or "<=" symbols and the proto‐
       col name or number.

       Hexadecimal  protocol  numbers  make  it possible to specify protocol bounds for TLS versions
       that are known to OpenSSL, but might not be known to Postfix.  They cannot be used  with  the
       legacy  exclusion  syntax.   Leading  "0"  or  "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent  to  "TLSv1".   Hexadecimal
       versions  unknown to OpenSSL will fail to set the upper or lower bound, and a warning will be
       logged.  Hexadecimal versions should only be used when Postfix is  linked  with  some  future
       version  of  OpenSSL  that supports TLS 1.4 or later, but Postfix does not yet support a sym‐
       bolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtpd_tls_mandatory_protocols = >=TLSv1.2, <=0305
           # Allow only TLS 1.2 and up:
           smtpd_tls_mandatory_protocols = >=0x0303

       With Postfix < 3.6 there is no support for a minimum or maximum  version,  and  the  protocol
       range   is   configured   via  protocol  exclusions.   To  require  at  least  TLS  1.0,  set
       "smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3".  Listing the protocols to  include,  rather
       than  protocols to exclude, is supported, but not recommended.  The exclusion form more accu‐
       rately matches the underlying OpenSSL interface.

       Support for  "TLSv1.3"  was  introduced  in  OpenSSL  1.1.1.   Disabling  this  protocol  via
       "!TLSv1.3"  is  supported  since  Postfix 3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and
       3.3.2).

       Example:

       # Preferred syntax with Postfix >= 3.6:
       smtpd_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
       # Legacy syntax:
       smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

       This feature is available in Postfix 2.3 and later.

**smtpd**___**tls**___**protocols** **(default:** **see** **postconf** **-d** **output)**
       TLS protocols accepted by the Postfix SMTP server with opportunistic TLS encryption.  If  the
       list is empty, the server supports all available TLS protocol versions.  A non-empty value is
       a list of protocol names to include or exclude, separated by whitespace, commas or colons.

       The valid protocol names (see **SSL**___**get**___**[version**(3)](https://www.chedong.com/phpMan.php/man/version/3/markdown)) are "SSLv2", "SSLv3",  "TLSv1",  "TLSv1.1",
       "TLSv1.2"  and  "TLSv1.3".   Starting with Postfix 3.6, the default value is ">=TLSv1", which
       sets TLS 1.0 as the lowest supported TLS protocol version (see below).   Older  releases  use
       the "!" exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set the
       lowest acceptable TLS protocol version and/or the highest acceptable  TLS  protocol  version.
       To  set the lower bound include an element of the form: ">=_version_" where _version_ is a either
       one of the TLS protocol names listed above, or a hexadecimal number corresponding to the  de‐
       sired  TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.).  For the upper bound,
       use "<=_version_".  There must be no whitespace between the ">=" or "<=" symbols and the proto‐
       col name or number.

       Hexadecimal  protocol  numbers  make  it possible to specify protocol bounds for TLS versions
       that are known to OpenSSL, but might not be known to Postfix.  They cannot be used  with  the
       legacy  exclusion  syntax.   Leading  "0"  or  "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent  to  "TLSv1".   Hexadecimal
       versions  unknown to OpenSSL will fail to set the upper or lower bound, and a warning will be
       logged.  Hexadecimal versions should only be used when Postfix is  linked  with  some  future
       version  of  OpenSSL  that supports TLS 1.4 or later, but Postfix does not yet support a sym‐
       bolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtpd_tls_protocols = >=TLSv1, <=0305
           # Allow only TLS 1.0 and up:
           smtpd_tls_protocols = >=0x0301

       With Postfix < 3.6 there is no support for a minimum or maximum  version,  and  the  protocol
       range   is   configured   via  protocol  exclusions.   To  require  at  least  TLS  1.0,  set
       "smtpd_tls_protocols = !SSLv2, !SSLv3".  Listing the protocols to include, rather than proto‐
       cols  to  exclude,  is  supported,  but  not recommended.  The exclusion form more accurately
       matches the underlying OpenSSL interface.

       Support for  "TLSv1.3"  was  introduced  in  OpenSSL  1.1.1.   Disabling  this  protocol  via
       "!TLSv1.3"  is  supported  since  Postfix 3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and
       3.3.2).

       Example:
       # Preferred syntax with Postfix >= 3.6:
       smtpd_tls_protocols = >=TLSv1, <=TLSv1.3
       # Legacy syntax:
       smtpd_tls_protocols = !SSLv2, !SSLv3

       This feature is available in Postfix 2.6 and later.

**smtpd**___**tls**___**received**___**header** **(default:** **no)**
       Request that the Postfix SMTP server produces Received:  message headers that include  infor‐
       mation  about  the protocol and cipher used, as well as the remote SMTP client CommonName and
       client certificate issuer CommonName.  This is disabled by default, as the information may be
       modified  in  transit  through other mail servers.  Only information that was recorded by the
       final destination can be trusted.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**req**___**ccert** **(default:** **no)**
       With mandatory TLS encryption, require a trusted remote SMTP client certificate in  order  to
       allow TLS connections to proceed.  This option implies "smtpd_tls_ask_ccert = yes".

       When  TLS  encryption is optional, this setting is ignored with a warning written to the mail
       log.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**security**___**level** **(default:** **empty)**
       The SMTP TLS security level for the Postfix SMTP server; when a non-empty value is specified,
       this overrides the obsolete parameters smtpd_use_tls and smtpd_enforce_tls. This parameter is
       ignored with "smtpd_tls_wrappermode = yes".

       Specify one of the following security levels:

       **none**   TLS will not be used.

       **may**    Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but  do  not  re‐
              quire that clients use TLS encryption.

### encrypt
              Mandatory  TLS  encryption:  announce STARTTLS support to remote SMTP clients, and re‐
              quire that clients use TLS encryption. According to RFC 2487 this MUST NOT be  applied
              in case of a publicly-referenced SMTP server. Instead, this option should be used only
              on dedicated servers.

       Note 1: the "fingerprint", "verify" and "secure" levels are not supported here.  The  Postfix
       SMTP server logs a warning and uses "encrypt" instead.  To verify remote SMTP client certifi‐
       cates, see TLS_README for a discussion of the smtpd_tls_ask_ccert,  smtpd_tls_req_ccert,  and
       permit_tls_clientcerts features.

       Note    2:    The    parameter   setting   "smtpd_tls_security_level   =   encrypt"   implies
       "smtpd_tls_auth_only = yes".

       Note 3: when invoked via "sendmail -bs", Postfix will never offer STARTTLS  due  to  insuffi‐
       cient privileges to access the server private key. This is intended behavior.

       This feature is available in Postfix 2.3 and later.

**smtpd**___**tls**___**session**___**cache**___**database** **(default:** **empty)**
       Name  of  the  file  containing the optional Postfix SMTP server TLS session cache. Specify a
       database type that supports enumeration, such as **btree** or **sdbm**; there is no need  to  support
       concurrent  access.   The  file is created if it does not exist. The [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) daemon does not
       use this parameter directly, rather the cache is implemented indirectly in the [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) dae‐
       mon.  This means that per-smtpd-instance master.cf overrides of this parameter are not effec‐
       tive. Note, that each of the cache databases supported by [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown)  daemon:  $smtpd_tls_ses‐
       sion_cache_database,   $smtp_tls_session_cache_database  (and  with  Postfix  2.3  and  later
       $lmtp_tls_session_cache_database), needs to be stored separately. It is not at this time pos‐
       sible to store multiple caches in a single database.

       Note: **dbm** databases are not suitable. TLS session objects are too large.

       As  of  version  2.5, Postfix no longer uses root privileges when opening this file. The file
       should now be stored under the Postfix-owned data_directory. As a migration aid,  an  attempt
       to open the file under a non-Postfix directory is redirected to the Postfix-owned data_direc‐
       tory, and a warning is logged.

       As of Postfix 2.11 the preferred mechanism for session resumption is  RFC  5077  TLS  session
       tickets, which don't require server-side storage.  Consequently, for Postfix >= 2.11 this pa‐
       rameter should generally be left empty.  TLS session tickets require an OpenSSL  library  (at
       least  version  0.9.8h)  that  provides  full  support  for  this  TLS  extension.   See also
       smtpd_tls_session_cache_timeout.

       Example:

       smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_scache

       This feature is available in Postfix 2.2 and later.

**smtpd**___**tls**___**session**___**cache**___**timeout** **(default:** **3600s)**
       The expiration time of Postfix SMTP server TLS session cache information. A cache cleanup  is
       performed    periodically    every    $smtpd_tls_session_cache_timeout   seconds.   As   with
       $smtpd_tls_session_cache_database, this parameter is implemented in the [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) daemon  and
       therefore per-smtpd-instance master.cf overrides are not possible.

       As of Postfix 2.11 this setting cannot exceed 100 days.  If set <= 0, session caching is dis‐
       abled, not just via the database, but also via RFC 5077 TLS session tickets, which don't  re‐
       quire server-side storage.  If set to a positive value less than 2 minutes, the minimum value
       of 2 minutes is used instead.  TLS session tickets require an OpenSSL library (at least  ver‐
       sion 0.9.8h) that provides full support for this TLS extension.

       This  feature  is available in Postfix 2.2 and later, and updated for TLS session ticket sup‐
       port in Postfix 2.11.

**smtpd**___**tls**___**wrappermode** **(default:** **no)**
       Run the Postfix SMTP server in the non-standard "wrapper" mode, instead of using the STARTTLS
       command.

       If  you  want  to  support  this service, enable a special port in master.cf, and specify "-o
       smtpd_tls_wrappermode=yes" on the SMTP server's command line. Port 465 (smtps) was once  cho‐
       sen for this purpose.

       This feature is available in Postfix 2.2 and later.

**smtpd**___**upstream**___**proxy**___**protocol** **(default:** **empty)**
       The  name  of  the  proxy protocol used by an optional before-smtpd proxy agent. When a proxy
       agent is used, this protocol conveys local and remote address and port information.   Specify
       "smtpd_upstream_proxy_protocol  =  haproxy" to enable the haproxy protocol; version 2 is sup‐
       ported with Postfix 3.5 and later.

       NOTE: To use the nginx proxy with [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown), enable the  XCLIENT  protocol  with  smtpd_autho‐
       rized_xclient_hosts.  This  supports  SASL authentication in the proxy agent (Postfix 2.9 and
       later).

       This feature is available in Postfix 2.10 and later.

**smtpd**___**upstream**___**proxy**___**timeout** **(default:** **5s)**
       The time limit for the proxy protocol specified with the smtpd_upstream_proxy_protocol param‐
       eter.

       This feature is available in Postfix 2.10 and later.

**smtpd**___**use**___**tls** **(default:** **no)**
       Opportunistic  TLS: announce STARTTLS support to remote SMTP clients, but do not require that
       clients use TLS encryption.

       Note: when invoked via "**sendmail** **-bs**", Postfix will never offer STARTTLS due to  insufficient
       privileges to access the server private key. This is intended behavior.

       This  feature  is  available  in  Postfix  2.2  and  later.  With  Postfix  2.3 and later use
       smtpd_tls_security_level instead.

**smtputf8**___**autodetect**___**classes** **(default:** **sendmail,** **verify)**
       Detect that a message requires SMTPUTF8 support for the specified mail origin classes.   This
       is a workaround to avoid chicken-and-egg problems during the initial SMTPUTF8 roll-out in en‐
       vironments with pre-existing mail flows that contain UTF8. Those mail flows should not  break
       because Postfix suddenly refuses to deliver such mail to down-stream MTAs that don't announce
       SMTPUTF8 support.

       The problem is that Postfix cannot rely solely on the sender's declaration that a message re‐
       quires SMTPUTF8 support, because UTF8 may be introduced during local processing (for example,
       the client hostname in Postfix's Received: header, adding @$myorigin or .$mydomain to an  in‐
       complete  address,  address  rewriting, alias expansion, automatic BCC recipients, local for‐
       warding, and changes made by header checks or Milter applications).

       For now, the default is to enable "SMTPUTF8 required" autodetection only for Postfix sendmail
       command-line submissions and address verification probes.  This may change once SMTPUTF8 sup‐
       port achieves world domination.  However, sites that add UTF8 content  via  local  processing
       (see above) should autodetect the need for SMTPUTF8 support for all email.

       Specify one or more of the following:

### sendmail
              Submission with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command.

        **smtpd** Mail received with the [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) daemon.

        **qmqpd** Mail received with the [**qmqpd**(8)](https://www.chedong.com/phpMan.php/man/qmqpd/8/markdown) daemon.

### forward
              Local  forwarding  or  aliasing.  When a message is received with "SMTPUTF8 required",
              then the forwarded (aliased) message always has "SMTPUTF8 required".

### bounce
              Submission by the [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown) daemon.  When a message is  received  with  "SMTPUTF8  re‐
              quired", then the delivery status notification always has "SMTPUTF8 required".

### notify
              Postmaster notification from the [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown) or [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) daemon.

### verify
              Address verification probe from the [**verify**(8)](https://www.chedong.com/phpMan.php/man/verify/8/markdown) daemon.

        **all**   Enable SMTPUTF8 autodetection for all mail.

       This feature is available in Postfix 3.0 and later.

**smtputf8**___**enable** **(default:** **yes)**
       Enable  preliminary  SMTPUTF8 support for the protocols described in RFC 6531..6533. This re‐
       quires that Postfix is built to support these protocols.

       This feature is available in Postfix 3.0 and later.

**soft**___**bounce** **(default:** **no)**
       Safety net to keep mail queued that would otherwise be returned to the sender.  This  parame‐
       ter  disables  locally-generated bounces, changes the handling of negative responses from re‐
       mote servers, content filters or plugins, and prevents the Postfix SMTP server from rejecting
       mail  permanently  by changing 5xx reply codes into 4xx.  However, soft_bounce is no cure for
       address rewriting mistakes or mail routing mistakes.

       Note: "soft_bounce = yes" is in some cases implemented by modifying server responses.  There‐
       fore, the response that Postfix logs may differ from the response that Postfix actually sends
       or receives.

       Example:

       soft_bounce = yes

**stale**___**lock**___**time** **(default:** **500s)**
       The time after which a stale exclusive mailbox lockfile is removed.  This is used for  deliv‐
       ery to file or mailbox.

       Time  units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time unit
       is s (seconds).

### stress (default: empty)
       This feature is documented in the STRESS_README document.

       This feature is available in Postfix 2.5 and later.

**strict**___**7bit**___**headers** **(default:** **no)**
       Reject mail with 8-bit text in message headers. This blocks mail from poorly written applica‐
       tions.

       This  feature should not be enabled on a general purpose mail server, because it is likely to
       reject legitimate email.

       This feature is available in Postfix 2.0 and later.

**strict**___**8bitmime** **(default:** **no)**
       Enable both strict_7bit_headers and strict_8bitmime_body.

       This feature should not be enabled on a general purpose mail server, because it is likely  to
       reject legitimate email.

       This feature is available in Postfix 2.0 and later.

**strict**___**8bitmime**___**body** **(default:** **no)**
       Reject  8-bit message body text without 8-bit MIME content encoding information.  This blocks
       mail from poorly written applications.

       Unfortunately, this also rejects majordomo approval requests when the included  request  con‐
       tains valid 8-bit MIME mail, and it rejects bounces from mailers that do not MIME encapsulate
       8-bit content (for example, bounces from qmail or from old versions of Postfix).

       This feature should not be enabled on a general purpose mail server, because it is likely  to
       reject legitimate email.

       This feature is available in Postfix 2.0 and later.

**strict**___**mailbox**___**ownership** **(default:** **yes)**
       Defer delivery when a mailbox file is not owned by its recipient.  The default setting is not
       backwards compatible.

       This feature is available in Postfix 2.5.3 and later.

**strict**___**mime**___**encoding**___**domain** **(default:** **no)**
       Reject mail with invalid Content-Transfer-Encoding: information for the message/*  or  multi‐
       part/* MIME content types.  This blocks mail from poorly written software.

       This  feature  should not be enabled on a general purpose mail server, because it will reject
       mail after a single violation.

       This feature is available in Postfix 2.0 and later.

**strict**___**rfc821**___**envelopes** **(default:** **no)**
       Require that addresses received in SMTP MAIL FROM and RCPT TO commands are enclosed with  <>,
       and  that  those addresses do not contain RFC 822 style comments or phrases.  This stops mail
       from poorly written software.

       By default, the Postfix SMTP server accepts RFC 822 syntax in  MAIL  FROM  and  RCPT  TO  ad‐
       dresses.

**strict**___**smtputf8** **(default:** **no)**
       Enable  stricter  enforcement  of the SMTPUTF8 protocol. The Postfix SMTP server accepts UTF8
       sender or recipient addresses only when the client requests an SMTPUTF8 mail transaction.

       This feature is available in Postfix 3.0 and later.

**sun**___**mailtool**___**compatibility** **(default:** **no)**
       Obsolete SUN mailtool compatibility feature. Instead, use "mailbox_delivery_lock = dotlock".

**swap**___**bangpath** **(default:** **yes)**
       Enable the rewriting of "site!user" into "user@site".  This is necessary if your  machine  is
       connected to UUCP networks.  It is enabled by default.

       Note: with Postfix version 2.2, message header address rewriting happens only when one of the
       following conditions is true:

       •      The message is received with the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) command,

       •      The  message  is  received  from  a  network  client  that  matches  $local_header_re‐
              write_clients,

       •      The message is received from the network, and the remote_header_rewrite_domain parame‐
              ter specifies a non-empty value.

       To get the behavior before  Postfix  version  2.2,  specify  "local_header_rewrite_clients  =
       static:all".

       Example:

       swap_bangpath = no

**syslog**___**facility** **(default:** **mail)**
       The  syslog facility of Postfix logging. Specify a facility as defined in syslog.[**conf**(5)](https://www.chedong.com/phpMan.php/man/conf/5/markdown). The
       default facility is "mail".

       Warning: a non-default syslog_facility setting takes effect only after a Postfix process  has
       completed  initialization.   Errors during process initialization will be logged with the de‐
       fault facility.  Examples are errors while parsing the command  line  arguments,  and  errors
       while accessing the Postfix main.cf configuration file.

**syslog**___**name** **(default:** **see** **postconf** **-d** **output)**
       A  prefix  that  is  prepended  to  the process name in syslog records, so that, for example,
       "smtpd" becomes "prefix/smtpd".

       Warning: a non-default syslog_name setting takes effect only after a Postfix process has com‐
       pleted  initialization.  Errors during process initialization will be logged with the default
       name. Examples are errors while parsing the command line arguments, and errors while  access‐
       ing the Postfix main.cf configuration file.

**tcp**___**windowsize** **(default:** **0)**
       An  optional workaround for routers that break TCP window scaling.  Specify a value > 0 and <
       65536 to enable this feature.  With Postfix TCP servers ([**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown), [**qmqpd**(8)](https://www.chedong.com/phpMan.php/man/qmqpd/8/markdown)), this feature is
       implemented by the Postfix [**master**(8)](https://www.chedong.com/phpMan.php/man/master/8/markdown) daemon.

       To  change  this  parameter without stopping Postfix, you need to first terminate all Postfix
       TCP servers:

           # postconf -e master_service_disable=inet
           # postfix reload

       This immediately terminates all processes that accept network connections.  Next, you  enable
       Postfix TCP servers with the updated tcp_windowsize setting:

           # postconf -e tcp_windowsize=65535 master_service_disable=
           # postfix reload

       If  you  skip  these steps with a running Postfix system, then the tcp_windowsize change will
       work only for Postfix TCP clients ([**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown), [**lmtp**(8)](https://www.chedong.com/phpMan.php/man/lmtp/8/markdown)).

       This feature is available in Postfix 2.6 and later.

**tls**___**append**___**default**___**CA** **(default:** **no)**
       Append the system-supplied default Certification Authority certificates to the ones specified
       with  *_tls_CApath or *_tls_CAfile.  The default is "no"; this prevents Postfix from trusting
       third-party certificates and giving them relay permission with permit_tls_all_clientcerts.

       This feature is available in Postfix 2.4.15, 2.5.11, 2.6.8, 2.7.2 and later versions. Specify
       "tls_append_default_CA = yes" for backwards compatibility, to avoid breaking certificate ver‐
       ification with sites that don't use permit_tls_all_clientcerts.

**tls**___**daemon**___**random**___**bytes** **(default:** **32)**
       The number of pseudo-random bytes that an [**smtp**(8)](https://www.chedong.com/phpMan.php/man/smtp/8/markdown)  or  [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown)  process  requests  from  the
       [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown)  server  in  order to seed its internal pseudo random number generator (PRNG).  The
       default of 32 bytes (equivalent to 256 bits) is sufficient to generate a 128bit  (or  168bit)
       session key.

       This feature is available in Postfix 2.2 and later.

**tls**___**dane**___**digest**___**agility** **(default:** **on)**
       Configure  RFC7671  DANE  TLSA digest algorithm agility.  Do not change this setting from its
       default value.

       See Section 8 of RFC7671 for correct key rotation procedures.

       This feature is available in Postfix 2.11 through 3.1.  Postfix 3.2  and  later  ignore  this
       configuration parameter and behave as though it were set to "on".

**tls**___**dane**___**digests** **(default:** **sha512** **sha256)**
       DANE TLSA (RFC 6698, RFC 7671, RFC 7672) resource-record "matching type" digest algorithms in
       descending preference order.  All the specified algorithms must be supported by the  underly‐
       ing OpenSSL library, otherwise the Postfix SMTP client will not support DANE TLSA security.

       Specify  a  list of digest names separated by commas and/or whitespace.  Each digest name may
       be followed by an optional "=<number>" suffix.  For example, "sha512" may instead  be  speci‐
       fied  as "sha512=2" and "sha256" may instead be specified as "sha256=1".  The optional number
       must   match   the   <a   href="<https://www.iana.org/assignments/dane-parameters/dane-parame>‐
       ters.xhtml#matching-types"  >IANA  assigned  TLSA matching type number the algorithm in ques‐
       tion.  Postfix will check this constraint for the  algorithms  it  knows  about.   Additional
       matching  type  algorithms  registered  with IANA can be added with explicit numbers provided
       they are supported by OpenSSL.

       Invalid list elements are logged with a warning and disable  DANE  support.   TLSA  RRs  that
       specify digests not included in the list are ignored with a warning.

       Note:  It  is  unwise to omit sha256 from the digest list.  This digest algorithm is the only
       mandatory to implement digest algorithm in RFC 6698, and many servers  are  expected  publish
       TLSA  records with just sha256 digests.  Unless one of the standard digests is seriously com‐
       promised and servers have had ample time to update their TLSA records you should not omit any
       standard digests, just arrange them in order from strongest to weakest.

       This feature is available in Postfix 2.11 and later.

**tls**___**dane**___**trust**___**anchor**___**digest**___**enable** **(default:** **yes)**
       Enable  support  for  RFC  6698 (DANE TLSA) DNS records that contain digests of trust-anchors
       with certificate usage "2".  Do not change this setting from its default value.

       This feature is available in Postfix 2.11 through 3.1.  It has been withdrawn in Postfix 3.2,
       as  trust-anchor  TLSA  records  are  now  widely used and have proved sufficiently reliable.
       Postfix 3.2 and later ignore this configuration parameter and behaves as though it  were  set
       to "yes".

**tls**___**disable**___**workarounds** **(default:** **see** **postconf** **-d** **output)**
       List or bit-mask of OpenSSL bug work-arounds to disable.

       The  OpenSSL toolkit includes a set of work-arounds for buggy SSL/TLS implementations. Appli‐
       cations, such as Postfix, that want to maximize interoperability ask the OpenSSL  library  to
       enable the full set of recommended work-arounds.

       From  time  to time, it is discovered that a work-around creates a security issue, and should
       no longer be used. If upgrading OpenSSL to a fixed version is not an option or an upgrade  is
       not available in a timely manner, or in closed environments where no buggy clients or servers
       exist, it may be  appropriate  to  disable  some  or  all  of  the  OpenSSL  interoperability
       work-arounds. This parameter specifies which bug work-arounds to disable.

       If  the  value  of  the  parameter  is a hexadecimal long integer starting with "0x", the bug
       work-arounds corresponding to the bits specified in its value are removed from the **SSL**___**OP**___**ALL**
       work-around  bit-mask  (see  openssl/ssl.h  and **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)). You can specify more
       bits than are present in SSL_OP_ALL, excess bits are ignored. Specifying 0xFFFFFFFF  disables
       all bug-workarounds on a 32-bit system. This should also be sufficient on 64-bit systems, un‐
       til OpenSSL abandons support for 32-bit systems and starts using the high 32 bits of a 64-bit
       bug-workaround mask.

       Otherwise,  the  parameter  is  a  white-space  or comma separated list of specific named bug
       work-arounds chosen from the list below. It is possible that your  OpenSSL  version  includes
       new  bug work-arounds added after your Postfix source code was last updated, in that case you
       can only disable one of these via the hexadecimal syntax above.

       **CRYPTOPRO**___**TLSEXT**___**BUG**
              New with GOST support in OpenSSL 1.0.0.

       **DONT**___**INSERT**___**EMPTY**___**FRAGMENTS**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **LEGACY**___**SERVER**___**CONNECT**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **MICROSOFT**___**BIG**___**SSLV3**___**BUFFER**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **MICROSOFT**___**SESS**___**ID**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **MSIE**___**SSLV2**___**RSA**___**PADDING**
              also aliased as **CVE-2005-2969**. Postfix 2.8 disables this work-around by  default  with
              OpenSSL versions that may predate the fix. Fixed in OpenSSL 0.9.7h and OpenSSL 0.9.8a.

       **NETSCAPE**___**CHALLENGE**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **NETSCAPE**___**REUSE**___**CIPHER**___**CHANGE**___**BUG**
              also  aliased  as **CVE-2010-4180**. Postfix 2.8 disables this work-around by default with
              OpenSSL versions that may predate the fix. Fixed in OpenSSL 0.9.8q and OpenSSL 1.0.0c.

       **SSLEAY**___**080**___**CLIENT**___**DH**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **SSLREF2**___**REUSE**___**CERT**___**TYPE**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **TLS**___**BLOCK**___**PADDING**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **TLS**___**D5**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)

       **TLS**___**ROLLBACK**___**BUG**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).  This is disabled  in  OpenSSL  0.9.7  and  later.  Nobody
              should still be using 0.9.6!

       **TLSEXT**___**PADDING**
              Postfix >= 3.4. See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       This feature is available in Postfix 2.8 and later.

**tls**___**eecdh**___**auto**___**curves** **(default:** **see** **postconf** **-d** **output)**
       The  prioritized  list  of  elliptic  curves supported by the Postfix SMTP client and server.
       These curves are used by the Postfix SMTP server when "smtpd_tls_eecdh_grade  =  auto".   The
       selected  curves  must be implemented by OpenSSL and be standardized for use in TLS (RFC 4492
       or its imminent successor).  It is unwise to list only "bleeding-edge" curves supported by  a
       small subset of clients.  The default list is suitable for most users.

       Postfix  skips  curve names that are unknown to OpenSSL, or that are known but not yet imple‐
       mented.  This makes it possible to "anticipate" support for curves that should be  used  once
       they  become  available.   In  particular,  in some OpenSSL versions, the new RFC 8031 curves
       "X25519" and "X448" may be known by name, but ECDH support for either or both may be missing.
       These  curves  may appear in the default value of this parameter, even though they'll only be
       usable with later versions of OpenSSL.

       This feature is available in Postfix 3.2 and later, when  it  is  compiled  and  linked  with
       OpenSSL 1.0.2 or later on platforms where EC algorithms have not been disabled by the vendor.

**tls**___**eecdh**___**strong**___**curve** **(default:** **prime256v1)**
       The elliptic curve used by the Postfix SMTP server for sensibly strong ephemeral ECDH key ex‐
       change. This curve is used by the Postfix SMTP server when "smtpd_tls_eecdh_grade =  strong".
       The  phrase  "sensibly  strong"  means approximately 128-bit security based on best known at‐
       tacks. The selected curve must be implemented by OpenSSL (as reported by [**ecparam**(1)](https://www.chedong.com/phpMan.php/man/ecparam/1/markdown) with  the
       "-list_curves"  option)  and  be  one  of the curves listed in Section 5.1.1 of RFC 4492. You
       should not generally change this setting.  Remote SMTP client  implementations  must  support
       this  curve  for EECDH key exchange to take place.  It is unwise to choose an "bleeding-edge"
       curve supported by only a small subset of clients.

       The default "strong" curve is rated in NSA Suite B for information classified up to SECRET.

       Note: elliptic curve names are poorly standardized; different standards groups are  assigning
       different names to the same underlying curves.  The curve with the X9.62 name "prime256v1" is
       also known under the SECG name "secp256r1", but OpenSSL does not recognize the latter name.

       If you want to take maximal advantage of ciphers that offer forward secrecy see  the  Getting
       started  section  of FORWARD_SECRECY_README.  The full document conveniently presents all in‐
       formation about Postfix "perfect" forward secrecy support in one place: what forward  secrecy
       is, how to tweak settings, and what you can expect to see when Postfix uses ciphers with for‐
       ward secrecy.

       This feature is available in Postfix 2.6 and later, when  it  is  compiled  and  linked  with
       OpenSSL 1.0.0 or later on platforms where EC algorithms have not been disabled by the vendor.

**tls**___**eecdh**___**ultra**___**curve** **(default:** **secp384r1)**
       The  elliptic  curve  used by the Postfix SMTP server for maximally strong ephemeral ECDH key
       exchange. This curve is used by the Postfix SMTP server when "smtpd_tls_eecdh_grade = ultra".
       The  phrase  "maximally  strong" means approximately 192-bit security based on best known at‐
       tacks.  This additional strength comes at a significant computational cost, most users should
       instead  set  "smtpd_tls_eecdh_grade  =  strong".   The selected curve must be implemented by
       OpenSSL (as reported by [**ecparam**(1)](https://www.chedong.com/phpMan.php/man/ecparam/1/markdown) with the "-list_curves" option) and be one of  the  curves
       listed in Section 5.1.1 of RFC 4492. You should not generally change this setting.

       This  default  "ultra" curve is rated in NSA Suite B for information classified up to TOP SE‐
       CRET.

       If you want to take maximal advantage of ciphers that offer forward secrecy see  the  Getting
       started  section  of FORWARD_SECRECY_README.  The full document conveniently presents all in‐
       formation about Postfix "perfect" forward secrecy support in one place: what forward  secrecy
       is, how to tweak settings, and what you can expect to see when Postfix uses ciphers with for‐
       ward secrecy.

       This feature is available in Postfix 2.6 and later, when  it  is  compiled  and  linked  with
       OpenSSL 1.0.0 or later on platforms where EC algorithms have not been disabled by the vendor.

**tls**___**export**___**cipherlist** **(default:** **see** **postconf** **-d** **output)**
       The  OpenSSL cipherlist for "export" or higher grade ciphers. This defines the meaning of the
       "export"  setting  in   smtpd_tls_ciphers,   smtpd_tls_mandatory_ciphers,   smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers,  lmtp_tls_ciphers,  and lmtp_tls_mandatory_ciphers.  With Postfix
       releases before the middle of 2015 this is  the  default  cipherlist  for  the  opportunistic
       ("may")  TLS  client  security level and also the default cipherlist for the SMTP server. You
       are strongly encouraged to not change this setting.

       This feature is available in Postfix 2.3 and later.

**tls**___**fast**___**shutdown**___**enable** **(default:** **yes)**
       A workaround for implementations that hang Postfix while shutting down a TLS  session,  until
       Postfix  times  out.  With this enabled, Postfix will not wait for the remote TLS peer to re‐
       spond to a TLS later.

**tls**___**high**___**cipherlist** **(default:** **see** **postconf** **-d** **output)**
       The OpenSSL cipherlist for "high" grade ciphers. This defines the meaning of the "high"  set‐
       ting  in  smtpd_tls_ciphers,  smtpd_tls_mandatory_ciphers,  smtp_tls_ciphers, smtp_tls_manda‐
       tory_ciphers, lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. You are  strongly  encouraged
       to not change this setting.

       This feature is available in Postfix 2.3 and later.

**tls**___**legacy**___**public**___**key**___**fingerprints** **(default:** **no)**
       A temporary migration aid for sites that use certificate _public-key_ fingerprints with Postfix
       2.9.0..2.9.5, which use an incorrect algorithm. This parameter has no effect on the  certifi‐
       cate fingerprint support that is available since Postfix 2.2.

       Specify "tls_legacy_public_key_fingerprints = yes" temporarily, pending a migration from con‐
       figuration files with incorrect Postfix 2.9.0..2.9.5 certificate public-key finger prints, to
       the correct fingerprints used by Postfix 2.9.6 and later.  To compute the correct certificate
       public-key fingerprints, see TLS_README.

       This feature is available in Postfix 2.9.6 and later.

**tls**___**low**___**cipherlist** **(default:** **see** **postconf** **-d** **output)**
       The OpenSSL cipherlist for "low" or higher grade ciphers. This defines  the  meaning  of  the
       "low"    setting   in   smtpd_tls_ciphers,   smtpd_tls_mandatory_ciphers,   smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers,  lmtp_tls_ciphers,  and   lmtp_tls_mandatory_ciphers.   You   are
       strongly encouraged to not change this setting.

       This feature is available in Postfix 2.3 and later.

**tls**___**medium**___**cipherlist** **(default:** **see** **postconf** **-d** **output)**
       The  OpenSSL cipherlist for "medium" or higher grade ciphers. This defines the meaning of the
       "medium"  setting  in   smtpd_tls_ciphers,   smtpd_tls_mandatory_ciphers,   smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers,  lmtp_tls_ciphers,  and  lmtp_tls_mandatory_ciphers.  This is the
       default cipherlist for mandatory TLS encryption in the TLS  client  (with  anonymous  ciphers
       disabled when verifying server certificates).  This is the default cipherlist for opportunis‐
       tic TLS with Postfix releases after the middle of 2015.  You are strongly encouraged  to  not
       change this setting.

       This feature is available in Postfix 2.3 and later.

**tls**___**null**___**cipherlist** **(default:** **eNULL:!aNULL)**
       The  OpenSSL  cipherlist for "NULL" grade ciphers that provide authentication without encryp‐
       tion. This  defines  the  meaning  of  the  "null"  setting  in  smtpd_mandatory_tls_ciphers,
       smtp_tls_mandatory_ciphers  and  lmtp_tls_mandatory_ciphers.   You are strongly encouraged to
       not change this setting.

       This feature is available in Postfix 2.3 and later.

**tls**___**preempt**___**cipherlist** **(default:** **no)**
       With SSLv3 and later, use the Postfix SMTP server's cipher preference order  instead  of  the
       remote client's cipher preference order.

       By  default,  the  OpenSSL  server selects the client's most preferred cipher that the server
       supports. With SSLv3 and later, the server may choose its own most preferred cipher  that  is
       supported  (offered) by the client. Setting "tls_preempt_cipherlist = yes" enables server ci‐
       pher preferences.

       While server cipher selection may in some cases lead to a more secure  or  performant  cipher
       choice,  there  is  some  risk of interoperability issues. In the past, some SSL clients have
       listed lower priority ciphers that they did not implement correctly. If the server chooses  a
       cipher  that  the  client prefers less, it may select a cipher whose client implementation is
       flawed. Most notably Windows 2003 Microsoft Exchange servers have flawed  implementations  of
       DES-CBC3-SHA,  which  OpenSSL  considers stronger than RC4-SHA.  Enabling server cipher-suite
       selection may create interoperability issues with Windows 2003 Microsoft Exchange clients.

       This feature is available in Postfix 2.8 and later, in combination  with  OpenSSL  0.9.7  and
       later.

**tls**___**random**___**bytes** **(default:** **32)**
       The number of bytes that [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) reads from $tls_random_source when (re)seeding the in-mem‐
       ory pseudo random number generator (PRNG) pool. The default of 32 bytes (256  bits)  is  good
       enough  for  128bit symmetric keys.  If using EGD or a device file, a maximum of 255 bytes is
       read.

       This feature is available in Postfix 2.2 and later.

**tls**___**random**___**exchange**___**name** **(default:** **see** **postconf** **-d** **output)**
       Name of the pseudo random number generator (PRNG) state file that is maintained by [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown).
       The file is created when it does not exist, and its length is fixed at 1024 bytes.

       As of version 2.5, Postfix no longer uses root privileges when opening this file, and the de‐
       fault  file  location  was  changed  from  ${config_directory}/prng_exch   to   ${data_direc‐
       tory}/prng_exch.   As a migration aid, an attempt to open the file under a non-Postfix direc‐
       tory is redirected to the Postfix-owned data_directory, and a warning is logged.

       This feature is available in Postfix 2.2 and later.

**tls**___**random**___**prng**___**update**___**period** **(default:** **3600s)**
       The time between attempts by [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) to save the state of the pseudo random number  genera‐
       tor (PRNG) to the file specified with $tls_random_exchange_name.

       This feature is available in Postfix 2.2 and later.

**tls**___**random**___**reseed**___**period** **(default:** **3600s)**
       The  maximal time between attempts by [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) to re-seed the in-memory pseudo random number
       generator (PRNG) pool from external sources.  The actual time between re-seeding attempts  is
       calculated using the PRNG, and is between 0 and the time specified.

       This feature is available in Postfix 2.2 and later.

**tls**___**random**___**source** **(default:** **see** **postconf** **-d** **output)**
       The external entropy source for the in-memory [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) pseudo random number generator (PRNG)
       pool. Be sure to specify a non-blocking source.  If this source is not a  regular  file,  the
       entropy source type must be prepended:  egd:/path/to/egd_socket for a source with EGD compat‐
       ible socket interface, or dev:/path/to/device for a device file.

       Note: on OpenBSD systems specify /dev/arandom when /dev/urandom gives timeout errors.

       This feature is available in Postfix 2.2 and later.

**tls**___**server**___**sni**___**maps** **(default:** **empty)**
       Optional lookup tables that map names received from remote SMTP clients via  the  TLS  Server
       Name Indication (SNI) extension to the appropriate keys and certificate chains.  This parame‐
       ter is implemented in the Postfix TLS library, and applies to  both  [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown)  and  the  SMTP
       server mode of [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown).

       When  this  parameter is non-empty, the Postfix SMTP server enables SNI extension processing,
       and logs SNI values that are invalid or don't match an entry in  the  the  specified  tables.
       When  an  entry  does  match, the SNI name is logged as part of the connection summary at log
       levels 1 and higher.

       The lookup key is either the verbatim SNI domain name or an ancestor domain prefixed  with  a
       leading dot.  For internationalized domains, the lookup key must be in IDNA 2008 A-label form
       (as required in the TLS SNI extension).

       The syntax of the lookup value is the same as with the  smtp_tls_chain_files  parameter  (see
       there  for  additional  details), but here scoped to just TLS connections in which the client
       sends a matching SNI domain name.

       Example:

           /etc/postfix/main.cf:
               #
               # The indexed SNI table must be created with "postmap -F"
               #
               indexed = ${default_database_type}:${config_directory}/
               tls_server_sni_maps = ${indexed}sni

           /etc/postfix/sni:
               #
               # The example.com domain has both an RSA and ECDSA certificate
               # chain.  The chain files MUST start with the private key,
               # with the certificate chain next, starting with the leaf
               # (server) certificate, and then the issuer certificates.
               #
               example.com /etc/postfix/sni-chains/rsa2048.example.com.pem,
                           /etc/postfix/sni-chains/ecdsa-p256.example.com.pem
               #
               # The example.net domain has a wildcard certificate, and two
               # additional DNS names.  So its certificate chain is also used
               # with any subdomain, plus the additional names.
               #
               example.net /etc/postfix/sni-chains/example.net.pem
               .example.net /etc/postfix/sni-chains/example.net.pem
               example.info /etc/postfix/sni-chains/example.net.pem
               example.org /etc/postfix/sni-chains/example.net.pem

       Note that the SNI lookup tables should also have entries for the domains that  correspond  to
       the  Postfix SMTP server's default certificate(s). This ensures that the remote SMTP client's
       TLS SNI extension gets a positive response when it specifies one of the Postfix SMTP server's
       default  domains,  and ensures that the Postfix SMTP server will not log an SNI name mismatch
       for such a domain.  The Postfix SMTP server's default certificates are then  only  used  when
       the  client sends no SNI or when it sends SNI with a domain that the server knows no certifi‐
       cate(s) for.

       The mapping from an SNI domain name to a certificate chain is indirect.  In the input  source
       files  for "cdb", "hash", "btree" or other tables that are converted to on-disk indexed files
       via [**postmap**(1)](https://www.chedong.com/phpMan.php/man/postmap/1/markdown), the value specified for each key is a list of filenames.  When [**postmap**(1)](https://www.chedong.com/phpMan.php/man/postmap/1/markdown)  is
       used  with  the  **-F** option, the generated table stores for each lookup key the base64-encoded
       contents of the associated files.  When querying tables via **postmap** **-Fq**, the table  value  is
       decoded from base64, yielding the original file content, plus a new line.

       With "regexp", "pcre", "inline", "texthash", "static" and similar tables that are interpreted
       at run-time, and don't have a separate source format, the table value is again a list  files,
       that are loaded into memory when the table is opened.

       With  tables  whose  content  is managed outside of Postfix, such as LDAP, MySQL, PostgreSQL,
       socketmap and tcp, the value must be a concatenation of the desired PEM keys and  certificate
       chains,  that is then further encoded to yield a single-line base64 string.  Creation of such
       tables and secure storage (the value includes private key material) are outside the responsi‐
       bility of Postfix.

       With  "socketmap"  and "tcp" the data will be transmitted in the clear, and there is no query
       access control, so these are generally unsuitable for storing SNI chains.  With LDAP and SQL,
       you should restrict read access and use TLS to protect the sensitive data in transit.

       Typically  there  is  only  one  private  key and its chain of certificates starting with the
       "leaf" certificate corresponding to that key, and continuing with the appropriate  intermedi‐
       ate  issuer  CA  certificates, with each certificate ideally followed by its issuer.  Servers
       that have keys and certificates for more than one algorithm (e.g.  both an  RSA  key  and  an
       ECDSA  key,  or  even  RSA, ECDSA and Ed25519) can use multiple chains concatenated together,
       with the key always listed before the corresponding certificates.

       This feature is available in Postfix 3.4 and later.

**tls**___**session**___**ticket**___**cipher** **(default:** **Postfix** **>=** **3.0:** **aes-256-cbc,** **Postfix** **<** **3.0:** **aes-128-cbc)**
       Algorithm used to encrypt RFC5077 TLS session tickets.  This algorithm  must  use  CBC  mode,
       have  a 128-bit block size, and must have a key length between 128 and 256 bits.  The default
       is aes-256-cbc.  Overriding the default to choose a different algorithm is discouraged.

       Setting this parameter empty disables session ticket support in the Postfix SMTP server.  An‐
       other way to disable session ticket support is via the tls_ssl_options parameter.

       This feature is available in Postfix 3.0 and later.

**tls**___**ssl**___**options** **(default:** **empty)**
       List or bit-mask of OpenSSL options to enable.

       The  OpenSSL  toolkit  provides  a  set  of  options that applications can enable to tune the
       OpenSSL behavior.  Some of these work around bugs in other implementations and are on by  de‐
       fault.   You can use the tls_disable_workarounds parameter to selectively disable some or all
       of the bug work-arounds, making OpenSSL more strict at the cost of non-interoperability  with
       SSL clients or servers that exhibit the bugs.

       Other  options  are  off by default, and typically enable or disable features rather than bug
       work-arounds.  These may be turned on (with care) via  the  tls_ssl_options  parameter.   The
       value  is  a white-space or comma separated list of named options chosen from the list below.
       The names are not case-sensitive, you can use lower-case if you prefer.  The upper case  val‐
       ues below match the corresponding macro name in the ssl.h header file with the SSL_OP_ prefix
       removed.  It is possible that your OpenSSL version includes  new  options  added  after  your
       Postfix  source  code was last updated, in that case you can only enable one of these via the
       hexadecimal syntax below.

       You should only enable features via the hexadecimal mask when the need to control the feature
       is  critical (to deal with a new vulnerability or a serious interoperability problem).  Post‐
       fix DOES NOT promise backwards compatible behavior with respect to the mask bits.  A  feature
       enabled via the mask in one release may be enabled by other means in a later release, and the
       mask bit will then be ignored.  Therefore, use of the hexadecimal mask is  only  a  temporary
       measure until a new Postfix or OpenSSL release provides a better solution.

       If  the  value of the parameter is a hexadecimal long integer starting with "0x", the options
       corresponding to the  bits  specified  in  its  value  are  enabled  (see  openssl/ssl.h  and
       **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown)).  You can only enable options not already controlled by other Postfix
       settings.  For example, you cannot disable protocols or enable server cipher preference.   Do
       not  attempt  to  turn  all  features by specifying 0xFFFFFFFF, this is unlikely to be a good
       idea.  Some bug work-arounds are also valid here, allowing  them  to  be  re-enabled  if/when
       they're no longer enabled by default.  The supported values include:

       **ENABLE**___**MIDDLEBOX**___**COMPAT**
              Postfix >= 3.4. See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       **LEGACY**___**SERVER**___**CONNECT**
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       **NO**___**TICKET**
              Enabled by default when needed in fully-patched Postfix >= 2.7.  Not needed at all for
              Postfix >= 2.11, unless for some reason you do not want to support TLS session resump‐
              tion.  Best not set explicitly.  See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       **NO**___**COMPRESSION**
              Disable  SSL  compression  even  if  supported by the OpenSSL library.  Compression is
              CPU-intensive, and compression before encryption does not always improve security.

       **NO**___**RENEGOTIATION**
              Postfix >= 3.4.  This can reduce opportunities for a potential CPU exhaustion  attack.
              See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       **NO**___**SESSION**___**RESUMPTION**___**ON**___**RENEGOTIATION**
              Postfix >= 3.4. See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       **PRIORITIZE**___**CHACHA**
              Postfix >= 3.4. See **SSL**___**CTX**___**set**___**[options**(3)](https://www.chedong.com/phpMan.php/man/options/3/markdown).

       This feature is available in Postfix 2.11 and later.

**tls**___**wildcard**___**matches**___**multiple**___**labels** **(default:** **yes)**
       Match multiple DNS labels with "*" in wildcard certificates.

       Some  mail service providers prepend the customer domain name to a base domain for which they
       have a wildcard TLS certificate.  For example, the MX records for example.com hosted by exam‐
       ple.net may be:

           example.com. IN MX 0 example.com.mx1.example.net.
           example.com. IN MX 0 example.com.mx2.example.net.

       and  the  TLS  certificate may be for "*.example.net". The "*" then corresponds with multiple
       labels in the mail server domain name.  While multi-label wildcards are not widely supported,
       and are not blessed by any standard, there is little to be gained by disallowing their use in
       this context.

       Notes:

       •      In a certificate name, the "*" is special only when it is used as the first label.

       •      While Postfix (2.11 or later) can match "*" with multiple domain  name  labels,  other
              implementations likely will not.

       •      Earlier  Postfix  implementations behave as if "tls_wildcard_matches_multiple_labels =
              no".

       This feature is available in Postfix 2.11 and later.

**tlsmgr**___**service**___**name** **(default:** **tlsmgr)**
       The name of the [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown) service entry in master.cf.  This  service  maintains  TLS  session
       caches and other information in support of TLS.

       This feature is available in Postfix 2.11 and later.

**tlsproxy**___**client**___**CAfile** **(default:** **$smtp**___**tls**___**CAfile)**
       A  file  containing CA certificates of root CAs trusted to sign either remote TLS server cer‐
       tificates or intermediate CA certificates.  See smtp_tls_CAfile for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**CApath** **(default:** **$smtp**___**tls**___**CApath)**
       Directory with PEM format Certification Authority certificates that the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)
       client  uses  to  verify a remote TLS server certificate. See smtp_tls_CApath for further de‐
       tails.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**cert**___**file** **(default:** **$smtp**___**tls**___**cert**___**file)**
       File  with  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)   client   RSA   certificate   in   PEM   format.   See
       smtp_tls_cert_file  for further details.  The preferred way to configure tlsproxy client keys
       and certificates is via the "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**chain**___**files** **(default:** **$smtp**___**tls**___**chain**___**files)**
       Files with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client keys and certificate  chains  in  PEM  format.  See
       smtp_tls_chain_files for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**dcert**___**file** **(default:** **$smtp**___**tls**___**dcert**___**file)**
       File   with   the   Postfix   [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)   client   DSA   certificate  in  PEM  format.  See
       smtp_tls_dcert_file for further details. DSA is obsolete and should not be used.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**dkey**___**file** **(default:** **$smtp**___**tls**___**dkey**___**file)**
       File  with  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  client  DSA   private   key   in   PEM   format.   See
       smtp_tls_dkey_file for further details. DSA is obsolete and should not be used.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**eccert**___**file** **(default:** **$smtp**___**tls**___**eccert**___**file)**
       File  with  the  Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client ECDSA certificate in PEM format. See smtp_tls_ec‐
       cert_file for further details. The preferred way to configure tlsproxy client keys  and  cer‐
       tificates is via the "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**eckey**___**file** **(default:** **$smtp**___**tls**___**eckey**___**file)**
       File   with   the   Postfix   [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  client  ECDSA  private  key  in  PEM  format.  See
       smtp_tls_eckey_file for further details.  The preferred way to configure tlsproxy client keys
       and certificates is via the "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**enforce**___**tls** **(default:** **$smtp**___**enforce**___**tls)**
       Enforcement  mode:  require  that  SMTP servers use TLS encryption.  See smtp_enforce_tls for
       further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**fingerprint**___**digest** **(default:** **$smtp**___**tls**___**fingerprint**___**digest)**
       The message digest algorithm used to construct remote TLS  server  certificate  fingerprints.
       See smtp_tls_fingerprint_digest for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**key**___**file** **(default:** **$smtp**___**tls**___**key**___**file)**
       File with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client RSA private key in PEM format. See smtp_tls_key_file
       for further details. The preferred way to configure tlsproxy client keys and certificates  is
       via the "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**loglevel** **(default:** **$smtp**___**tls**___**loglevel)**
       Enable  additional  Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client logging of TLS activity. See smtp_tls_loglevel
       for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**loglevel**___**parameter** **(default:** **smtp**___**tls**___**loglevel)**
       The name of the parameter that provides the tlsproxy_client_loglevel value.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**per**___**site** **(default:** **$smtp**___**tls**___**per**___**site)**
       Optional lookup tables with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client TLS usage policy by next-hop  des‐
       tination and by remote TLS server hostname.  See smtp_tls_per_site for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**policy**___**maps** **(default:** **$smtp**___**tls**___**policy**___**maps)**
       Optional  lookup  tables  with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client TLS security policy by next-hop
       destination. See smtp_tls_policy_maps for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**scert**___**verifydepth** **(default:** **$smtp**___**tls**___**scert**___**verifydepth)**
       The verification depth for remote TLS server  certificates.   See  smtp_tls_scert_verifydepth
       for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**security**___**level** **(default:** **$smtp**___**tls**___**security**___**level)**
       The  default  TLS  security  level  for  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) client. See smtp_tls_secu‐
       rity_level for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**client**___**use**___**tls** **(default:** **$smtp**___**use**___**tls)**
       Opportunistic mode: use TLS when a remote server announces TLS support. See smtp_use_tls  for
       further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**enforce**___**tls** **(default:** **$smtpd**___**enforce**___**tls)**
       Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require that clients use
       TLS encryption. See smtpd_enforce_tls for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**service**___**name** **(default:** **tlsproxy)**
       The name of the [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) service entry in master.cf. This service performs  plaintext  <=>
       TLS ciphertext conversion.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**CAfile** **(default:** **$smtpd**___**tls**___**CAfile)**
       A file containing (PEM format) CA certificates of root CAs trusted to sign either remote SMTP
       client certificates or intermediate CA certificates.  See smtpd_tls_CAfile  for  further  de‐
       tails.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**CApath** **(default:** **$smtpd**___**tls**___**CApath)**
       A directory containing (PEM format) CA certificates of root CAs trusted to sign either remote
       SMTP client certificates or intermediate CA certificates. See  smtpd_tls_CApath  for  further
       details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**always**___**issue**___**session**___**ids** **(default:** **$smtpd**___**tls**___**always**___**issue**___**session**___**ids)**
       Force the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server to issue a TLS session id, even when TLS session caching
       is turned off. See smtpd_tls_always_issue_session_ids for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**ask**___**ccert** **(default:** **$smtpd**___**tls**___**ask**___**ccert)**
       Ask a remote SMTP client for a client certificate. See smtpd_tls_ask_ccert  for  further  de‐
       tails.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**ccert**___**verifydepth** **(default:** **$smtpd**___**tls**___**ccert**___**verifydepth)**
       The verification depth for remote SMTP client certificates. A depth of 1 is sufficient if the
       issuing CA is listed in a local CA file. See smtpd_tls_ccert_verifydepth for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**cert**___**file** **(default:** **$smtpd**___**tls**___**cert**___**file)**
       File with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server RSA certificate in PEM format.  This file  may  also
       contain  the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server private RSA key.  See smtpd_tls_cert_file for further
       details.  With Postfix >= 3.4 the preferred way to configure tlsproxy server  keys  and  cer‐
       tificates is via the "tlsproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**chain**___**files** **(default:** **$smtpd**___**tls**___**chain**___**files)**
       Files  with  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  server keys and certificate chains in PEM format. See
       smtpd_tls_chain_files for further details.

       This feature is available in Postfix 3.4 and later.

**tlsproxy**___**tls**___**ciphers** **(default:** **$smtpd**___**tls**___**ciphers)**
       The minimum TLS cipher grade that the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server will use with  opportunistic
       TLS encryption. See smtpd_tls_ciphers for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**dcert**___**file** **(default:** **$smtpd**___**tls**___**dcert**___**file)**
       File  with  the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server DSA certificate in PEM format.  This file may also
       contain the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server private DSA key.  DSA is obsolete and  should  not  be
       used.  See smtpd_tls_dcert_file for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**dh1024**___**param**___**file** **(default:** **$smtpd**___**tls**___**dh1024**___**param**___**file)**
       File  with  DH  parameters that the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server should use with non-export EDH
       ciphers. See smtpd_tls_dh1024_param_file for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**dh512**___**param**___**file** **(default:** **$smtpd**___**tls**___**dh512**___**param**___**file)**
       File with DH parameters that the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server should use with export-grade  EDH
       ciphers.  See smtpd_tls_dh512_param_file for further details.  The default SMTP server cipher
       grade is "medium" with Postfix releases after the  middle  of  2015,  and  as  a  result  ex‐
       port-grade cipher suites are by default not used.

       With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no longer supported, and this
       parameter is silently ignored.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**dkey**___**file** **(default:** **$smtpd**___**tls**___**dkey**___**file)**
       File with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server DSA private key in PEM format.   This  file  may  be
       combined   with   the   Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  server  DSA  certificate  file  specified  with
       $smtpd_tls_dcert_file.  DSA is obsolete and should not be used.  See smtpd_tls_dkey_file  for
       further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**eccert**___**file** **(default:** **$smtpd**___**tls**___**eccert**___**file)**
       File with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server ECDSA certificate in PEM format.  This file may also
       contain the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server private ECDSA key.  See smtpd_tls_eccert_file for fur‐
       ther  details.   With  Postfix >= 3.4 the preferred way to configure tlsproxy server keys and
       certificates is via the "tlsproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**eckey**___**file** **(default:** **$smtpd**___**tls**___**eckey**___**file)**
       File with the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server ECDSA private key in PEM format.  This file  may  be
       combined   with  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  server  ECDSA  certificate  file  specified  with
       $smtpd_tls_eccert_file.  See smtpd_tls_eckey_file for further details.  With Postfix  >=  3.4
       the  preferred  way  to  configure  tlsproxy  server  keys  and  certificates is via the "tl‐
       sproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**eecdh**___**grade** **(default:** **$smtpd**___**tls**___**eecdh**___**grade)**
       The Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server security grade  for  ephemeral  elliptic-curve  Diffie-Hellman
       (EECDH) key exchange. See smtpd_tls_eecdh_grade for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**exclude**___**ciphers** **(default:** **$smtpd**___**tls**___**exclude**___**ciphers)**
       List of ciphers or cipher types to exclude from the [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server cipher list at all TLS
       security levels. See smtpd_tls_exclude_ciphers for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**fingerprint**___**digest** **(default:** **$smtpd**___**tls**___**fingerprint**___**digest)**
       The message digest algorithm to construct remote SMTP  client-certificate  fingerprints.  See
       smtpd_tls_fingerprint_digest for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**key**___**file** **(default:** **$smtpd**___**tls**___**key**___**file)**
       File  with  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server RSA private key in PEM format.  This file may be
       combined  with  the  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  server  RSA  certificate   file   specified   with
       $smtpd_tls_cert_file.   See  smtpd_tls_key_file for further details.  With Postfix >= 3.4 the
       preferred  way  to  configure  tlsproxy  server  keys  and  certificates  is  via  the   "tl‐
       sproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**loglevel** **(default:** **$smtpd**___**tls**___**loglevel)**
       Enable  additional  Postfix  [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)  server logging of TLS activity.  Each logging level
       also includes the information that is logged at a lower logging level. See smtpd_tls_loglevel
       for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**mandatory**___**ciphers** **(default:** **$smtpd**___**tls**___**mandatory**___**ciphers)**
       The  minimum TLS cipher grade that the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server will use with mandatory TLS
       encryption. See smtpd_tls_mandatory_ciphers for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**mandatory**___**exclude**___**ciphers** **(default:** **$smtpd**___**tls**___**mandatory**___**exclude**___**ciphers)**
       Additional list of ciphers or cipher types to exclude from the [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server cipher list
       at  mandatory  TLS  security levels.  See smtpd_tls_mandatory_exclude_ciphers for further de‐
       tails.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**mandatory**___**protocols** **(default:** **$smtpd**___**tls**___**mandatory**___**protocols)**
       The SSL/TLS protocols accepted by the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server with mandatory  TLS  encryp‐
       tion. If the list is empty, the server supports all available SSL/TLS protocol versions.  See
       smtpd_tls_mandatory_protocols for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**protocols** **(default:** **$smtpd**___**tls**___**protocols)**
       List of TLS protocols that the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server will exclude or include with oppor‐
       tunistic TLS encryption. See smtpd_tls_protocols for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**req**___**ccert** **(default:** **$smtpd**___**tls**___**req**___**ccert)**
       With  mandatory  TLS encryption, require a trusted remote SMTP client certificate in order to
       allow TLS connections to proceed.  See smtpd_tls_req_ccert for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**security**___**level** **(default:** **$smtpd**___**tls**___**security**___**level)**
       The SMTP TLS security level for the Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server; when  a  non-empty  value  is
       specified,  this  overrides  the obsolete parameters smtpd_use_tls and smtpd_enforce_tls. See
       smtpd_tls_security_level for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**tls**___**session**___**cache**___**timeout** **(default:** **$smtpd**___**tls**___**session**___**cache**___**timeout)**
       Obsolete expiration time of Postfix [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) server TLS session cache  information.  Since
       the cache is shared with [**smtpd**(8)](https://www.chedong.com/phpMan.php/man/smtpd/8/markdown) and managed by [**tlsmgr**(8)](https://www.chedong.com/phpMan.php/man/tlsmgr/8/markdown), there is only one expiration time
       for the SMTP server cache shared by all three services, namely  smtpd_tls_session_cache_time‐
       out.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**use**___**tls** **(default:** **$smtpd**___**use**___**tls)**
       Opportunistic  TLS: announce STARTTLS support to remote SMTP clients, but do not require that
       clients use TLS encryption. See smtpd_use_tls for further details.

       This feature is available in Postfix 2.8 and later.

**tlsproxy**___**watchdog**___**timeout** **(default:** **10s)**
       How much time a [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown) process may take to process local or remote I/O before it is ter‐
       minated  by  a built-in watchdog timer.  This is a safety mechanism that prevents [**tlsproxy**(8)](https://www.chedong.com/phpMan.php/man/tlsproxy/8/markdown)
       from becoming non-responsive due to a bug in Postfix itself or in system software.  To  avoid
       false alarms and unnecessary cache corruption this limit cannot be set under 10s.

       Specify  a  non-zero  time  value  (an integral value plus an optional one-letter suffix that
       specifies the time unit).  Time units: s (seconds), m  (minutes),  h  (hours),  d  (days),  w
       (weeks).

       This feature is available in Postfix 2.8 and later

**trace**___**service**___**name** **(default:** **trace)**
       The  name of the trace service. This service is implemented by the [**bounce**(8)](https://www.chedong.com/phpMan.php/man/bounce/8/markdown) daemon and main‐
       tains a record of mail deliveries and produces a mail delivery report when  verbose  delivery
       is requested with "**sendmail** **-v**".

       This feature is available in Postfix 2.1 and later.

**transport**___**delivery**___**slot**___**cost** **(default:** **$default**___**delivery**___**slot**___**cost)**
       A  transport-specific  override  for  the  default_delivery_slot_cost  parameter value, where
       _transport_ is the master.cf name of the message delivery transport.

       Note: _transport__delivery_slot_cost parameters will not show up in "postconf"  command  output
       before  Postfix version 2.9.  This limitation applies to many parameters whose name is a com‐
       bination of a  master.cf  service  name  and  a  built-in  suffix  (in  this  case:  "_deliv‐
       ery_slot_cost").

**transport**___**delivery**___**slot**___**discount** **(default:** **$default**___**delivery**___**slot**___**discount)**
       A  transport-specific  override for the default_delivery_slot_discount parameter value, where
       _transport_ is the master.cf name of the message delivery transport.

       Note: _transport__delivery_slot_discount parameters will not show up in "postconf" command out‐
       put  before  Postfix version 2.9.  This limitation applies to many parameters whose name is a
       combination of a master.cf service name  and  a  built-in  suffix  (in  this  case:  "_deliv‐
       ery_slot_discount").

**transport**___**delivery**___**slot**___**loan** **(default:** **$default**___**delivery**___**slot**___**loan)**
       A  transport-specific  override  for  the  default_delivery_slot_loan  parameter value, where
       _transport_ is the master.cf name of the message delivery transport.

       Note: _transport__delivery_slot_loan parameters will not show up in "postconf"  command  output
       before  Postfix version 2.9.  This limitation applies to many parameters whose name is a com‐
       bination of a  master.cf  service  name  and  a  built-in  suffix  (in  this  case:  "_deliv‐
       ery_slot_loan").

**transport**___**destination**___**concurrency**___**failed**___**cohort**___**limit**     **(default:**     **$default**___**destination**___**concur**‐‐
       **rency**___**failed**___**cohort**___**limit)**
       A transport-specific override for the default_destination_concurrency_failed_cohort_limit pa‐
       rameter value, where _transport_ is the master.cf name of the message delivery transport.

       Note:  some _transport__destination_concurrency_failed_cohort_limit parameters will not show up
       in "postconf" command output before Postfix version 2.9.  This limitation applies to many pa‐
       rameters  whose  name  is a combination of a master.cf service name and a built-in suffix (in
       this case: "_destination_concurrency_failed_cohort_limit").

       This feature is available in Postfix 2.5 and later.

**transport**___**destination**___**concurrency**___**limit** **(default:** **$default**___**destination**___**concurrency**___**limit)**
       A transport-specific override for the default_destination_concurrency_limit parameter  value,
       where _transport_ is the master.cf name of the message delivery transport.

       Note:  some _transport__destination_concurrency_limit parameters will not show up in "postconf"
       command output before Postfix version 2.9.  This limitation applies to many parameters  whose
       name is a combination of a master.cf service name and a built-in suffix (in this case: "_des‐
       tination_concurrency_limit").

**transport**___**destination**___**concurrency**___**negative**___**feedback** **(default:** **$default**___**destination**___**concurrency**___**nega**‐‐
       **tive**___**feedback)**
       A  transport-specific  override for the default_destination_concurrency_negative_feedback pa‐
       rameter value, where _transport_ is the master.cf name of the message delivery transport.

       Note: some _transport__destination_concurrency_negative_feedback parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation applies to many param‐
       eters whose name is a combination of a master.cf service name and a built-in suffix (in  this
       case: "_destination_concurrency_negative_feedback").

       This feature is available in Postfix 2.5 and later.

**transport**___**destination**___**concurrency**___**positive**___**feedback** **(default:** **$default**___**destination**___**concurrency**___**posi**‐‐
       **tive**___**feedback)**
       A transport-specific override for the  default_destination_concurrency_positive_feedback  pa‐
       rameter value, where _transport_ is the master.cf name of the message delivery transport.

       Note: some _transport__destination_concurrency_positive_feedback parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation applies to many param‐
       eters  whose name is a combination of a master.cf service name and a built-in suffix (in this
       case: "_destination_concurrency_positive_feedback").

       This feature is available in Postfix 2.5 and later.

**transport**___**destination**___**rate**___**delay** **(default:** **$default**___**destination**___**rate**___**delay)**
       A transport-specific override for the default_destination_rate_delay parameter  value,  where
       _transport_ is the master.cf name of the message delivery transport.

       Note: some _transport__destination_rate_delay parameters will not show up in "postconf" command
       output before Postfix version 2.9.  This limitation applies to many parameters whose name  is
       a  combination  of  a  master.cf service name and a built-in suffix (in this case: "_destina‐
       tion_rate_delay").

       This feature is available in Postfix 2.5 and later.

**transport**___**destination**___**recipient**___**limit** **(default:** **$default**___**destination**___**recipient**___**limit)**
       A transport-specific override for the  default_destination_recipient_limit  parameter  value,
       where _transport_ is the master.cf name of the message delivery transport.

       Note:  some  _transport__destination_recipient_limit  parameters will not show up in "postconf"
       command output before Postfix version 2.9.  This limitation applies to many parameters  whose
       name is a combination of a master.cf service name and a built-in suffix (in this case: "_des‐
       tination_recipient_limit").

**transport**___**extra**___**recipient**___**limit** **(default:** **$default**___**extra**___**recipient**___**limit)**
       A transport-specific override for the default_extra_recipient_limit  parameter  value,  where
       _transport_ is the master.cf name of the message delivery transport.

       Note:  _transport__extra_recipient_limit parameters will not show up in "postconf" command out‐
       put before Postfix version 2.9.  This limitation applies to many parameters whose name  is  a
       combination  of a master.cf service name and a built-in suffix (in this case: "_extra_recipi‐
       ent_limit").

**transport**___**initial**___**destination**___**concurrency** **(default:** **$initial**___**destination**___**concurrency)**
       A transport-specific override for the initial_destination_concurrency parameter value,  where
       _transport_ is the master.cf name of the message delivery transport.

       Note:  some  _transport__initial_destination_concurrency  parameters will not show up in "post‐
       conf" command output before Postfix version 2.9.  This limitation applies to many  parameters
       whose  name is a combination of a master.cf service name and a built-in suffix (in this case:
       "_initial_destination_concurrency").

       This feature is available in Postfix 2.5 and later.

**transport**___**maps** **(default:** **empty)**
       Optional lookup tables with mappings from recipient address to (message  delivery  transport,
       next-hop destination).  See [**transport**(5)](https://www.chedong.com/phpMan.php/man/transport/5/markdown) for details.

       Specify  zero  or  more  "type:table" lookup tables, separated by whitespace or comma. Tables
       will be searched in the specified order until a match is found.  If you use this feature with
       local files, run "**postmap** **/etc/postfix/transport**" after making a change.

       Pattern matching of domain names is controlled by the presence or absence of "transport_maps"
       in the parent_domain_matches_subdomains parameter value.

       For safety reasons, as of Postfix 2.3 this feature does not allow  $number  substitutions  in
       regular expression maps.

       Examples:

       transport_maps = dbm:/etc/postfix/transport
       transport_maps = hash:/etc/postfix/transport

**transport**___**minimum**___**delivery**___**slots** **(default:** **$default**___**minimum**___**delivery**___**slots)**
       A  transport-specific  override for the default_minimum_delivery_slots parameter value, where
       _transport_ is the master.cf name of the message delivery transport.

       Note: _transport__minimum_delivery_slots parameters will not show up in "postconf" command out‐
       put  before  Postfix version 2.9.  This limitation applies to many parameters whose name is a
       combination of a master.cf service name and a built-in suffix (in this case: "_minimum_deliv‐
       ery_slots").

**transport**___**recipient**___**limit** **(default:** **$default**___**recipient**___**limit)**
       A  transport-specific  override for the default_recipient_limit parameter value, where _trans__‐
       _port_ is the master.cf name of the message delivery transport.

       Note: some _transport__recipient_limit parameters will not show up in "postconf" command output
       before  Postfix version 2.9.  This limitation applies to many parameters whose name is a com‐
       bination of a master.cf  service  name  and  a  built-in  suffix  (in  this  case:  "_recipi‐
       ent_limit").

**transport**___**recipient**___**refill**___**delay** **(default:** **$default**___**recipient**___**refill**___**delay)**
       A  transport-specific  override for the default_recipient_refill_delay parameter value, where
       _transport_ is the master.cf name of the message delivery transport.

       Note: _transport__recipient_refill_delay parameters will not show up in "postconf" command out‐
       put  before  Postfix version 2.9.  This limitation applies to many parameters whose name is a
       combination of a master.cf service name and a built-in suffix (in this case:  "_recipient_re‐
       fill_delay").

       This feature is available in Postfix 2.4 and later.

**transport**___**recipient**___**refill**___**limit** **(default:** **$default**___**recipient**___**refill**___**limit)**
       A  transport-specific  override for the default_recipient_refill_limit parameter value, where
       _transport_ is the master.cf name of the message delivery transport.

       Note: _transport__recipient_refill_limit parameters will not show up in "postconf" command out‐
       put  before  Postfix version 2.9.  This limitation applies to many parameters whose name is a
       combination of a master.cf service name and a built-in suffix (in this case:  "_recipient_re‐
       fill_limit").

       This feature is available in Postfix 2.4 and later.

**transport**___**retry**___**time** **(default:** **60s)**
       The  time  between  attempts by the Postfix queue manager to contact a malfunctioning message
       delivery transport.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**transport**___**time**___**limit** **(default:** **$command**___**time**___**limit)**
       A  transport-specific override for the command_time_limit parameter value, where _transport_ is
       the master.cf name of the message delivery transport.

       Note: _transport__time_limit parameters will not show up in "postconf"  command  output  before
       Postfix  version 2.9.  This limitation applies to many parameters whose name is a combination
       of a master.cf service name and a built-in suffix (in this case: "_time_limit").

**transport**___**transport**___**rate**___**delay** **(default:** **$default**___**transport**___**rate**___**delay)**
       A transport-specific override for the default_transport_rate_delay parameter value, where the
       initial  _transport_ in the parameter name is the master.cf name of the message delivery trans‐
       port.

**trigger**___**timeout** **(default:** **10s)**
       The time limit for sending a trigger to a Postfix  daemon  (for  example,  the  [**pickup**(8)](https://www.chedong.com/phpMan.php/man/pickup/8/markdown)  or
       [**qmgr**(8)](https://www.chedong.com/phpMan.php/man/qmgr/8/markdown) daemon). This time limit prevents programs from getting stuck when the mail system is
       under heavy load.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).  The default time  unit
       is s (seconds).

**undisclosed**___**recipients**___**header** **(default:** **see** **postconf** **-d** **output)**
       Message  header  that the Postfix [**cleanup**(8)](https://www.chedong.com/phpMan.php/man/cleanup/8/markdown) server inserts when a message contains no To: or
       Cc: message header. With Postfix 2.8 and later, the default  value  is  empty.  With  Postfix
       2.4-2.7, specify an empty value to disable this feature.

       Example:

       # Default value before Postfix 2.8.
       # Note: the ":" and ";" are both required.
       undisclosed_recipients_header = To: undisclosed-recipients:;

**unknown**___**address**___**reject**___**code** **(default:** **450)**
       The  numerical  response  code when the Postfix SMTP server rejects a sender or recipient ad‐
       dress because its domain is unknown.  This is one of the possible replies from  the  restric‐
       tions reject_unknown_sender_domain and reject_unknown_recipient_domain.

       Do not change this unless you have a complete understanding of RFC 5321.

**unknown**___**address**___**tempfail**___**action** **(default:** **$reject**___**tempfail**___**action)**
       The  Postfix SMTP server's action when reject_unknown_sender_domain or reject_unknown_recipi‐
       ent_domain fail due to a temporary error condition. Specify "defer" to defer the remote  SMTP
       client  request  immediately.  With  the  default  "defer_if_permit" action, the Postfix SMTP
       server continues to look for opportunities to reject mail, and defers the client request only
       if it would otherwise be accepted.

       This feature is available in Postfix 2.6 and later.

**unknown**___**client**___**reject**___**code** **(default:** **450)**
       The  numerical Postfix SMTP server response code when a client without valid address <=> name
       mapping is rejected by the reject_unknown_client_hostname restriction. The SMTP server always
       replies with 450 when the mapping failed due to a temporary error condition.

       Do not change this unless you have a complete understanding of RFC 5321.

**unknown**___**helo**___**hostname**___**tempfail**___**action** **(default:** **$reject**___**tempfail**___**action)**
       The  Postfix  SMTP server's action when reject_unknown_helo_hostname fails due to a temporary
       error condition. Specify "defer" to defer the remote SMTP client  request  immediately.  With
       the  default "defer_if_permit" action, the Postfix SMTP server continues to look for opportu‐
       nities to reject mail, and defers the client request only if it would otherwise be accepted.

       This feature is available in Postfix 2.6 and later.

**unknown**___**hostname**___**reject**___**code** **(default:** **450)**
       The numerical Postfix SMTP server response code when the hostname specified with the HELO  or
       EHLO command is rejected by the reject_unknown_helo_hostname restriction.

       Do not change this unless you have a complete understanding of RFC 5321.

**unknown**___**local**___**recipient**___**reject**___**code** **(default:** **550)**
       The  numerical  Postfix SMTP server response code when a recipient address is local, and $lo‐
       cal_recipient_maps specifies a list of lookup tables that does not match  the  recipient.   A
       recipient  address  is  local  when  its  domain matches $mydestination, $proxy_interfaces or
       $inet_interfaces.

       The default setting is 550 (reject mail) but it is safer to  initially  use  450  (try  again
       later) so you have time to find out if your local_recipient_maps settings are OK.

       Example:

       unknown_local_recipient_reject_code = 450

       This feature is available in Postfix 2.0 and later.

**unknown**___**relay**___**recipient**___**reject**___**code** **(default:** **550)**
       The numerical Postfix SMTP server reply code when a recipient address matches $relay_domains,
       and relay_recipient_maps specifies a list of lookup tables that does not match the  recipient
       address.

       This feature is available in Postfix 2.0 and later.

**unknown**___**virtual**___**alias**___**reject**___**code** **(default:** **550)**
       The  Postfix  SMTP server reply code when a recipient address matches $virtual_alias_domains,
       and $virtual_alias_maps specifies a list of lookup tables that does not match  the  recipient
       address.

       This feature is available in Postfix 2.0 and later.

**unknown**___**virtual**___**mailbox**___**reject**___**code** **(default:** **550)**
       The Postfix SMTP server reply code when a recipient address matches $virtual_mailbox_domains,
       and $virtual_mailbox_maps specifies a list of lookup tables that does not match the recipient
       address.

       This feature is available in Postfix 2.0 and later.

**unverified**___**recipient**___**defer**___**code** **(default:** **450)**
       The numerical Postfix SMTP server response when a recipient address probe fails due to a tem‐
       porary error condition.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

       Do not change this unless you have a complete understanding of RFC 5321.

       This feature is available in Postfix 2.6 and later.

**unverified**___**recipient**___**reject**___**code** **(default:** **450)**
       The numerical Postfix SMTP server response when a recipient address is rejected  by  the  re‐
       ject_unverified_recipient restriction.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

       Do not change this unless you have a complete understanding of RFC 5321.

       This feature is available in Postfix 2.1 and later.

**unverified**___**recipient**___**reject**___**reason** **(default:** **empty)**
       The  Postfix SMTP server's reply when rejecting mail with reject_unverified_recipient. Do not
       include the numeric SMTP reply code or the enhanced status code. By default, the response in‐
       cludes actual address verification details.

       Example:

       unverified_recipient_reject_reason = Recipient address lookup failed

       This feature is available in Postfix 2.6 and later.

**unverified**___**recipient**___**tempfail**___**action** **(default:** **$reject**___**tempfail**___**action)**
       The  Postfix  SMTP  server's action when reject_unverified_recipient fails due to a temporary
       error condition. Specify "defer" to defer the remote SMTP client  request  immediately.  With
       the  default "defer_if_permit" action, the Postfix SMTP server continues to look for opportu‐
       nities to reject mail, and defers the client request only if it would otherwise be accepted.

       This feature is available in Postfix 2.6 and later.

**unverified**___**sender**___**defer**___**code** **(default:** **450)**
       The numerical Postfix SMTP server response code when a sender address probe fails  due  to  a
       temporary error condition.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

       Do not change this unless you have a complete understanding of RFC 5321.

       This feature is available in Postfix 2.6 and later.

**unverified**___**sender**___**reject**___**code** **(default:** **450)**
       The  numerical  Postfix SMTP server response code when a recipient address is rejected by the
       reject_unverified_sender restriction.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

       Do not change this unless you have a complete understanding of RFC 5321.

       This feature is available in Postfix 2.1 and later.

**unverified**___**sender**___**reject**___**reason** **(default:** **empty)**
       The Postfix SMTP server's reply when rejecting mail with reject_unverified_sender. Do not in‐
       clude  the  numeric SMTP reply code or the enhanced status code. By default, the response in‐
       cludes actual address verification details.

       Example:

       unverified_sender_reject_reason = Sender address lookup failed

       This feature is available in Postfix 2.6 and later.

**unverified**___**sender**___**tempfail**___**action** **(default:** **$reject**___**tempfail**___**action)**
       The Postfix SMTP server's action when reject_unverified_sender fails due to a temporary error
       condition.  Specify "defer" to defer the remote SMTP client request immediately. With the de‐
       fault "defer_if_permit" action, the Postfix SMTP server continues to look  for  opportunities
       to reject mail, and defers the client request only if it would otherwise be accepted.

       This feature is available in Postfix 2.6 and later.

**verp**___**delimiter**___**filter** **(default:** **-=+)**
       The  characters  Postfix accepts as VERP delimiter characters on the Postfix [**sendmail**(1)](https://www.chedong.com/phpMan.php/man/sendmail/1/markdown) com‐
       mand line and in SMTP commands.

       This feature is available in Postfix 1.1 and later.

**virtual**___**alias**___**address**___**length**___**limit** **(default:** **1000)**
       The maximal length of an email address after virtual alias  expansion.   This  stops  virtual
       aliasing loops that increase the address length exponentially.

       This feature is available in Postfix 3.0 and later.

**virtual**___**alias**___**domains** **(default:** **$virtual**___**alias**___**maps)**
       Postfix  is  final  destination for the specified list of virtual alias domains, that is, do‐
       mains for which all addresses are aliased to addresses in other local or remote domains.  The
       SMTP  server  validates recipient addresses with $virtual_alias_maps and rejects non-existent
       recipients. See also the virtual alias domain class in the ADDRESS_CLASS_README file

       This feature is available in Postfix 2.0 and later. The default value is backwards compatible
       with Postfix version 1.1.

       The  default  value is $virtual_alias_maps so that you can keep all information about virtual
       alias domains in one place.  If you have many users, it is  better  to  separate  information
       that changes more frequently (virtual address -> local or remote address mapping) from infor‐
       mation that changes less frequently (the list of virtual domain names).

       Specify a list of host or domain names, "/file/name" or "type:table" patterns,  separated  by
       commas  and/or whitespace. A "/file/name" pattern is replaced by its contents; a "type:table"
       lookup table is matched when a table entry matches a lookup string (the lookup result is  ig‐
       nored).  Continue long lines by starting the next line with whitespace. Specify "!pattern" to
       exclude a host or domain name from the list. The form  "!/file/name"  is  supported  only  in
       Postfix version 2.4 and later.

       See also the VIRTUAL_README and ADDRESS_CLASS_README documents for further information.

       Example:

       virtual_alias_domains = virtual1.tld virtual2.tld

**virtual**___**alias**___**expansion**___**limit** **(default:** **1000)**
       The  maximal number of addresses that virtual alias expansion produces from each original re‐
       cipient.

       This feature is available in Postfix 2.1 and later.

**virtual**___**alias**___**maps** **(default:** **$virtual**___**maps)**
       Optional lookup tables that alias specific mail addresses or domains to other local or remote
       address.  The table format and lookups are documented in [**virtual**(5)](https://www.chedong.com/phpMan.php/man/virtual/5/markdown). For an overview of Post‐
       fix address manipulations see the ADDRESS_REWRITING_README document.

       This feature is available in Postfix 2.0 and later. The default value is backwards compatible
       with Postfix version 1.1.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.  Note: these  lookups  are  recur‐
       sive.

       If you use this feature with indexed files, run "**postmap** **/etc/postfix/virtual**" after changing
       the file.

       Examples:

       virtual_alias_maps = dbm:/etc/postfix/virtual
       virtual_alias_maps = hash:/etc/postfix/virtual

**virtual**___**alias**___**recursion**___**limit** **(default:** **1000)**
       The maximal nesting depth of virtual alias expansion.  Currently the recursion limit  is  ap‐
       plied  only  to  the  left branch of the expansion graph, so the depth of the tree can in the
       worst case reach the sum of the expansion and recursion limits.  This may change in  the  fu‐
       ture.

       This feature is available in Postfix 2.1 and later.

**virtual**___**delivery**___**status**___**filter** **(default:** **$default**___**delivery**___**status**___**filter)**
       Optional  filter  for the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent to change the delivery status code or ex‐
       planatory text of successful or unsuccessful deliveries.  See  default_delivery_status_filter
       for details.

       This feature is available in Postfix 3.0 and later.

**virtual**___**destination**___**concurrency**___**limit** **(default:** **$default**___**destination**___**concurrency**___**limit)**
       The maximal number of parallel deliveries to the same destination via the virtual message de‐
       livery transport. This limit is enforced by the queue manager. The message delivery transport
       name is the first field in the entry in the master.cf file.

**virtual**___**destination**___**recipient**___**limit** **(default:** **$default**___**destination**___**recipient**___**limit)**
       The maximal number of recipients per message for the virtual message delivery transport. This
       limit is enforced by the queue manager. The message delivery  transport  name  is  the  first
       field in the entry in the master.cf file.

       Setting  this  parameter  to  a value of 1 changes the meaning of virtual_destination_concur‐
       rency_limit from concurrency per domain into concurrency per recipient.

**virtual**___**gid**___**maps** **(default:** **empty)**
       Lookup tables with the per-recipient group ID for [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) mailbox delivery.

       This parameter is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply when mail  is
       delivered with a different mail delivery program.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       In a lookup table, specify a left-hand side of "@domain.tld" to match any user in the  speci‐
       fied domain that does not have a specific "<user@domain.tld>" entry.

       When  a  recipient  address has an optional address extension (user+<foo@domain.tld>), the **vir**‐‐
       [**tual**(8)](https://www.chedong.com/phpMan.php/man/tual/8/markdown) delivery agent looks up the full address first, and when the lookup fails,  it  looks
       up the unextended address (<user@domain.tld>).

       Note 1: for security reasons, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent disallows regular expression sub‐
       stitution of $1 etc. in regular expression lookup tables, because that would open a  security
       hole.

       Note  2: for security reasons, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent will silently ignore requests to
       use the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) server. Instead it will open the table directly. Before  Postfix  version
       2.2, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent will terminate with a fatal error.

**virtual**___**mailbox**___**base** **(default:** **empty)**
       A  prefix  that  the  [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown)  delivery  agent prepends to all pathname results from $vir‐
       tual_mailbox_maps table lookups.  This is a safety measure to ensure that an out  of  control
       map  doesn't  litter the file system with mailboxes.  While virtual_mailbox_base could be set
       to "/", this setting isn't recommended.

       This parameter is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply when mail  is
       delivered with a different mail delivery program.

       Example:

       virtual_mailbox_base = /var/mail

**virtual**___**mailbox**___**domains** **(default:** **$virtual**___**mailbox**___**maps)**
       Postfix  is  final  destination  for the specified list of domains; mail is delivered via the
       $virtual_transport mail delivery transport.  By default this is the Postfix [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) deliv‐
       ery  agent.  The SMTP server validates recipient addresses with $virtual_mailbox_maps and re‐
       jects mail for non-existent recipients.  See also the virtual mailbox domain class in the AD‐
       DRESS_CLASS_README file.

       This parameter expects the same syntax as the mydestination configuration parameter.

       This feature is available in Postfix 2.0 and later. The default value is backwards compatible
       with Postfix version 1.1.

**virtual**___**mailbox**___**limit** **(default:** **51200000)**
       The maximal size in bytes of an individual [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) mailbox or maildir file,  or  zero  (no
       limit).

       This  parameter is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply when mail is
       delivered with a different mail delivery program.

**virtual**___**mailbox**___**lock** **(default:** **see** **postconf** **-d** **output)**
       How to lock a UNIX-style [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) mailbox before attempting delivery.  For a list of avail‐
       able file locking methods, use the "**postconf** **-l**" command.

       This  parameter is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply when mail is
       delivered with a different mail delivery program.

       This setting is ignored with **maildir** style delivery, because such deliveries are safe without
       application-level locks.

       Note  1:  the  **dotlock**  method requires that the recipient UID or GID has write access to the
       parent directory of the recipient's mailbox file.

       Note 2: the default setting of this parameter is system dependent.

**virtual**___**mailbox**___**maps** **(default:** **empty)**
       Optional lookup tables with all valid addresses in  the  domains  that  match  $virtual_mail‐
       box_domains.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       In a lookup table, specify a left-hand side of "@domain.tld" to match any user in the  speci‐
       fied domain that does not have a specific "<user@domain.tld>" entry.

       The  remainder  of this text is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply
       when mail is delivered with a different mail delivery program.

       The [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent uses this table to look up the per-recipient mailbox or maildir
       pathname.  If the lookup result ends in a slash ("/"), maildir-style delivery is carried out,
       otherwise the path is assumed to specify a UNIX-style mailbox file.  Note that $virtual_mail‐
       box_base is unconditionally prepended to this path.

       When  a  recipient  address has an optional address extension (user+<foo@domain.tld>), the **vir**‐‐
       [**tual**(8)](https://www.chedong.com/phpMan.php/man/tual/8/markdown) delivery agent looks up the full address first, and when the lookup fails,  it  looks
       up the unextended address (<user@domain.tld>).

       Note 1: for security reasons, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent disallows regular expression sub‐
       stitution of $1 etc. in regular expression lookup tables, because that would open a  security
       hole.

       Note  2: for security reasons, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent will silently ignore requests to
       use the [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) server. Instead it will open the table directly. Before  Postfix  version
       2.2, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent will terminate with a fatal error.

**virtual**___**maps** **(default:** **empty)**
       Optional  lookup  tables  with a) names of domains for which all addresses are aliased to ad‐
       dresses in other local or remote domains, and b) addresses that are aliased to  addresses  in
       other  local  or  remote domains.  Available before Postfix version 2.0. With Postfix version
       2.0 and later,  this  is  replaced  by  separate  controls:  virtual_alias_domains  and  vir‐
       tual_alias_maps.

**virtual**___**minimum**___**uid** **(default:** **100)**
       The  minimum  user ID value that the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent accepts as a result from $vir‐
       tual_uid_maps table lookup.  Returned values less than this will be rejected, and the message
       will be deferred.

       This  parameter is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply when mail is
       delivered with a different mail delivery program.

**virtual**___**transport** **(default:** **virtual)**
       The default mail delivery transport and next-hop destination for final  delivery  to  domains
       listed  with  $virtual_mailbox_domains.   This  information  can be overruled with the **trans**‐‐
       [**port**(5)](https://www.chedong.com/phpMan.php/man/port/5/markdown) table.

       Specify a string of the form _transport:nexthop_, where _transport_ is the name of a mail  deliv‐
       ery transport defined in master.cf.  The _:nexthop_ destination is optional; its syntax is doc‐
       umented in the manual page of the corresponding delivery agent.

       This feature is available in Postfix 2.0 and later.

**virtual**___**uid**___**maps** **(default:** **empty)**
       Lookup tables with the per-recipient user ID that the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery  agent  uses  while
       writing to the recipient's mailbox.

       This  parameter is specific to the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent.  It does not apply when mail is
       delivered with a different mail delivery program.

       Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
       be searched in the specified order until a match is found.

       In  a lookup table, specify a left-hand side of "@domain.tld" to match any user in the speci‐
       fied domain that does not have a specific "<user@domain.tld>" entry.

       When a recipient address has an optional address extension  (user+<foo@domain.tld>),  the  **vir**‐‐
       [**tual**(8)](https://www.chedong.com/phpMan.php/man/tual/8/markdown)  delivery  agent looks up the full address first, and when the lookup fails, it looks
       up the unextended address (<user@domain.tld>).

       Note 1: for security reasons, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent disallows regular expression sub‐
       stitution  of $1 etc. in regular expression lookup tables, because that would open a security
       hole.

       Note 2: for security reasons, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent will silently ignore requests  to
       use  the  [**proxymap**(8)](https://www.chedong.com/phpMan.php/man/proxymap/8/markdown) server. Instead it will open the table directly. Before Postfix version
       2.2, the [**virtual**(8)](https://www.chedong.com/phpMan.php/man/virtual/8/markdown) delivery agent will terminate with a fatal error.

## SEE ALSO
       [postconf(1)](https://www.chedong.com/phpMan.php/man/postconf/1/markdown), Postfix configuration parameter maintenance
       [master(5)](https://www.chedong.com/phpMan.php/man/master/5/markdown), Postfix daemon configuration maintenance

## LICENSE
       The Secure Mailer license must be distributed with this software.

**AUTHOR(S)**
       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA

       Wietse Venema
       Google, Inc.
       111 8th Avenue
       New York, NY 10011, USA

       Viktor Dukhovni



                                                                                         [POSTCONF(5)](https://www.chedong.com/phpMan.php/man/POSTCONF/5/markdown)