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POSTCONF(5)                            File Formats Manual                            POSTCONF(5)

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  parame-
       ters; parameters not specified are left at their default values.

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

       o      Each  logical  line has the form "parameter = value".  Whitespace around the "=" is
              ignored, as is whitespace at the end of a logical line.

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

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

       o      A parameter value may refer to other parameters.

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

              o      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.

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

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

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

              o      Each  "value" is subject to recursive named parameter and relational expres-
                     sion evaluation, except where noted.

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

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

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

       o      When the same parameter is defined multiple times, only the last instance is remem-
              bered.

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

       The  remainder  of this document is a description of all Postfix configuration parameters.
       Default 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. Un-
       necessary 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) map "defer" action, in-
       cluding "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) 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) address verification database cleanup runs. This fea-
       ture 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) 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) 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  cor-
       rupted,  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  di-
       rectory 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 en-
       abled, the cache may pollute quickly with garbage.  When this feature is disabled, Postfix
       will generate 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
       maximum  size  (qmgr_message_active_limit).  The queue manager enforces the limit by temp-
       failing requests that exceed the limit. This affects only unknown addresses  and  inactive
       addresses  that  have expired, because the verify(8) daemon automatically refreshes an ac-
       tive 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) service for the completion of an  address  verifica-
       tion request in progress.

       By  default,  the  Postfix SMTP server polls the verify(8) 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) service up to three times by default.

       Specify 1 to implement a crude form of greylisting, that is, always defer the first deliv-
       ery 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
       address 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  informa-
       tion can be overruled with the transport(5) 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  ad-
       dress probes, the Postfix SMTP server excludes the probe sender address from all SMTPD ac-
       cess blocks.

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

       Examples:

       address_verify_sender = <>
       address_verify_sender = postmaster AT 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 veri-
       fication 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 addresses 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 inte-
       gral 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) 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)  delivery  that are updated with "newaliases" or with
       "sendmail -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) delivery. See aliases(5) for syntax de-
       tails.  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 recursive.

       The default list is system dependent.  On systems with NIS, the default is to  search  the
       local 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)  delivery  agent  disallows  regular  expression  substitution of $1 etc. in
       alias_maps, because that would open a security hole.

       The local(8) delivery agent will silently ignore requests to use  the  proxymap(8)  server
       within  alias_maps.  Instead  it will open the table directly. Before Postfix version 2.2,
       the local(8) 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) mail delivery to external commands.  The default is to disallow delivery
       to "|command" in :include:  files (see aliases(5) for the text that defines this terminol-
       ogy).

       Specify zero or more of: alias,  forward  or  include,  in  order  to  allow  commands  in
       aliases(5), .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) mail delivery to external files. The default is to disallow "/file/name"
       destinations in :include:  files (see aliases(5) for the text that defines this  terminol-
       ogy).

       Specify  zero or more of: alias, forward or include, in order to allow "/file/name" desti-
       nations in aliases(5), .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 com-
       mand 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 termi-
       nator 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).   With  ear-
       lier  versions  this  feature  was implemented by qmgr(8) and was limited to recipient ad-
       dresses only.

allow_percent_hack (default: yes)
       Enable the rewriting of the form "user%domain" to "user@domain".  This is enabled  by  de-
       fault.

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

       o      The message is received with the Postfix sendmail(1) command,

       o      The message is received  from  a  network  client  that  matches  $local_header_re-
              write_clients,

       o      The  message is received from the network, and the remote_header_rewrite_domain pa-
              rameter 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 ta-
       bles, because the address is ambiguous.

alternate_config_directories (default: empty)
       A list of non-default Postfix configuration directories that may  be  specified  with  "-c
       config_directory"  on the command line (in the case of sendmail(1), 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) and postdrop(1).

       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
       parameter setting.  Earlier Postfix versions always add these headers; this may break DKIM
       signatures  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 un-
       deliverable.

       Note: automatic BCC recipients are produced only for new mail.  To avoid mailer loops, au-
       tomatic  BCC recipients are not generated after Postfix forwards mail internally, or after
       Postfix 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) 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) 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) connection and rate limiting server logs peak  usage  informa-
       tion.

       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 "@$myorigin" to mail addresses without  do-
       main  information.  With  remotely  submitted mail, append the string "@$remote_header_re-
       write_domain" instead.

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

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

       o      The message is received with the Postfix sendmail(1) command,

       o      The message is received  from  a  network  client  that  matches  $local_header_re-
              write_clients,

       o      The  message is received from the network, and the remote_header_rewrite_domain pa-
              rameter 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 ".$mydomain" to addresses that have no
       ".domain"  information.  With  remotely  submitted  mail,   append   the   string   ".$re-
       mote_header_rewrite_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:

       o      The message is received with the Postfix sendmail(1) command,

       o      The  message  is  received  from  a  network  client that matches $local_header_re-
              write_clients,

       o      The message is received from the network, and the remote_header_rewrite_domain  pa-
              rameter 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) command waits for a request to enter the Postfix  daemon  process
       input 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 in-
       voking  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 corre-
       sponding  login  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 in-
       voking 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 corre-
       sponding login 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) command (and with the
       privileged postdrop(1) 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
       parameter 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  ad-
       dresses  contain  the  ":"  character, and would otherwise be confused with a "type:table"
       pattern.

backwards_bounce_logfile_compatibility (default: yes)
       Produce additional bounce(8) 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.
       Specify 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
       myself"  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 re-
       turns such mail as undeliverable.

       Specify, for example, "best_mx_transport = local" to pass the mail from the  Postfix  SMTP
       client to the local(8) delivery agent. You can specify any message delivery "transport" or
       "transport:nexthop" that is defined in the master.cf file.  See  the  transport(5)  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) 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 interac-
       tive  users,  the biff service can be a performance drain.  Specify "biff = no" in main.cf
       to disable.

body_checks (default: empty)
       Optional lookup tables for content inspection as specified in  the  body_checks(5)  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) 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 origi-
       nal) or as text/rfc822-headers (the headers only).  With Postfix version 2.4 and  earlier,
       a  message  is always returned as message/rfc822 and is truncated when it exceeds the size
       limit.

       Notes:

       o      If you increase this limit, then you should increase the  mime_nesting_limit  value
              proportionally.

       o      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) manual page
       describes how to edit and test template files.

       Template message body text may contain $name references to Postfix  configuration  parame-
       ters.  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
       address  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_recipi-
       ent

       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
       applied to both sender and recipient addresses, in both envelopes and in headers, as  con-
       trolled with the canonical_classes parameter. This is typically used to clean up dirty ad-
       dresses from legacy mail systems, or to replace login names  by  Firstname.Lastname.   The
       table  format  and  lookups are documented in canonical(5). For an overview of Postfix ad-
       dress 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
       recursive.

       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
       "postfix reload" to eliminate the delay.

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

       o      The message is received with the Postfix sendmail(1) command,

       o      The  message  is  received  from  a  network  client that matches $local_header_re-
              write_clients,

       o      The message is received from the network, and the remote_header_rewrite_domain  pa-
              rameter 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
       prevent outbound SMTP smuggling, and to make the evaluation of Postfix-added DKIM or other
       signatures 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  dif-
       ferent  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) service. This service  rewrites  addresses  into  the  standard
       form, and performs canonical(5) address mapping and virtual(5) 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)  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_di-
       rectory before the directory is used. Expansion happens in the context of the delivery re-
       quest.  The result of $name expansion is filtered with the character set that is specified
       with the execution_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) delivery agent allows  in  $name  expansions  of
       $mailbox_command and $command_execution_directory.  Characters outside the allowed set are
       replaced by underscores.

command_time_limit (default: 1000s)
       Time limit for delivery to external commands. This limit is used by the local(8)  delivery
       agent, and is the default time limit for delivery by the pipe(8) delivery agent.

       Note:  if  you set this time limit to a large value you must update the global ipc_timeout
       parameter 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
       default 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-com-
       patible 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) 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 compat-
       ibility levels are 0, 1 and 2.

       NOTE: this also introduces support for the "<level", "<=level",  and  other  operators  to
       compare  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:

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

       o      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  alter-
       nate_config_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
       network 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
       enforced 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) 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) 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) 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:destina-
       tion. The transport name specifies the first field of a mail delivery agent definition  in
       master.cf;  the  syntax of the next-hop destination is described in the manual page of the
       corresponding delivery agent.  More information about external content filters is  in  the
       Postfix FILTER_README file.

       Notes:

       o      This  setting has lower precedence than a FILTER action that is specified in an ac-
              cess(5), header_checks(5) or body_checks(5) table.

       o      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.
              Specify "default_filter_nexthop = $myhostname" for compatibility with  Postfix  2.6
              or  earlier,  or  specify a content_filter value with an explicit next-hop destina-
              tion.

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 in-
       voked 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  degrada-
       tion or immediate termination.

        no  (default)
              Gradual  degradation: a daemon process logs a message of type "error" and continues
              execution with reduced functionality. Features that do not depend on  the  unavail-
              able  table 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
              immediately.   This  option reduces the number of possible code paths through Post-
              fix, 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
       lifetime 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 termi-
       nated 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 num-
       bers).  This directory must be owned by the mail_owner account, and  must  not  be  shared
       with non-Postfix 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 speci-
       fied 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  op-
       tion.

       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 be-
       fore starting Postfix.

       Note:  the  command is subject to $name expansion, before it is passed to the default com-
       mand 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), postalias(1) and postmap(1)  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 mes-
       sage 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  controls  how  often  is  the  counter  incremented - it happens after each de-
       fault_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 wait-
       ing until the full amount of delivery slots required is available, the preemption can hap-
       pen  when  transport_delivery_slot_discount  percent  of  the  required amount plus trans-
       port_delivery_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
       transport 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 wait-
       ing until the full amount of delivery slots required is available, the preemption can hap-
       pen when transport_delivery_slot_discount percent  of  the  required  amount  plus  trans-
       port_delivery_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
       unsuccessful 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 de-
       livery is successful, when delivery is rejected with 5XX, or when there are no more alter-
       nate 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
       specified 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  unsuc-
       cessful  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 num-
       ber 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
       destination command or filename when a remote sender requests confirmation  of  successful
       delivery.

           /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:

       o      This feature will NOT override the soft_bounce safety net.

       o      This  feature  will  change the enhanced status code and text that is logged to the
              maillog 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 des-
       tination 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 de-
       livery completes 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
       default  limit  for  delivery  via  the  lmtp(8), pipe(8), smtp(8) and virtual(8) delivery
       agents.  With per-destination recipient limit > 1, a destination is a domain, otherwise it
       is a recipient.

       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
       completes  with  a  connection or handshake failure. Feedback values are in the range 0..1
       inclusive. With negative feedback, concurrency is decremented at the beginning  of  a  se-
       quence of length 1/feedback. This is unlike positive feedback, where concurrency is incre-
       mented at the end of a sequence of length 1/feedback.

       As of Postfix version 2.5, negative feedback cannot reduce delivery concurrency  to  zero.
       Instead,  a  destination  is  marked  dead  (further  delivery suspended) after the failed
       pseudo-cohort  count  reaches   $default_destination_concurrency_failed_cohort_limit   (or
       $transport_destination_concurrency_failed_cohort_limit).  To make the scheduler completely
       immune to connection 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 set-
              ting 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) af-
              ter 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 concur-
              rency 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
       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_positive_feedback (default: 1)
       The per-destination amount of delivery concurrency positive  feedback,  after  a  delivery
       completes  without  connection or handshake failure. Feedback values are in the range 0..1
       inclusive.  The concurrency increases until it reaches the per-destination maximal concur-
       rency  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 set-
              ting 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  concur-
              rency 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
       override, 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.

       o      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
              parallel, subject to the process limits specified in master.cf.

       o      With  a  corresponding  per-destination  recipient limit equal to 1, the rate delay
              specifies the time between deliveries to the same recipient.  Different  recipients
              are delivered 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
       transport 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), pipe(8), smtp(8) and virtual(8) delivery agents.

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

       o      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.

       o      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.

       o      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 re-
       cipients.  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 recip-
       ients slots for the chosen message in order to avoid performance degradation.

       Use transport_extra_recipient_limit to specify a transport-specific override, where trans-
       port 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
       $default_filter_nexthop instead; when that value is empty, use the domain in the recipient
       address.   Specify  "default_filter_nexthop  = $myhostname" for compatibility with Postfix
       version 2.6 and earlier, or specify  an  explicit  next-hop  destination  with  each  con-
       tent_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
       scheduling algorithm at all.  Messages which would never accumulate at least this many de-
       livery slots (subject to slot cost parameter as well) are never preempted.

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

default_privs (default: nobody)
       The default rights used by the local(8) delivery agent for delivery to  external  file  or
       command.  These rights are used when delivery is requested from an aliases(5) 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 de-
              tails.

       $reverse_client_name
              The client hostname  from  address->name  lookup,  or  "unknown".   See  reject_un-
              known_reverse_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  con-
              figuration  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  informa-
              tion 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.

       o      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.

       o      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 recip-
              ient 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
       recipients 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
       transport 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  re-
       sult  in recipient batches lower than this when this limit is too high for too slow deliv-
       eries.

       Use transport_recipient_refill_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 2.4 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) 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  de-
       livery  transport  defined in master.cf.  The :nexthop destination is optional; its syntax
       is documented 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
       initial 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. Spec-
       ify 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 re-
       jected 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)  daemon  and
       maintains 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 is-
       sues "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
       delay_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:

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

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

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

       o      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
       delivered 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)
       logfile.

deliver_lock_delay (default: 1s)
       The time between attempts to acquire an exclusive lock on a mailbox file or bounce(8) 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:  mes-
       sage 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  head-
       ers is considered 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 re-
       quested record does not exist. If this promise is broken, specify "yes" to enable a  work-
       around 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)  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 Post-
       fix receives TLSA records with secure TLS server certificate info. When DNSSEC  validation
       is  unavailable,  mail deliveries using opportunistic DANE will not be protected by server
       certificate 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, Post-
       fix 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:

       o      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).

       o      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) 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)
       or virtual(5) alias expansion, or for showq(8) 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
       address.

       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  ad-
       dresses  in SMTP commands, but they may still be created locally as the result of configu-
       ration 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: mes-
       sage  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
       domain names to/from the ASCII form that is used for DNS lookups. Specify "yes"  for  com-
       patibility  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://uni-
       code.org/cldr/utility/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:

       o      Existing queue file names are not affected.

       o      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  sec-
              onds, 4 characters for the time in microseconds, the 'z'; the remainder is the file
              inode number encoded in the first 51 characters of the 52-character alphabet.

       o      New messages have a Message-ID header with queueID@myhostname.

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

       o      The hash_queue_depth algorithm uses the first characters of the queue file creation
              time in microseconds, after conversion into hexadecimal representation.  This  pro-
              duces  the  same  queue hashing behavior as if the queue file name was created with
              "enable_long_queue_ids = no".

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

       o      Existing long queue file names are renamed to the short form (while running  "post-
              fix reload" or "postsuper").

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

       o      New messages have a Message-ID header with YYYYMMDDHHMMSS.queueid@myhostname, where
              YYYYMMDDHHMMSS are the year, month, day, hour, minute and second.

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

       o      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  dif-
       ferent 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
              different recipients that share the same mailbox.

       Recipient deduplication
              With  "enable_original_recipient  =  yes", the cleanup(8) daemon performs duplicate
              recipient elimination based on the content of (original recipient,  maybe-rewritten
              recipient)  pairs.   Otherwise,  the cleanup(8) daemon performs duplicate recipient
              elimination based only on the maybe-rewritten recipient address.

       Note: with Postfix <= 3.2 the "setting enable_original_recipient = no" breaks address ver-
       ification  for  addresses  that  are  aliased or otherwise rewritten (Postfix is unable to
       store the address verification result under the original probe  destination  address;  in-
       stead, 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 orig-
       inal 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  re-
              lated 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
       notify_classes parameter.

error_service_name (default: error)
       The  name of the error(8) pseudo delivery agent. This service always returns mail as unde-
       liverable.

       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) delivery agent allows  in  $name  expansions  of
       $command_execution_directory.   Characters  outside the allowed set are replaced by under-
       scores.

       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,
       Postfix 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
       opening  "{" 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 un-
       reachable. 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
       specify 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  fall-
       back_relay host when the final destination is unavailable.

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

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

       o      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) delivery agent should use for  names
       that are not found in the aliases(5) or UNIX password database.

       The precedence of local(8) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,   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) delivery agent could not find in the aliases(5) or UNIX password database.

       The precedence of local(8) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,   mail_spool_directory,   fallback_transport_maps,   fallback_transport  and
       luser_relay.

       For safety reasons, this feature does not allow $number substitutions in  regular  expres-
       sion 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) manual).

       Specify a list of hosts or domains, "/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  ta-
       ble 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_domains" 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 mes-
       sages 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) 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) delivery agent allows in $name expansions of
       $forward_path.  Characters outside the allowed set are replaced by underscores.

forward_path (default: see postconf -d output)
       The local(8) delivery agent search list for finding a .forward  file  with  user-specified
       delivery methods. The first file that is found is used.

       The  forward_path value is not subject to Postfix configuration parameter $name expansion.
       Instead, the following $name expansions are done on forward_path before the  search  actu-
       ally  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) 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 Deliv-
       ered-To: address while expanding aliases or .forward files.

       This feature is available in Postfix 2.3 and later. With older Postfix releases,  the  be-
       havior  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 Deliv-
       ered-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
       parameter. 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
       microseconds.

       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 re-
       sult is used for queue hashing.  The base 16 encoding gives finer control over the  number
       of subdirectories 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
       "postfix 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
       "postfix 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) server.

header_checks (default: empty)
       Optional  lookup  tables  for  content  inspection of primary non-MIME message headers, as
       specified in the header_checks(5) 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) command.

       Specify one of the following:

       standard (default)
              Produce a header formatted as "From: name <address>".  This is the  default  as  of
              Postfix 3.3.

       obsolete
              Produce a header formatted as "From: address (name)". This is the behavior prior to
              Postfix 3.3.

       Notes:

       o      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.

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

       o      The  Postfix sendmail(1) command gets name information from the -F command-line op-
              tion, 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) 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) user's home directory.

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

       The precedence of local(8) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,   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
       headers. 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  de-
       fers  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  vio-
       lates 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 envi-
       ronment.  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
       opening "{" 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
       message 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
       receive  mail  on all network interfaces (default), and "loopback-only" to receive mail on
       loopback network interfaces only (Postfix version 2.2 and later).  The parameter also con-
       trols 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
       address, the Postfix SMTP client will use this address as the IP source address  for  out-
       bound 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
       "outside" 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 potential 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 preserves 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  dif-
       ferent $myhostname setting).

       See also the proxy_interfaces parameter, for network addresses that are forwarded to Post-
       fix 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  imple-
       ments 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 work-
       around 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 sys-
       tems 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  original  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 pro-
       tocol 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 lo-
       calpart will be quoted, for example:

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

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

               from=<name with spaces AT 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  ad-
       dress  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  destina-
       tion.   With  per-destination recipient limit > 1, a destination is a domain, otherwise it
       is a recipient.

       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 follow-
       ing, separated by whitespace or comma.

       bounce Inspect the content of delivery status notifications.

       notify Inspect the content of postmaster notifications by the smtp(8)  and  smtpd(8)  pro-
              cesses.

       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  pa-
       rameter 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  chan-
       nel.   The  purpose  is to break out of deadlock situations. If the time limit is exceeded
       the software 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 pur-
       pose 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) 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
       delivery, and send "delivered" delivery status notifications instead of "relayed". The de-
       fault 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
       details.

       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_connection_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
       connections are closed under any of the following conditions:

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

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

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

       o      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 oper-
       ating 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  parame-
       ter.  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 parame-
       ter.  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 connec-
       tion 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  parame-
       ter.  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 de-
       livery transport. This limit is enforced by the queue manager. The message delivery trans-
       port 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 de-
       tails. The table is not indexed by hostname for consistency  with  smtpd_discard_ehlo_key-
       word_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 Post-
       fix LMTP client will ignore in the LHLO response from a remote LMTP server.

       This feature is available in Postfix 2.3 and later.

       Notes:

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

       o      Use the lmtp_discard_lhlo_keyword_address_maps feature to discard LHLO keywords se-
              lectively.

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
       details.

       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 un-
       reachable.  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, Post-
       fix 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
       details.

       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
       initial 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  parame-
       ter.  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 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_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
       remote 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 re-
       mote LMTP server response. The LMTP client sends RSET in order to finish a  recipient  ad-
       dress 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
       domain.   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 implementa-
       tion that is selected with lmtp_sasl_type.  Typically this specifies the name of a config-
       uration 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 at-
              tacks.

       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 pa-
       rameter.  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
       available 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) 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 param-
       eter.  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)) or a numeric port.

lmtp_tls_CAfile (default: empty)
       The LMTP-specific version of the smtp_tls_CAfile configuration parameter.  See  there  for
       details.

       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
       details.

       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
       details.

       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
       parameter.  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 parame-
       ter.  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)  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 distribu-
       tion).

       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) delivery agent to change the status code  or  explanatory
       text of successful or unsuccessful deliveries.  See default_delivery_status_filter for de-
       tails.

       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  trans-
       port.  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 ad-
       dresses with the domain name in $myorigin or $mydomain; either don't rewrite message head-
       ers  from other clients at all, or rewrite message headers and update incomplete addresses
       with the domain 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
              authenticated via the RFC 4954 (AUTH) protocol.

       permit_tls_clientcerts
              Append the domain name in $myorigin or $mydomain when the remote  SMTP  client  TLS
              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).
              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 cryptanaly-
              sis 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.

       permit_tls_all_clientcerts
              Append  the  domain  name in $myorigin or $mydomain when the remote SMTP client TLS
              certificate 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 in-
       formation 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
       remote 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
       enforced by the Postfix postdrop(1) command. The default  is  backwards-compatible:  every
       user may specify any sender envelope address.

       When  no  UNIX login name is available, the postdrop(1) command will prepend "uid:" to the
       numerical 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  pat-
       tern.

        *     This pattern allows any envelope sender address.

        <>    This  pattern  allows  the empty envelope sender address. See the empty_address_lo-
              cal_login_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 AT 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  lo-
       cal  when its domain matches $mydestination, $inet_interfaces or $proxy_interfaces.  Spec-
       ify @domain as a wild-card for domains that do not have a valid recipient  list.   Techni-
       cally,  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 lo-
       cal delivery. You need to update the local_recipient_maps setting if:

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

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

       o      You use the "luser_relay", "mailbox_transport", or "fallback_transport" feature  of
              the Postfix local(8) 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) service, in order to overcome chroot access restrictions. The alternative,
       maintaining 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) 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  de-
       livery  transport  defined in master.cf.  The :nexthop destination is optional; its syntax
       is documented 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) recipients.  By default, mail for  un-
       known  recipients  in domains that match $mydestination, $inet_interfaces or $proxy_inter-
       faces is returned as undeliverable.

       The luser_relay value is not subject to Postfix configuration parameter  $name  expansion.
       Instead, 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) delivery agent.

       Note:  if  you  use this feature for accounts not in the UNIX password file, then you must
       specify "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 AT other.host
       luser_relay = $local AT other.host
       luser_relay = admin+$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.
       Specify 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 particu-
       lar, 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
       Postfix 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) 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.  Experi-
       mental  releases also include the release date. The version string can be used in, for ex-
       ample, the SMTP greeting banner.

mailbox_command (default: empty)
       Optional external command that the local(8) delivery agent should use for  mailbox  deliv-
       ery.   The  command is run with the user ID and the primary group ID privileges of the re-
       cipient.  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
              version 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 sub-
       jected to $name substitutions. This is to make it easier to specify shell syntax (see  ex-
       ample below).

       If  you  can, avoid shell meta characters because they will force Postfix to run an expen-
       sive shell process. If you're delivering via "procmail" then running a shell won't make  a
       noticeable 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) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,  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) mailbox
       delivery.  Behavior is as with mailbox_command.

       The precedence of local(8) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,  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) mailbox before  attempting  delivery.   For  a  list  of
       available 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
       parent 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) 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,  includ-
       ing files written by external commands that are executed by the local(8) 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) delivery agent should use for mail-
       box delivery to all local recipients, whether or not they are found  in  the  UNIX  passwd
       database.

       The precedence of local(8) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,   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)
       mailbox delivery, whether or not the recipients are found in the UNIX passwd database.

       The precedence of local(8) delivery features from high to low is: aliases, .forward files,
       mailbox_transport_maps,    mailbox_transport,    mailbox_command_maps,    mailbox_command,
       home_mailbox,   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 expres-
       sion 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)  service.  An
       empty  value  selects  logging  to syslogd(8).  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), 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,  sepa-
       rated 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) 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) 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_recip-
       ient

masquerade_domains (default: empty)
       Optional  list  of  domains  whose  subdomain  structure will be stripped off in email ad-
       dresses.

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

           masquerade_domains = foo.example.com example.com

       strips   "user AT any.com"   to    "user AT foo.com",    but    strips
       "user AT any.com" to "user AT 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 AT any.com"  or "user AT foo.com", but strips
       "user AT any.com" to "user AT example.com".

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

       o      The message is received with the Postfix sendmail(1) command,

       o      The  message  is  received  from  a  network  client that matches $local_header_re-
              write_clients,

       o      The message is received from the network, and the remote_header_rewrite_domain  pa-
              rameter 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) 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 ex-
       clude a service from the list. By default, all master(8) listener ports are enabled.

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

       Examples:

       # With Postfix 2.6..2.10 use '.' instead of '/'.
       # Turn on all master(8) 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 con-
       nection 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  be-
       fore  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)  daemon  will  remove  after   applying
       header_checks(5) and before invoking Milter applications.  The default setting is compati-
       ble with Postfix < 3.0.

       Specify a list of header names, separated by comma or  space.   Names  are  matched  in  a
       case-insensitive  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  es-
       cape  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
       message size limit.

message_strip_characters (default: empty)
       The set of characters that Postfix will remove from message content.  The usual C-like es-
       cape 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.   Typi-
       cally, 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  =
       $daemon_directory" in main.cf before installing or upgrading Postfix, or specify "meta_di-
       rectory = /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
       receiving 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
       protocol 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
       receiving 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 Mil-
       ter applications.  See the header_checks(5)  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
       Postfix  may send to Milter applications.  These defaults are used when there is no corre-
       sponding 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
       enclosing "{", 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
       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_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
       version 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  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_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
       meanings.

       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
       distinguish between boundary  strings  that  do  not  differ  in  the  first  $mime_bound-
       ary_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 de-
       scribed in the header_checks(5) 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
       default 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 be-
       long to additional Postfix instances that share the Postfix executable files and  documen-
       tation  with  the  default Postfix instance, and that are started, stopped, etc., together
       with the default Postfix instance.  Specify a list of  pathnames  separated  by  comma  or
       whitespace.

       When  $multi_instance_directories is empty, the postfix(1) command runs in single-instance
       mode and operates on a single Postfix instance only.  Otherwise,  the  postfix(1)  command
       runs  in  multi-instance  mode  and  invokes the multi-instance manager specified with the
       multi_instance_wrapper parameter. The multi-instance manager in turn  executes  postfix(1)
       commands  for the default instance and for all Postfix instances in $multi_instance_direc-
       tories.

       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
       inadvertently.  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-re-
       lated  Postfix instances that the multi-instance manager can start, stop, etc., as a unit.
       This parameter 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) command invokes when
       the multi_instance_directories parameter value is non-empty. The pathname may be  followed
       by  initial command arguments separated by whitespace; shell metacharacters such as quotes
       are not supported in this context.

       The postfix(1) command invokes the manager command with the postfix(1) non-option  command
       arguments  on the manager command line, and with all installation configuration parameters
       exported into the manager command process environment. The manager command in turn invokes
       the  postfix(1)  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) delivery agent which looks up  all  recipients  in
       /etc/passwd  and  /etc/aliases.  The  SMTP  server validates recipient addresses with $lo-
       cal_recipient_maps and rejects non-existent recipients. See also the local domain class in
       the ADDRESS_CLASS_README file.

       The default mydestination value specifies names for the local machine only.  On a mail do-
       main 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_interfaces and proxy_interfaces parameters).

       Warnings:

       o      Do not specify the names of virtual domains - those  domains  are  specified  else-
              where. See VIRTUAL_README for more information.

       o      Do  not  specify  the names of domains that this machine is backup MX host for. See
              STANDARD_CONFIGURATION_README for how to set up backup MX hosts.

       o      By default, the Postfix SMTP server rejects mail for recipients not listed with the
              local_recipient_maps  parameter.   See  the postconf(5) 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 do-
       main 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
       parameters.

       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) 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
       "!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  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.

       o      Specify  "mynetworks_style  =  host" when Postfix should "trust" only the local ma-
              chine.

       o      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.

       o      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
              Postfix to "trust" your entire provider's network.  Instead,  specify  an  explicit
              mynetworks 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 AT that.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) 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) command.
       This command can be used to rebuild the local(8) aliases(5) 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 restric-
       tion.

non_smtpd_milters (default: empty)
       A list of Milter (mail filter) applications for new mail that  does  not  arrive  via  the
       Postfix  smtpd(8) server. This includes local submission via the sendmail(1) command line,
       new mail that arrives via the Postfix qmqpd(8) server, and old mail  that  is  re-injected
       into  the  queue  with  "postsuper -r".  Specify space or comma as separator. See the MIL-
       TER_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
       relaying) 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 infor-
       mation 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
              specified with the bounce_notice_recipient configuration parameter (default:  post-
              master).

       2bounce
              Send  undeliverable bounced mail to the postmaster. The notification is sent to the
              address specified with the 2bounce_notice_recipient  configuration  parameter  (de-
              fault: postmaster).

       data   Send the postmaster a transcript of the SMTP session with an error because a criti-
              cal 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
              configuration 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
              protocol errors. The notification is sent to the address  specified  with  the  er-
              ror_notice_recipient configuration parameter (default: postmaster).

       resource
              Inform  the postmaster of mail not delivered due to resource problems.  The notifi-
              cation is sent to the address specified with the error_notice_recipient  configura-
              tion parameter (default: postmaster).

       software
              Inform  the postmaster of mail not delivered due to software problems.  The notifi-
              cation is sent to the address specified with the error_notice_recipient  configura-
              tion 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 ad-
       dress 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_unknown_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).  This is used by the "postfix
       tls"  command  to  create private keys, certificate signing requests, self-signed certifi-
       cates, and to compute public key digests for DANE TLSA records.  In  multi-instance  envi-
       ronments,  this parameter is always determined from the configuration of the default Post-
       fix 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)  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  ex-
       ample.com, instead of requiring an explicit ".example.com" pattern.  This is planned back-
       wards compatibility:  eventually, all Postfix features are expected  to  require  explicit
       ".example.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,  re-
              lay_domains, 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
       mynetworks 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) 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) delivery agent to change the delivery status code  or  ex-
       planatory text of successful or unsuccessful deliveries.  See default_delivery_status_fil-
       ter 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)  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) 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) 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)  commands  that the postmulti(1) instance manager treats as "control" com-
       mands, 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) commands that the postmulti(1) 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) commands that the postmulti(1) instance manager treats as "stop" commands.
       For these commands, disabled instances are skipped, and enabled instances are processed in
       reverse 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) searches this
       list immediately after a remote SMTP client connects.  Specify a comma- or whitespace-sep-
       arated  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
              $mynetworks.   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, ex-
              cept 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  sub-
              string matching like smtpd(8). 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 con-
              figured with the postscreen_denylist_action configuration parameter.

        dunno All postscreen(8) access lists implicitly have this command at the end.
              When  dunno is executed inside a lookup table, return from  the  lookup  table  and
              evaluate 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) server IP addresses  where  a  non-allowlisted  remote  SMTP
       client  can obtain postscreen(8)'s temporary allowlist status. This status is required be-
       fore the client can talk to a Postfix SMTP server process.  By default, a client  can  ob-
       tain postscreen(8)'s allowlist status on any local postscreen(8) server IP address.

       When  postscreen(8)  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  ad-
       dresses 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  ad-
       dresses  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) takes when a remote SMTP client sends a bare newline charac-
       ter, 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 re-
              ply, 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) server. These tests are ex-
       pensive: 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) 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) cache cleanup runs.  Cache cleanup increases the
       load on the cache database and should therefore not be run frequently.  This  feature  re-
       quires  that the cache database supports the "delete" and "sequence" operators.  Specify a
       zero interval to disable cache cleanup.

       After each cache cleanup run, the postscreen(8) 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) server decisions.

       To  share  a  postscreen(8)  cache   between   multiple   postscreen(8)   instances,   use
       "postscreen_cache_map  = proxy:btree:/path/to/file".  This requires Postfix version 2.9 or
       later; earlier proxymap(8) implementations don't support cache cleanup. For an alternative
       approach see the memcache_table(5) manpage.

       This feature is available in Postfix 2.8.

postscreen_cache_retention_time (default: 7d)
       The  amount of time that postscreen(8) will cache an expired temporary allowlist entry be-
       fore it is removed. This prevents clients from being logged as "NEW"  just  because  their
       cache  entry  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)  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) 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)'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 commands.

       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)'s built-in SMTP  protocol
       engine.

       This feature is available in Postfix 2.8.

postscreen_denylist_action (default: ignore)
       The  action  that  postscreen(8) 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  re-
              ply, 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) daemon.  See disable_vrfy_command for
       details.

       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) server will not
       send in the EHLO response to a remote SMTP client. See smtpd_discard_ehlo_keywords for de-
       tails.  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) 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) 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  collect-
              ing statistics without blocking mail.

       enforce
              Allow  other tests to complete. Reject attempts to deliver mail with a 550 SMTP re-
              ply, 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
       disabled   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  up-
       dated 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)  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)  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 do-
       main name that postscreen will reply with when it rejects mail.  When no mapping is found,
       the actual DNSBL domain will be used.

       For maximal stability it is best to use a file that is read into  memory  such  as  pcre:,
       regexp:  or  texthash:  (texthash:  is similar to hash:, except a) there is no need to run
       postmap(1) 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) daemon will query these domains with the IP addresses of  remote
       SMTP clients, and postscreen(8) will update an SMTP client's DNSBL score with each non-er-
       ror 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) can drop the connection  with  the  remote  SMTP
       client.

       Specify a list of domain=filter*weight entries, separated by comma or whitespace.

       o      When  no  "=filter" is specified, postscreen(8) will use any non-error DNSBL reply.
              Otherwise, postscreen(8) 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 con-
              tains one or more ";"-separated numbers or number..number ranges.

       o      When no "*weight" is specified, postscreen(8) increments the remote  SMTP  client's
              DNSBL  score  by  1.   Otherwise,  the  weight  must  be  an  integral  number, and
              postscreen(8) adds the specified weight to the remote SMTP  client's  DNSBL  score.
              Specify a negative number for allowlisting.

       o      When   one   postscreen_dnsbl_sites   entry   produces  multiple  DNSBL  responses,
              postscreen(8) applies the weight at most once.

       Examples:

       To use example.com as a high-confidence blocklist, and to block mail with example.net  and
       example.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) daemon which are defined by system resolver(3) routines.

       This feature is available in Postfix 3.0.

postscreen_dnsbl_ttl (default: 1h)
       The amount of time that postscreen(8) will use the result from a successful DNS-based rep-
       utation 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) server considers in violation of the  SMTP  proto-
       col.  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) 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 collect-
              ing statistics without blocking mail.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP  re-
              ply, 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) 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) 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 dis-
       able this feature.

       This feature is available in Postfix 2.8.

postscreen_greet_ttl (default: 1d)
       The amount of time that postscreen(8) will use the result from a successful PREGREET test.
       During  this  time, the client IP address is excluded from this test. The default is rela-
       tively 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) will wait for an SMTP client to send a  command  be-
       fore  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) 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  re-
              ply, 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) 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) 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,  be-
       fore 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) takes when a remote SMTP client sends multiple commands in-
       stead 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 re-
              ply, 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) server. These tests  are  ex-
       pensive:  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) will use the result from a  successful  "pipelining"
       SMTP  protocol  test.  During this time, the client IP address is excluded from this test.
       The default 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-allowlisted 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)  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)
       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) server; when a non-empty value is speci-
       fied,  this  overrides  the  obsolete  parameters  postscreen_use_tls  and  postscreen_en-
       force_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;
       version 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_proto-
       col 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) 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) 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) 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 forward-
       ing  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  pa-
       rameter.

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) mapping of "joe AT example.com =>  joe.user AT example.net",  the
       address "joe+foo AT example.com" would rewrite to "joe.user+foo AT example.net".

       Specify  zero  or  more  of  canonical, virtual, alias, forward, include or generic. These
       cause address extension propagation with canonical(5), virtual(5),  and  aliases(5)  maps,
       with  local(8) .forward and :include: file lookups, and with smtp(8) generic maps, respec-
       tively.

       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
       mailing 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) server is allowed to access for the read-only ser-
       vice.

       Specify zero or more "type:name" lookup tables, separated by whitespace or  comma.   Table
       references 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) server is allowed to access for the read-write ser-
       vice. Postfix-owned local database files should be stored under the Postfix-owned data_di-
       rectory.  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) 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  im-
       plemented by the proxymap(8) 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
       active 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
       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).

       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) 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  com-
       munication channel.  The purpose is to break out of deadlock situations. If the time limit
       is exceeded 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 max-
       imal 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
       relay 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 instead.

       Patterns are separated by whitespace and/or commas. In order to reverse the  result,  pre-
       cede  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
       operation blocks for more than $qmqpd_timeout seconds the Postfix QMQP server gives up and
       disconnects.

       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 Post-
       fix 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) 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 re-
       ceive 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
       minimum 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 de-
       fault 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) service. This service manages the Postfix queue and schedules  de-
       livery 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 do-
       main name. By default, Postfix uses  the  default  template  as  specified  with  the  de-
       fault_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
       specific Postfix subsystem or feature.

receive_override_options (default: empty)
       Enable  or  disable  recipient validation, built-in content filtering, or address mapping.
       Typically, these are specified in master.cf as command-line arguments  for  the  smtpd(8),
       qmqpd(8) or pickup(8) daemons.

       Specify  zero or more of the following options.  The options override main.cf settings and
       are either implemented by smtpd(8), qmqpd(8), or pickup(8) themselves, or  they  are  for-
       warded to the cleanup server.

       no_unknown_recipient_checks
              Do  not  try  to  reject  unknown recipients (SMTP server only).  This is typically
              specified AFTER an external content filter.

       no_address_mappings
              Disable canonical address mapping, virtual alias map expansion, address  masquerad-
              ing,  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
              filter.

       no_milters
              Disable Milter (mail filter) applications. This is typically specified AFTER an ex-
              ternal 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
       master.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:

       o      Look  up the "user+extension AT domain.tld" address including the optional address ex-
              tension.

       o      Look up the "user AT domain.tld" address without the optional address extension.

       o      Look up the "user+extension" address local part when the  recipient  domain  equals
              $myorigin, $mydestination, $inet_interfaces or $proxy_interfaces.

       o      Look  up  the "user" address local part when the recipient domain equals $myorigin,
              $mydestination, $inet_interfaces or $proxy_interfaces.

       o      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, au-
       tomatic BCC recipients are not generated after Postfix forwards mail internally, or  after
       Postfix 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, re-
       cipient_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
       table format and lookups are documented in canonical(5).

       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  .for-
       ward file name from its extension.  For example, with "recipient_delimiter = +", the soft-
       ware tries user+foo AT example.com before trying  user AT 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
       extension may then be used to generate an extended .forward file name. This implementation
       recognizes 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 charac-
       ter.

       See canonical(5), local(8), relocated(5) and virtual(5) for the effects  of  recipient_de-
       limiter  on lookups in aliases, canonical, virtual, and relocated maps, and see the propa-
       gate_unmatched_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_de-
       limiter parameter value (Postfix 2.10 and earlier).

       The recipient_delimiter is not applied to the mailer-daemon address,  the  postmaster  ad-
       dress,  or  the double-bounce address. With the default "owner_request_special = yes" set-
       ting, the recipient_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 re-
       jected 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
       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 oppor-
       tunities  to  reject mail, and defers the client request only if it would otherwise be ac-
       cepted.

       For  finer  control,  see:  unverified_recipient_tempfail_action,  unverified_sender_temp-
       fail_action, 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 be-
       ing deprecated in favor of sha256.  However, as long as there are no known "second pre-im-
       age" attacks against the older algorithms, their use in this context,  though  not  recom-
       mended, 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)
       policy 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
       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.

       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 de-
       tails about how the  relay_domains  value  is  used,  see  the  description  of  the  per-
       mit_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
       rejects  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)
       manual page.

       Specify a list of host or domain names, "/file/name" patterns or "type:table"  lookup  ta-
       bles,  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 domain 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_do-
       mains" 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
       reject_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  $re-
       lay_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, then the Postfix SMTP server will reject mail to unknown
       relay 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  do-
       mains  listed with $relay_domains. In order of decreasing precedence, the nexthop destina-
       tion is taken from $relay_transport, $sender_dependent_relayhost_maps, $relayhost, or from
       the recipient domain. This information can be overruled with the transport(5) table.

       Specify  a string of the form transport:nexthop, where transport is the name of a mail de-
       livery transport defined in master.cf.  The :nexthop destination is optional;  its  syntax
       is documented 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
       addresses.  This  information  is  overruled  with  relay_transport,  sender_dependent_de-
       fault_transport_maps,  default_transport,  sender_dependent_relayhost_maps  and  with  the
       transport(5) table.

       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
       domain  name,  hostname,  hostname:port,  [hostname]:port,   [hostaddress]   or   [hostad-
       dress]:port,  separated by comma or whitespace.  The form [hostname] turns off MX lookups.
       Multiple destinations 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
       exist.  The table format and lookups are documented in relocated(5).

       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;
       otherwise, rewrite message headers and append the specified domain name to incomplete  ad-
       dresses.  The local_header_rewrite_clients parameter controls what clients Postfix consid-
       ers 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)  recipient's  home directory exists before mail delivery is at-
       tempted. By default this test is disabled.  It can be useful for environments that  import
       home directories to the mail server (IMPORTING HOME DIRECTORIES IS NOT RECOMMENDED).

reset_owner_alias (default: no)
       Reset  the  local(8)  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  be-
       havior is as if this parameter is set to "yes".

       As  documented  in  aliases(5), 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)  delivery
       agent  to  write  local and remote addresses from alias expansion to a new queue file, in-
       stead of attempting 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 han-
       dling of temporary delivery errors: errors with one local member have no effect on  deliv-
       eries  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  ad-
       dress 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 temporary 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  decision to create a new queue file, not the decision to override the envelope sender
       address. The local(8) 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 ex-
       pansion 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 speci-
       fied, 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
       domain.

resolve_numeric_domain (default: no)
       Resolve  "user@ipaddress"  as  "user@[ipaddress]", instead of rejecting the address as in-
       valid.

       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) 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
       authcid's password.

       The  non-default  setting "yes" enables the behavior of older Postfix versions.  These al-
       ways send a SASL authzid that is equal to the SASL authcid, but this causes interoperabil-
       ity problems 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
       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:

       o      Look  up the "user+extension AT domain.tld" address including the optional address ex-
              tension.

       o      Look up the "user AT domain.tld" address without the optional address extension.

       o      Look up the "user+extension" address local part when the sender domain equals $myo-
              rigin, $mydestination, $inet_interfaces or $proxy_interfaces.

       o      Look up the "user" address local part when the sender domain equals $myorigin, $my-
              destination, $inet_interfaces or $proxy_interfaces.

       o      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 un-
       deliverable.

       Note: automatic BCC recipients are produced only for new mail.  To avoid mailer loops, au-
       tomatic BCC recipients are not generated after Postfix forwards mail internally, or  after
       Postfix 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 ta-
       ble format and lookups are documented in canonical(5).

       Example: you want to rewrite the SENDER  address  "user AT ugly.domain"  to  "user AT pretty.do-
       main", while still being able to send mail to the RECIPIENT address "user AT 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  termi-
       nates  the search without overriding the global default_transport parameter setting.  This
       information is overruled with the transport(5) 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  expres-
       sion 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_de-
       fault_transport_maps, default_transport and with the transport(5) 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  expres-
       sion 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  complete-
              ness 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)
       command. 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) waits before forking a server that appears to be malfunc-
       tioning.

       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
       Postfix 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
       location of Postfix database plugins (postfix-*.so) that have a relative pathname  in  the
       dynamicmaps.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
       typically defaults to /usr/lib/postfix or /usr/local/lib/postfix.

       Notes:

       o      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  pro-
              grams  is not supported.  Postfix dynamically-linked libraries and database plugins
              implement a Postfix-internal API that changes without maintaining compatibility.

       o      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) 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 effect 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:

       o      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.

       o      The  setting  "smtp_address_preference = any" is safe. With this, mail will eventu-
              ally 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 trans-
       port_maps to apply 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
       connection 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
       Postfix).

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_address_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
       destination 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
       virtual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces
       documentation 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
       virtual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces
       documentation 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) 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 deci-
       sions, 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 oper-
       ating 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 con-
       nection caching, a connection is not closed immediately after completion of a mail  trans-
       action.   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 performance.

       Specify a comma or white space separated list of destinations or pseudo-destinations:

       o      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),

       o      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,

       o      if mail is sent via a UNIX-domain socket: a pathname (without the unix: prefix),

       o      a /file/name with domain names and/or relay host names as defined above,

       o      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_connection_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 connec-
       tion 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
       limitation 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
       latency, 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 de-
       livery latency 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
       limiting the amount of time during which a connection can be used repeatedly  (instead  of
       limiting  the  number of deliveries over that connection), Postfix not only restores fair-
       ness in the distribution of simultaneous connections across a set of MX hosts, it also fa-
       vors 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  messages per cached connection. It may prove necessary to lower the limit to avoid in-
       teroperability issues with MTAs that exhibit bugs when many messages are delivered  via  a
       single  connection.  A lower reuse time limit risks losing the benefit of connection reuse
       when the average 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 connec-
       tion makes no progress for more than  $smtp_data_xfer_timeout  seconds  the  Postfix  SMTP
       client terminates 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
       default  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) delivery agent to change the delivery status code or ex-
       planatory text of successful or unsuccessful deliveries.  See default_delivery_status_fil-
       ter 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 de-
       livery transport. This limit is enforced by the queue manager. The message delivery trans-
       port 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  de-
       tails.  The  table is not indexed by hostname for consistency with smtpd_discard_ehlo_key-
       word_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 Post-
       fix SMTP client will ignore in the EHLO response from a remote SMTP server.

       This feature is available in Postfix 2.2 and later.

       Notes:

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

       o      Use the smtp_discard_ehlo_keyword_address_maps feature to discard EHLO keywords se-
              lectively.

smtp_dns_reply_filter (default: empty)
       Optional  filter  for Postfix SMTP client DNS lookup results.  Specify zero or more lookup
       tables.  The lookup tables are searched in the given order for a match with the DNS lookup
       result, 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  im-
       plemented.

       Notes:

       o      Postfix  DNS  reply  filters  have  no  effect on implicit DNS lookups through nss-
              witch.conf or equivalent mechanisms.

       o      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_unveri-
              fied_sender and reject_unverified_recipient.

       o      The  Postfix SMTP/LMTP client defers mail delivery when a filter removes all lookup
              results from a successful query.

       o      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_*.

       o      The  Postfix  SMTP  server logs a warning or defers mail delivery when a filter re-
              moves all lookup results from a successful query.

       Example: ignore Google AAAA records in Postfix SMTP client  DNS  lookups,  because  Google
       sometimes 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
       options, 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  behav-
              ior 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 enabled 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 de-
              liver mail to the public Internet.  Some obsolete how-to documents  recommend  dis-
              abling  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  sub-
              ject  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:

       o      Any MX lookups will set RES_USE_DNSSEC and RES_USE_EDNS0  to  request  DNSSEC-vali-
              dated responses. If the MX response is DNSSEC-validated the corresponding hostnames
              are considered validated.

       o      The address lookups of validated hostnames are also validated, (provided of  course
              "smtp_host_lookup" includes "dns", see below).

       o      Temporary  failures in DNSSEC-enabled hostname-to-address resolution block any "na-
              tive" lookups.   Additional  "native"  lookups  only  happen  when  DNSSEC  lookups
              hard-fail (NODATA or NXDOMAIN).

       The  Postfix  SMTP  client  considers non-MX "[nexthop]" and "[nexthop]:port" destinations
       equivalent 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 sub-
       ject to MX lookups.

       The "dnssec" setting is recommended only if you plan to use the dane or dane-only TLS  se-
       curity  level, otherwise enabling DNSSEC support in Postfix offers no additional security.
       Postfix DNSSEC support relies on an upstream recursive nameserver  that  validates  DNSSEC
       signatures.   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
       configured DNSSEC-validating recursive nameserver to perform all DNSSEC validation.  Since
       this  nameserver's DNSSEC-validated responses will be fully trusted, it is strongly recom-
       mended that the MTA host have a local DNSSEC-validating recursive caching nameserver  lis-
       tening  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 recursive nameserver

       DNSSEC  support requires a version of Postfix compiled against a reasonably-modern DNS re-
       solver(3) 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 infor-
       mation 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 ver-
       ify or the hostname 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 SubjectAlter-
       nativeName.  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 un-
       reachable. 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  de-
       layed 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
       specify 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, typi-
       cally 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); 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) 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 parame-
       ter 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
       inserting  "<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 earlier.

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) tables for the Postfix  SMTP  client.  These  tables  are
       searched while mail is being delivered.  Actions that change the delivery time or destina-
       tion 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 ig-
       nores sessions 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) tables for the Postfix SMTP client.  These  tables  are
       searched while mail is being delivered.  Actions that change the delivery time or destina-
       tion 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 param-
       eter.

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 be-
       havior 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 sec-
       onds. In other words, the workaround is normally turned off for  the  first  delivery  at-
       tempt.

       Specify 0 to enable the PIX firewall "<CR><LF>.<CR><LF>" bug workaround upon the first de-
       livery attempt.

smtp_pix_workarounds (default: disable_esmtp, delay_dotcrlf)
       A list that specifies zero or more workarounds for CISCO PIX firewall  bugs.  These  work-
       arounds  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_work-
              around_threshold_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
       compatible with earlier Postfix versions.

smtp_quit_timeout (default: 300s)
       The Postfix SMTP client time limit for sending the QUIT command, and for receiving the re-
       mote 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) 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 Post-
       fix SMTP client.  Other uses involve fault injection to test Postfix's handling of invalid
       responses.

       Notes:

       o      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.

       o      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  re-
       moved.  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 re-
       mote SMTP server response. The SMTP client sends RSET in order to finish a  recipient  ad-
       dress 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 remote 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 de-
       livery 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, de-
       fer 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 mecha-
       nism 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 ver-
       sion 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,
       remote  hostname  or next-hop domain. Per-sender lookup is done only when sender-dependent
       authentication 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  speci-
       fies 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 fea-
       tures 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 ver-
              sion 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 en-
       crypted 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  en-
       crypted 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
       delivery 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
       plaintext mechanisms when a secure connection  to  the  server  is  available.  Submission
       servers  subject 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
       available 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-authenti-
       cated 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 an-
       nounces 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
       filter and downstream queuing SMTP server. This can produce more useful logging  than  lo-
       calhost[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
       different 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
       remote 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 shut-
       down 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)) or a numeric port.

smtp_tls_CAfile (default: empty)
       A  file  containing  CA certificates of root CAs trusted to sign either remote SMTP server
       certificates or intermediate CA certificates.  These are loaded  into  memory  before  the
       smtp(8)  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) client is chrooted. This file may also be used to augment the
       client certificate 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 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_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 neces-
       sary "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
       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_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 malicious 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  con-
       tain  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  con-
       figure 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
       configurations 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 cer-
       tificates  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
       itself  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  white-
       space.   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, in-
       clude 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.  There-
       fore, 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
       multiple  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  certifi-
       cate  and issuer chain, it can be in a separate file, so long as that file is listed imme-
       diately 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  reli-
       able.   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 con-
       figuration 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 (typi-
       cally 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  fea-
       ture applies to all TLS security levels, it is easy to create interoperability problems by
       choosing 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 appropri-
       ate  for  an internal MTA, where one can exert some control over the TLS software and set-
       tings 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
       encryption. Cipher types listed in smtp_tls_exclude_ciphers are  excluded  from  the  base
       definition  of  the selected cipher grade.   The default value is "medium" for Postfix re-
       leases after the middle of 2015, "export" for older releases.

       When TLS is mandatory the cipher grade is chosen via the  smtp_tls_mandatory_ciphers  con-
       figuration  parameter, see there for syntax details. See smtp_tls_policy_maps for informa-
       tion 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
       "export" 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)
       service to encrypt an SMTP connection, uses the scache(8) service to save that connection,
       and relies on hints from the qmgr(8) 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  secu-
       rity  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", authentica-
              tion  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  al-
              ways "dane".

       Though  with  "insecure" MX records an active attacker can compromise SMTP transport secu-
       rity by returning forged MX records, such attacks are "tamper-evident" since any forged MX
       hostnames  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 con-
       tain 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 com-
       bined   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
       contain 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  com-
       bined  with  the  Postfix  SMTP client ECDSA certificate file specified with $smtp_tls_ec-
       cert_file.  With Postfix >= 3.4 the preferred way to configure client  keys  and  certifi-
       cates 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
       information  in  the  remote SMTP server certificate.  As of RFC 2487 the requirements for
       hostname checking for MTA clients are not specified.

       This option can be set to "no" to disable strict peer name checking. This setting  has  no
       effect 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
       cipher 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  dis-
       ables  ciphers  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  ex-
       change 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
       security level (smtp_tls_security_level = fingerprint). At this security level, Certifica-
       tion  Authorities  are  not  used,  and certificate expiration times are ignored. Instead,
       server certificates 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_finger-
       print_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 corre-
       sponding destination. Multiple fingerprints can be combined with a "|" delimiter in a sin-
       gle match attribute, 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
       relayhost may be in the process of switching from one set of private/public  keys  to  an-
       other, 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 exam-
       ple 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  finger-
       prints.  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 certificate (or its public key) using the selected algorithm. With a digest al-
       gorithm 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
       public key from the certificate and compute the appropriate digest of its DER (ASN.1)  en-
       coding.  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
       configured 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 la-
       tency 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 com-
       bined   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
       includes 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 veri-
              fication  is  not  required.  With Postfix 2.8 and earlier, log the summary message
              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 "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 en-
       cryption.  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_policy_maps for information on how to configure ciphers on a per-destination  ba-
       sis.

       The following cipher grades are supported:

       export Enable  "EXPORT"  grade  or  better  OpenSSL ciphers.  The underlying cipherlist is
              specified 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 better OpenSSL ciphers.  The underlying cipherlist is speci-
              fied via the tls_low_cipherlist configuration parameter, which you are strongly en-
              couraged to not change.  This choice is insecure and SHOULD NOT be used.

       medium Enable  "MEDIUM"  grade  or  better  OpenSSL ciphers.  The underlying cipherlist is
              specified via the tls_medium_cipherlist  configuration  parameter,  which  you  are
              strongly encouraged 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  re-
              layhost)  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 en-
              cryption.  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" ciphers. The underlying cipherlist is specified via the tls_null_ci-
              pherlist configuration 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 certificates.  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 ci-
       phers 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 anony-
       mous 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-des-
       tination  basis via the TLS policy "exclude" attribute. See smtp_tls_policy_maps for notes
       and examples.

       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
       "protocols" 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))  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  ei-
       ther  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  up-
       per  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  re-
       quired.   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 symbolic 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
       accurately 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
       example:

           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 infor-
       mation 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 destina-
       tion 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 se-
       cure 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  do-
       main  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
       allowed. The next-hop destination is either the recipient domain, or the destination spec-
       ified with a transport(5) table, the relayhost parameter, or the  relay_transport  parame-
       ter.  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 al-
              ternate host or  next-hop  lookup  key,  and  overrides  the  global  smtp_use_tls,
              smtp_enforce_tls, and smtp_tls_enforce_peername settings.

       MAY    Try  to use TLS if the server announces support, otherwise use the unencrypted con-
              nection. 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
              certificate was issued by a trusted CA. This overrides a less secure NONE 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.

       MUST   Require  TLS  encryption,  require that the remote SMTP server hostname matches the
              information 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
       Postfix 2.3, and independently of how the policy is specified, the  smtp_tls_mandatory_ci-
       phers  and smtp_tls_mandatory_protocols parameters apply when TLS encryption is mandatory.
       Connections for which encryption is optional typically enable all "export" grade and  bet-
       ter 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 desti-
       nation; when a non-empty value is specified, this overrides the obsolete smtp_tls_per_site
       parameter.  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  re-
       cipient  domain,  or the verbatim next-hop specified in the transport table, $local_trans-
       port, $virtual_transport, $relay_transport or $default_transport. This  includes  any  en-
       closing  square  brackets  and  any  non-default  destination server port suffix. The LMTP
       socket type prefix (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) table, the parent domain starting with a leading "." is matched  re-
       cursively. 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 se-
       curity 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",
              "exclude", and "protocols" attributes (available for opportunistic TLS with Postfix
              >=   2.6)   and   "connection_reuse"   attribute  (Postfix  >=  3.4)  override  the
              "smtp_tls_ciphers",    "smtp_tls_exclude_ciphers",    "smtp_tls_protocols",     and
              "smtp_tls_connection_reuse"  configuration parameters. When opportunistic TLS hand-
              shakes fail, Postfix retries the connection with TLS disabled.   This  allows  mail
              delivery to sites with non-interoperable TLS implementations.

       encrypt
              Mandatory TLS encryption. At this level and higher, the optional "protocols" attri-
              bute 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_manda-
              tory_exclude_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 values 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 op-
              tional  "connection_reuse"  attribute  (Postfix  >=  3.4)  overrides  the   main.cf
              smtp_tls_connection_reuse parameter.

       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 op-
              tional  "connection_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 at-
              tribute, or else the main.cf smtp_tls_fingerprint_cert_match parameter,  lists  the
              certificate  fingerprints  or the public key fingerprint (Postfix 2.9 and later) of
              the valid server certificate. The digest algorithm used to  calculate  the  finger-
              print  is  selected  by the smtp_tls_fingerprint_digest parameter. Multiple finger-
              prints can be combined with a "|" delimiter in a single match attribute, or  multi-
              ple  match attributes can be employed. 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_con-
              nection_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 ob-
              tained indirectly via unauthenticated DNS MX lookups.  The optional "match"  attri-
              bute  overrides the main.cf smtp_tls_verify_cert_match parameter. In the policy ta-
              ble, multiple match patterns and strategies must be separated by colons.  In  prac-
              tice  explicit  control  over matching is more common with the "secure" policy, de-
              scribed below. The optional "connection_reuse" attribute (Postfix >= 3.4) overrides
              the main.cf smtp_tls_connection_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 se-
              cure  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_se-
              cure_cert_match parameter. In the policy table, multiple match patterns and strate-
              gies 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 op-
              tional  "connection_reuse"  attribute  (Postfix  >=  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_se-
       cure_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 configura-
       tions 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
       "protocols" 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))  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 ver-
       sion.  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 "<=" sym-
       bols 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  re-
       quired.   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 symbolic 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_protocols = !SSLv2, !SSLv3".  Listing the protocols to include, rather than pro-
       tocols to exclude, is supported, but not recommended.  The exclusion form more  accurately
       matches the underlying OpenSSL interface.

       When  using  the  exclusion syntax, take care to ensure that the range of protocols adver-
       tised 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 ac-
       tually  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
       security 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, white-
       space 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
       parameter. 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,  other-
              wise  use  plaintext.  Since sending in the clear is acceptable, demanding stronger
              than default 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  re-
              leases  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 rea-
              sonable 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 security 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
              containing  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  validate  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 algorithm, 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  selec-
              tion,  the  "dane" security level is treated like a "mandatory" TLS security level,
              and weak ciphers and protocols are disabled.  Since DANE authenticates server  cer-
              tificates  the  "aNULL"  cipher-suites are transparently 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  cer-
              tificate fingerprint or public key fingerprint (Postfix 2.9 and later) of the valid
              server certificate. The digest algorithm used to calculate the fingerprint  is  se-
              lected 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 ob-
              tained  indirectly  via  unauthenticated  DNS   MX   lookups.   The   smtp_tls_ver-
              ify_cert_match  parameter controls how the server name is verified. In practice ex-
              plicit 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_secure_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 domain. 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)
       extension.  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
       reduce 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 se-
       lect a default 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 host-
       name or nexthop, which select either the remote hostname or  the  nexthop  domain  respec-
       tively.   DNS  names  for SNI must be in A-label (punycode) form.  Invalid DNS names log a
       configuration error warning and mail delivery is deferred.

       Except when using a relayhost to forward all email, the only  sensible  non-empty  main.cf
       setting  for  this  parameter is hostname.  Other non-empty values are only practical on a
       per-destination 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  sup-
       port concurrent access.  The file is created if it does not exist. The smtp(8) daemon does
       not use this parameter directly,  rather  the  cache  is  implemented  indirectly  in  the
       tlsmgr(8)  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)  daemon:
       $smtpd_tls_session_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 possible 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  at-
       tempt  to  open  the file under a non-Postfix directory is redirected to the Postfix-owned
       data_directory, 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_session_cache_database, this parameter is implemented in  the  tlsmgr(8)  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
       disabled.  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
       parameter 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-an-
       chors 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 is-
       suing parent certificates 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 mate-
       rial, and must be readable by the non-privileged $mail_owner user.  This  allows  destina-
       tions  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 specified on a per-destination basis via the "tafile" attribute  of  the  "verify"  and
       "secure" levels 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
       security 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, white-
       space 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 cer-
              tificate 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
              transport  next-hop  configured for the domain stripped of any optional socket type
              prefix, enclosing square brackets and trailing port. When MX lookups are  not  sup-
              pressed, 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  $myhostname.  This strategy is suitable for use with the "secure" policy.
              Case is ignored.

       dot-nexthop
              As above, but match server certificate names that are subdomains  of  the  next-hop
              domain. 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 $myhost-
              name.  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
       using 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, other-
       wise send the mail in the clear. Beware: some SMTP servers offer STARTTLS even  if  it  is
       not  configured.   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 com-
       patible 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  ad-
       dresses  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
       remote  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 ad-
       dresses 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
       information that is used to improve logging after  SMTP-based  content  filters.  See  the
       XFORWARD_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 ad-
       dresses 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 proto-
       col.

       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
       legitimate 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  de-
       fault, 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
       legitimate 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 pa-
       rameter.

       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
       legitimate 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
       mynetworks parameter description for the parameter value syntax.

       By default, clients in trusted networks are excluded. Specify a list  of  network  blocks,
       hostnames 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
       version  6  addresses  contain  the  ":" character, and would otherwise be confused with a
       "type:table" 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 mes-
       sages.  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
       legitimate 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  al-
       lowed  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  re-
       jected.

       WARNING:  The  purpose  of this feature is to limit abuse. It must not be used to regulate
       legitimate 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 ser-
       vice per time unit, regardless of whether or not Postfix actually  accepts  those  recipi-
       ents.  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
       legitimate 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 con-
       nection request.  See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access  re-
       striction 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
       specified; the first restriction that matches wins.

       The  following  restrictions are specific to client hostname or client network address in-
       formation.

       check_ccert_access type:table
              By default use the remote SMTP client certificate fingerprint  or  the  public  key
              fingerprint (Postfix 2.9 and later) as lookup key for the specified access(5) data-
              base; with Postfix version 2.2, also require that the remote SMTP  client  certifi-
              cate  is  verified  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 cryptanaly-
              sis 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.
              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_finger-
              print } }
              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 access(5) manual page for details.

       check_client_a_access type:table
              Search  the  specified access(5) database for the IP addresses for the client host-
              name, 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) 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) database for the DNS servers for  the  client  host-
              name,  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,
              parent domains, client IP address, or networks obtained by stripping least signifi-
              cant  octets. See the access(5) manual page for details.  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 2.6 and later.

       check_reverse_client_hostname_a_access type:table
              Search the specified access(5) 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) 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) database for the DNS servers 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 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)
              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 ad-
              dress 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 success-
              fully.  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 cer-
              tificate 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  fin-
              gerprint  digest algorithm is configurable via the smtpd_tls_fingerprint_digest pa-
              rameter (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 cryptanaly-
              sis 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.

       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  num-
              bers 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 re-
              quests (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 ad-
              dress 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"
              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..num-
              ber  ranges (Postfix version 2.8 and later).  If no "=d.d.d.d" is specified, reject
              the request when the client hostname is listed with any A record under  rbl_domain.
              See  the  reject_rbl_client description above for additional RBL related configura-
              tion parameters.  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 re-
              sults.

       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"
              under  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  ex-
              ceptions.
              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
              hostname  is  listed  with any A record under rbl_domain. See the reject_rbl_client
              description above for additional RBL related configuration parameters.   This  fea-
              ture 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
              requests  (default:  450).  The  reply  is  always 450 in case the address->name or
              name->address 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
              requests  (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 de-
              tails. 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  use-
              ful 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 fea-
              ture 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
              NOTIFY=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
              rejected  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  re-
              quests (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
              deliveries.
              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 speci-
              fies 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 restric-
              tion 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 ac-
              cepted by some later access  restriction).  This  feature  has  no  effect  on  de-
              fer_if_reject restrictions.

       Other restrictions that are valid in this context:

       o      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_re-
              strictions 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
       received 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:

       o      Use "resolve_numeric_domain = yes" to accept "user@ipaddress".

       o      Postfix already accepts the correct form "user@[ipaddress]". Use virtual_alias_maps
              or canonical_maps to translate these into domain names if necessary.

       o      Use "strict_rfc821_envelopes =  no"  to  accept  "RCPT  TO:<User  Name  <user@exam-
              ple.com>>". Postfix will ignore the "User Name" part and deliver to the <user@exam-
              ple.com> address.

       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
       restriction 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
       specified; the first restriction that matches wins.

       The following restrictions are valid in this context:

       o      Generic restrictions that can be used in any SMTP command context, described  under
              smtpd_client_restrictions.

       o      SMTP  command  specific  restrictions  described  under  smtpd_client_restrictions,
              smtpd_helo_restrictions, smtpd_sender_restrictions or smtpd_recipient_restrictions.

       o      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 be-
              fore.

       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_restrictions  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 in-
       formation 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 de-
       tails.  The tables 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 Post-
       fix 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:

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

       o      Use the smtpd_discard_ehlo_keyword_address_maps feature to  discard  EHLO  keywords
              selectively.

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 restriction 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 pub-
       licly-referenced 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 with-
       out delivering 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  Post-
       fix "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
       specified; 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  do-
              mains. See the access(5) manual page for details.

       Other restrictions that are valid in this context:

       o      Generic  restrictions that can be used in any SMTP command context, described under
              smtpd_client_restrictions.

       o      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
       expansion.

       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  stan-
       dard  <CR><LF>.  Such  line endings are commonly allowed with UNIX-based SMTP servers, but
       they violate 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 com-
              mand 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 stan-
              dard 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
              message  content  when a line contains bare <LF>, log a "bare <LF> received" error,
              and reply 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 contain-
              ing a bare newline (RFC 3030 Section 3 requires canonical MIME format for text mes-
              sage 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_dis-
              card_ehlo_keyword_address_maps,  or  globally disabled with smtpd_discard_ehlo_key-
              words).

       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 compati-
              ble, but is not recommended for an Internet-facing SMTP server, because it is  vul-
              nerable 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 system. 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 send-
       ing 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
       specified; 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) 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) database for the IP addresses for the HELO  or  EHLO
              hostname,  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
              restriction  (without  "smtpd_helo_required  =  yes",  a  client  can  simply  skip
              check_helo_a_access  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) 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  restriction  (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
              Postfix 2.1 and later.

       check_helo_ns_access type:table
              Search  the  specified  access(5) database for the DNS servers for the HELO or EHLO
              hostname, 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
              restriction  (without  "smtpd_helo_required  =  yes",  a  client  can  simply  skip
              check_helo_ns_access 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_required  = 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  re-
              quests (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
              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  speci-
              fied, 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
              configuration  parameters.   Note: specify "smtpd_helo_required = yes" to fully en-
              force this restriction (without "smtpd_helo_required = yes", a  client  can  simply
              skip  reject_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
              respective parameter descriptions for details.
              Note: specify "smtpd_helo_required = yes" to fully enforce this restriction  (with-
              out "smtpd_helo_required = yes", a client can simply skip reject_unknown_helo_host-
              name by not sending HELO or EHLO).

       Other restrictions that are valid in this context:

       o      Generic restrictions that can be used in any SMTP command context, described  under
              smtpd_client_restrictions.

       o      Client   hostname   or   network  address  specific  restrictions  described  under
              smtpd_client_restrictions.

       o      SMTP command specific restrictions  described  under  smtpd_sender_restrictions  or
              smtpd_recipient_restrictions.  When sender or recipient restrictions are listed un-
              der 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
       before  the  Postfix SMTP server starts to increment the error counter with each junk com-
       mand.  The junk command count is reset after mail is delivered.  See  also  the  smtpd_er-
       ror_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 allows 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).   Con-
       tinue  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)  server.  Specify space or comma as separator. See the MILTER_README document for
       details.

       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) 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 veri-
       fied, 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 proto-
       col 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  be-
       havior 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:

       o      This  parameter may specify any value that would be a valid SMTPD policy server re-
              sponse (or access(5) map lookup result).  An access(5) map or policy server in this
              parameter  value  may  need to be declared in advance with a restriction_class set-
              ting.

       o      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" attri-
       bute 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-domain 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  as-
       sumed.  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
              received.  This reduces the number of simultaneous before-queue content filter pro-
              cesses.

       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 $mes-
       sage_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 infor-
       mation 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  speci-
       fied  with  $smtpd_recipient_limit, before the Postfix SMTP server increments the per-ses-
       sion 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
       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
       combined under smtpd_recipient_restrictions, resulting in error-prone  configuration.   As
       of  Postfix  2.10,  relay permission rules are preferably implemented with smtpd_relay_re-
       strictions, 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 parame-
       ter must specify at least one of the following restrictions. Otherwise Postfix will refuse
       to receive 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
       specified; 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) database for the resolved RCPT TO  address,  domain,
              parent domains, or localpart@, and execute the corresponding action.

       check_recipient_a_access type:table
              Search  the  specified  access(5) database for the IP addresses for the RCPT TO do-
              main, 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) 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) 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:

       o      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),

       o      Postfix  is  the final destination: the resolved RCPT TO domain matches $mydestina-
              tion,  $inet_interfaces,  $proxy_interfaces,   $virtual_alias_domains,   or   $vir-
              tual_mailbox_domains,   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
              definition).

       o      Safety: permit_mx_backup does not accept addresses that have sender-specified rout-
              ing information (example: user@elsewhere@domain).

       o      Safety: permit_mx_backup can be vulnerable to mis-use when access is not restricted
              with permit_mx_backup_networks.

       o      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).

       o      Limitation:  mail  may  be  rejected in case of a temporary DNS lookup problem with
              Postfix 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 re-
              quests (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..num-
              ber  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  re-
              quests (default: 554); the default_rbl_reply parameter specifies the default server
              reply; and the rbl_reply_maps parameter specifies tables with  server  replies  in-
              dexed 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:

       o      Postfix  is mail forwarder: the resolved RCPT TO domain matches $relay_domains or a
              subdomain thereof, and contains  no  sender-specified  routing  (user@elsewhere@do-
              main),

       o      Postfix  is  the final destination: the resolved RCPT TO domain matches $mydestina-
              tion,  $inet_interfaces,  $proxy_interfaces,   $virtual_alias_domains,   or   $vir-
              tual_mailbox_domains,  and contains no sender-specified routing (user@elsewhere@do-
              main).
              The relay_domains_reject_code parameter specifies the response  code  for  rejected
              requests (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 re-
              cipients for its domain class. See  the  smtpd_reject_unlisted_recipient  parameter
              description 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) 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
              confident 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 ad-
              dress probe failure due to a temporary problem (default: defer_if_permit).
              This feature breaks for aliased addresses  with  "enable_original_recipient  =  no"
              (Postfix <= 3.2).
              This feature is available in Postfix 2.1 and later.

       Other restrictions that are valid in this context:

       o      Generic  restrictions that can be used in any SMTP command context, described under
              smtpd_client_restrictions.

       o      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 literally.

       /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 delivery status notification.

       The template text is not subject to Postfix configuration parameter $name  expansion.  In-
       stead,  this  feature  supports  a  limited number of $name attributes in the footer text.
       These attributes 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_reject_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
              multiple  MTAs (perhaps behind a load-balancer), where the server name can help the
              server support team to quickly find the right log files.

       Notes:

       o      NOT SUPPORTED are other attributes such as sender, recipient,  or  main.cf  parame-
              ters.

       o      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  re-
       ply 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-charac-
       ter 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 re-
       ject 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) alias or a canoni-
       cal(5) mapping.

       o      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.

       o      The recipient domain matches $virtual_alias_domains but the recipient is not listed
              in $virtual_alias_maps.

       o      The  recipient  domain  matches  $virtual_mailbox_domains  but the recipient is not
              listed in $virtual_mailbox_maps, and $virtual_mailbox_maps is not null.

       o      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
       explosion of forged mail from worms or viruses.

       An  address  is  always considered "known" when it matches a virtual(5) alias or a canoni-
       cal(5) mapping.

       o      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.

       o      The sender domain matches $virtual_alias_domains but the sender is  not  listed  in
              $virtual_alias_maps.

       o      The  sender domain matches $virtual_mailbox_domains but the sender is not listed in
              $virtual_mailbox_maps, and $virtual_mailbox_maps is not null.

       o      The sender domain matches $relay_domains but the sender is not listed in $relay_re-
              cipient_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
       documented behavior.

       Background:  the  smtpd_relay_restrictions feature is primarily designed to enforce a mail
       relaying 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 sup-
       port the same features.

       This feature is available in Postfix 3.6 and later.

smtpd_relay_restrictions    (default:    permit_mynetworks,    permit_sasl_authenticated,     de-
       fer_unauth_destination)
       Access  restrictions  for  mail  relay control that the Postfix SMTP server applies in the
       context of the  RCPT  TO  command,  before  smtpd_recipient_restrictions.   See  SMTPD_AC-
       CESS_README,  section  "Delayed evaluation of SMTP access restriction lists" for a discus-
       sion of evaluation context and time.

       With Postfix versions before 2.10, the rules for relay permission and spam  blocking  were
       combined  under  smtpd_recipient_restrictions, resulting in error-prone configuration.  As
       of Postfix 2.10, relay permission rules are preferably  implemented  with  smtpd_relay_re-
       strictions,  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:

       o      Mail from clients whose IP address matches $mynetworks, or:

       o      Mail to remote destinations that match $relay_domains, except  for  addresses  that
              contain sender-specified routing (user@elsewhere@domain), or:

       o      Mail  to  local destinations that match $inet_interfaces or $proxy_interfaces, $my-
              destination, $virtual_alias_domains, or $virtual_mailbox_domains.

       IMPORTANT: Either the smtpd_relay_restrictions or the smtpd_recipient_restrictions parame-
       ter must specify at least one of the following restrictions. Otherwise Postfix will refuse
       to receive 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) ta-
       ble.

       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,  corre-
       sponding 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) 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
       password 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 con-
       tents; 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 white-
       space. 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_ex-
       ceptions_networks 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:

       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 EX-
       TERNAL 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  speci-
       fies 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  fea-
       tures 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  anony-
       mous 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 en-
       crypted 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
       available 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
              $myorigin, $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 re-
       striction 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
       specified; 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) database for the MAIL FROM address, domain, parent
              domains, or localpart@, and execute the corresponding action.

       check_sender_a_access type:table
              Search the specified access(5) database for the IP addresses for the MAIL FROM  do-
              main,  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) 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) database for the DNS servers for the MAIL FROM do-
              main, 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  ver-
              sion 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  re-
              quests (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"
              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..num-
              ber ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is  specified,  reject
              the request 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 re-
              quests (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
              recipients for its domain class. See the smtpd_reject_unlisted_sender parameter de-
              scription 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  managed  by  the verify(8) 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 con-
              fident that it is safe to do so).
              The unverified_sender_defer_code specifies the numerical response code when an  ad-
              dress 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"
              (Postfix <= 3.2).
              This feature is available in Postfix 2.1 and later.

       Other restrictions that are valid in this context:

       o      Generic  restrictions that can be used in any SMTP command context, described under
              smtpd_client_restrictions.

       o      SMTP command specific restrictions described  under  smtpd_client_restrictions  and
              smtpd_helo_restrictions.

       o      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) hands off allowed connections to. In a future ver-
       sion 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 be-
       fore the Postfix SMTP server slows down all its responses.

       o      With Postfix version 2.1 and later, the Postfix SMTP server delays all responses by
              $smtpd_error_sleep_time seconds.

       o      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
       shutdown handshake procedures. The current default value is stress-dependent. Before Post-
       fix version 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)  server  enters  the  chroot jail. If the number of trusted roots is
       large, consider using smtpd_tls_CApath instead, but note that the latter directory must be
       present  in the chroot jail if the smtpd(8) 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 directly in the server certificate file.

       Specify  "smtpd_tls_CAfile  = /path/to/system_CA_file" 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_CAfile should remain empty. If you do make use of client certificates, the  dis-
       tinguished  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 multi-
       ple  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 re-
       mote SMTP client certificates or intermediate CA certificates. Do not forget to create the
       necessary "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  ch-
       root jail.

       Specify  "smtpd_tls_CApath = /path/to/system_CA_directory" 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_CApath should remain empty. In contrast to smtpd_tls_CAfile, DNs  of  Certifica-
       tion 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 Cer-
       tification 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 almost 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 interop-
       erability 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 certifi-
       cate 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 ac-
       tually 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  con-
       tain  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 certifi-
       cate. 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  sup-
       ported by typical SMTP clients. Since some clients may not fall back to plain text after a
       TLS handshake failure, a certificate-less Postfix SMTP server will be  unable  to  receive
       email  from  some TLS-enabled clients. To avoid accidental configurations with no certifi-
       cates, Postfix enables certificate-less operation only when the  administrator  explicitly
       sets  "smtpd_tls_cert_file  =  none". This ensures that new Postfix SMTP server configura-
       tions 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_proto-
       cols" and perhaps also "smtpd_tls_mandatory_protocols".  It is  simpler  instead  to  just
       configure a certificate 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 cer-
       tificates  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
       itself   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
       certificates  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  white-
       space.   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, in-
       clude 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.  There-
       fore, 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
       multiple  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  certifi-
       cate  and issuer chain, it can be in a separate file, so long as that file is listed imme-
       diately 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  reli-
       able.   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 con-
       figuration 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 (typi-
       cally 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
       algorithms 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-default TLS cipherlist for MX hosts on the public Internet. Clients that begin the TLS
       handshake, 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
       encryption.  Cipher  types  listed in smtpd_tls_exclude_ciphers are excluded from the base
       definition of the selected cipher grade.  The default value is "medium"  for  Postfix  re-
       leases after the middle of 2015, "export" for older releases.

       When  TLS is mandatory the cipher grade is chosen via the smtpd_tls_mandatory_ciphers con-
       figuration 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
       "export" 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  con-
       tain  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  ci-
       phers.

       The  best-practice  choice of parameters uses a 2048-bit prime.  This is fine, despite the
       historical "1024" in the parameter name.  Do not be tempted to  use  much  larger  values,
       performance  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 parame-
       ters.

       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
       commands:

           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
       prefer, you can generate separate parameters for each instance.

       If  you  want to take maximal advantage of ciphers that offer forward secrecy see the Get-
       ting started section of FORWARD_SECRECY_README.  The full document  conveniently  presents
       all information 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  ci-
       phers with forward 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
       middle 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  com-
       bined   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
       contain  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 com-
       bined with the Postfix SMTP server ECDSA certificate file  specified  with  $smtpd_tls_ec-
       cert_file.   With  Postfix  >= 3.4 the preferred way to configure server keys and certifi-
       cates 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
              default 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 Get-
       ting started section of FORWARD_SECRECY_README.  The full document  conveniently  presents
       all information 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  ci-
       phers with forward 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 ven-
       dor.

smtpd_tls_exclude_ciphers (default: empty)
       List of ciphers or cipher types to exclude from the SMTP server cipher list at all TLS se-
       curity levels. Excluding valid ciphers can create interoperability problems.  DO  NOT  ex-
       clude  ciphers 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 excluded.

       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 dis-
       ables ciphers 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 ex-
       change 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
       public   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
       public key from the certificate and compute the appropriate digest of its DER (ASN.1)  en-
       coding.  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  com-
       bined   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
       includes 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
              certificate  trust-chain  verification errors if client certificate verification is
              not required.  With Postfix 2.8 and earlier, log the summary message, peer certifi-
              cate summary 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  en-
       cryption.  The default grade ("medium") is sufficiently strong that any benefit from glob-
       ally restricting 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 typi-
       cally 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
              specified 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 speci-
              fied via the tls_low_cipherlist configuration parameter, which you are strongly en-
              couraged 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 sym-
              metric bulk-encryption keys. This is the default minimum strength for mandatory TLS
              encryption.  The  underlying  cipherlist is specified via the tls_medium_cipherlist
              configuration 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 encour-
              aged to not change.

       null   Enable only the "NULL" OpenSSL ciphers, these provide  authentication  without  en-
              cryption.   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 cipher 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 certificates.  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 ci-
       phers even when Postfix does not need or use peer certificates, set "smtpd_tls_exclude_ci-
       phers   =   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))  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 ver-
       sion.  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 "<=" sym-
       bols 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  re-
       quired.   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 symbolic 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 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_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))  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 ver-
       sion.  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  "<="  sym-
       bols 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 re-
       quired.  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 symbolic 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
       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_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  in-
       formation 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  speci-
       fied, this overrides the obsolete parameters smtpd_use_tls and smtpd_enforce_tls. This pa-
       rameter 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  ap-
              plied  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 Post-
       fix  SMTP  server logs a warning and uses "encrypt" instead.  To verify remote SMTP client
       certificates,   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  sup-
       port  concurrent  access.   The  file is created if it does not exist. The smtpd(8) daemon
       does not use this parameter directly, rather the cache is implemented  indirectly  in  the
       tlsmgr(8) daemon. This means that per-smtpd-instance master.cf overrides of this parameter
       are not effective. Note, that each of the cache databases supported by  tlsmgr(8)  daemon:
       $smtpd_tls_session_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 possible 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  at-
       tempt  to  open  the file under a non-Postfix directory is redirected to the Postfix-owned
       data_directory, 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
       parameter 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)  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
       disabled, not just via the database, but also via RFC  5077  TLS  session  tickets,  which
       don't  require  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  li-
       brary (at least version 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
       support 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
       chosen 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.  Spec-
       ify "smtpd_upstream_proxy_protocol = haproxy" to enable the haproxy protocol; version 2 is
       supported with Postfix 3.5 and later.

       NOTE: To use the nginx proxy with smtpd(8), 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 pa-
       rameter.

       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  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.

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 environments 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
       requires SMTPUTF8 support, because UTF8 may be introduced during local processing (for ex-
       ample, the client hostname in Postfix's Received: header, adding @$myorigin or  .$mydomain
       to  an  incomplete  address, address rewriting, alias expansion, automatic BCC recipients,
       local forwarding, and changes made by header checks or Milter applications).

       For now, the default is to enable "SMTPUTF8 required" autodetection only for Postfix send-
       mail  command-line submissions and address verification probes.  This may change once SMT-
       PUTF8 support 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) command.

        smtpd Mail received with the smtpd(8) daemon.

        qmqpd Mail received with the qmqpd(8) 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) 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) or smtpd(8) daemon.

        verify
              Address verification probe from the verify(8) 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
       requires 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  pa-
       rameter  disables  locally-generated  bounces,  changes the handling of negative responses
       from remote 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.
       Therefore,  the response that Postfix logs may differ from the response that Postfix actu-
       ally 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  de-
       livery 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  ap-
       plications.

       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
       contains  valid  8-bit MIME mail, and it rejects bounces from mailers that do not MIME en-
       capsulate 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  mul-
       tipart/* 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 re-
       ject 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  =  dot-
       lock".

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:

       o      The message is received with the Postfix sendmail(1) command,

       o      The  message  is  received  from  a  network  client that matches $local_header_re-
              write_clients,

       o      The message is received from the network, and the remote_header_rewrite_domain  pa-
              rameter 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).
       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  default  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
       completed initialization. Errors during process initialization will be logged with the de-
       fault name. Examples are errors while parsing the command line arguments, and errors while
       accessing 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), qmqpd(8)), this fea-
       ture is implemented by the Postfix master(8) 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 en-
       able 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), lmtp(8)).

       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 speci-
       fied with *_tls_CApath or *_tls_CAfile.  The default is "no"; this prevents  Postfix  from
       trusting   third-party   certificates   and   giving   them  relay  permission  with  per-
       mit_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 cer-
       tificate verification 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) or smtpd(8) process  requests  from  the
       tlsmgr(8) 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 un-
       derlying OpenSSL library, otherwise the Postfix SMTP client will not support DANE TLSA se-
       curity.

       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
       specified 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-pa-
       rameters.xhtml#matching-types" >IANA assigned TLSA matching type number the  algorithm  in
       question.   Postfix  will  check this constraint for the algorithms it knows about.  Addi-
       tional 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
       compromised  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  reli-
       able.   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.  Ap-
       plications,  such  as  Postfix, that want to maximize interoperability ask the OpenSSL li-
       brary 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 in-
       teroperability 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)).  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, until 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)

       LEGACY_SERVER_CONNECT
              See SSL_CTX_set_options(3)

       MICROSOFT_BIG_SSLV3_BUFFER
              See SSL_CTX_set_options(3)

       MICROSOFT_SESS_ID_BUG
              See SSL_CTX_set_options(3)

       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)

       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)

       SSLREF2_REUSE_CERT_TYPE_BUG
              See SSL_CTX_set_options(3)

       TLS_BLOCK_PADDING_BUG
              See SSL_CTX_set_options(3)

       TLS_D5_BUG
              See SSL_CTX_set_options(3)

       TLS_ROLLBACK_BUG
              See  SSL_CTX_set_options(3).   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).

       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 sup-
       ported 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  im-
       plemented.   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 ven-
       dor.

tls_eecdh_strong_curve (default: prime256v1)
       The  elliptic curve used by the Postfix SMTP server for sensibly strong ephemeral ECDH key
       exchange. 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 attacks. The selected curve must be implemented  by  OpenSSL  (as  reported  by  ec-
       param(1)  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 implementa-
       tions  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  SE-
       CRET.

       Note: elliptic curve names are poorly standardized; different standards groups are assign-
       ing 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  Get-
       ting  started  section of FORWARD_SECRECY_README.  The full document conveniently presents
       all information 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 ci-
       phers with forward 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 ven-
       dor.

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 = ul-
       tra". The phrase "maximally strong" means approximately 192-bit  security  based  on  best
       known  attacks.   This additional strength comes at a significant computational cost, most
       users should instead set "smtpd_tls_eecdh_grade = strong".  The selected curve must be im-
       plemented by OpenSSL (as reported by ecparam(1) 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
       SECRET.

       If you want to take maximal advantage of ciphers that offer forward secrecy see  the  Get-
       ting  started  section of FORWARD_SECRECY_README.  The full document conveniently presents
       all information 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 ci-
       phers with forward 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 ven-
       dor.

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  Post-
       fix  releases before the middle of 2015 this is the default cipherlist for the opportunis-
       tic ("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, un-
       til Postfix times out. With this enabled, Postfix will not wait for the remote TLS peer to
       respond 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"
       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_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 certificate fingerprint support that is available since Postfix 2.2.

       Specify "tls_legacy_public_key_fingerprints = yes" temporarily, pending a  migration  from
       configuration  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  cor-
       rect 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  oppor-
       tunistic  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  en-
       cryption.  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 cipher 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 implementa-
       tions of DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA.  Enabling server ci-
       pher-suite  selection  may  create interoperability issues with Windows 2003 Microsoft Ex-
       change 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) reads from $tls_random_source when (re)seeding the
       in-memory 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).  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
       default  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  di-
       rectory 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) to save the state of the pseudo random number gen-
       erator (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) to re-seed the in-memory pseudo random num-
       ber  generator  (PRNG) pool from external sources.  The actual time between re-seeding at-
       tempts 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)  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 compatible 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  pa-
       rameter  is  implemented  in the Postfix TLS library, and applies to both smtpd(8) and the
       SMTP server mode of tlsproxy(8).

       When this parameter is non-empty, the Postfix SMTP server enables SNI  extension  process-
       ing, and logs SNI values that are invalid or don't match an entry in the the specified ta-
       bles.  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 certificate(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  in-
       dexed files via postmap(1), the value specified for each key is a list of filenames.  When
       postmap(1) 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 inter-
       preted 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  certifi-
       cate  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 responsibility 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 interme-
       diate 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 to-
       gether, 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  de-
       fault  is aes-256-cbc.  Overriding the default to choose a different algorithm is discour-
       aged.

       Setting this parameter empty disables session ticket support in the Postfix  SMTP  server.
       Another 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
       default.  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-interoperabil-
       ity 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 be-
       low.  The names are not case-sensitive, you can use lower-case if you prefer.   The  upper
       case  values  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 fea-
       ture is critical (to deal with a new vulnerability or a serious interoperability problem).
       Postfix 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 re-
       lease, 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 solu-
       tion.

       If the value of the parameter is a hexadecimal long integer starting with  "0x",  the  op-
       tions  corresponding to the bits specified in its value are enabled (see openssl/ssl.h and
       SSL_CTX_set_options(3)).  You can only enable options  not  already  controlled  by  other
       Postfix settings.  For example, you cannot disable protocols or enable server cipher pref-
       erence.  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).

       LEGACY_SERVER_CONNECT
              See SSL_CTX_set_options(3).

       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
              resumption.  Best not set explicitly.  See SSL_CTX_set_options(3).

       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 at-
              tack.  See SSL_CTX_set_options(3).

       NO_SESSION_RESUMPTION_ON_RENEGOTIATION
              Postfix >= 3.4. See SSL_CTX_set_options(3).

       PRIORITIZE_CHACHA
              Postfix >= 3.4. See SSL_CTX_set_options(3).

       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 example.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 sup-
       ported, and are not blessed by any standard, there is little to be gained  by  disallowing
       their use in this context.

       Notes:

       o      In a certificate name, the "*" is special only when it is used as the first label.

       o      While Postfix (2.11 or later) can match "*" with multiple domain name labels, other
              implementations likely will not.

       o      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) 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
       certificates 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 tl-
       sproxy(8) client uses to verify a remote TLS server certificate. See  smtp_tls_CApath  for
       further details.

       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)  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) 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)  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)  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) client ECDSA certificate in PEM format. See smtp_tls_ec-
       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_eckey_file (default: $smtp_tls_eckey_file)
       File  with  the  Postfix  tlsproxy(8)  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)  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)   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) client TLS usage policy by next-hop
       destination 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) 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)  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) 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 fur-
       ther details.

       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 re-
       mote  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) 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) server RSA certificate in PEM  format.   This  file  may
       also  contain the Postfix tlsproxy(8) 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 certificates 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) 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) server will use with opportunis-
       tic 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) server DSA certificate in PEM format.  This file may
       also contain the Postfix tlsproxy(8) 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) 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) 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
       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.

       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) server DSA private key in PEM format.  This file may  be
       combined  with  the  Postfix  tlsproxy(8)  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) server ECDSA certificate in PEM format.  This file may
       also contain the Postfix tlsproxy(8) server private ECDSA key.  See  smtpd_tls_eccert_file
       for  further  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) server ECDSA private key in PEM format.  This  file  may
       be  combined  with  the  Postfix  tlsproxy(8) 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) 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) 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) server RSA private key in PEM format.  This file may be
       combined  with  the  Postfix  tlsproxy(8)  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) 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) 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)  server  cipher
       list  at  mandatory TLS security levels.  See smtpd_tls_mandatory_exclude_ciphers for fur-
       ther details.

       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) server with  mandatory  TLS  en-
       cryption.  If  the  list is empty, the server supports all available SSL/TLS protocol ver-
       sions.  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) server will exclude or include with op-
       portunistic 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) 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)  server  TLS  session  cache  information.
       Since  the cache is shared with smtpd(8) and managed by tlsmgr(8), there is only one expi-
       ration time for the SMTP server cache shared by all three services, namely  smtpd_tls_ses-
       sion_cache_timeout.

       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) process may take to process local or remote I/O before  it  is
       terminated  by  a  built-in  watchdog timer.  This is a safety mechanism that prevents tl-
       sproxy(8) from becoming non-responsive due to a bug in Postfix itself or in  system  soft-
       ware.  To avoid false alarms and unnecessary cache corruption this limit cannot be set un-
       der 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)  daemon  and
       maintains a record of mail deliveries and produces a mail delivery report when verbose de-
       livery 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 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_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
       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: "_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 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_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
       parameter 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 parameters 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  "post-
       conf"  command output before Postfix version 2.9.  This limitation applies to many parame-
       ters whose name is a combination of a master.cf service name and  a  built-in  suffix  (in
       this case: "_destination_concurrency_limit").

transport_destination_concurrency_negative_feedback     (default:    $default_destination_concur-
       rency_negative_feedback)
       A transport-specific override  for  the  default_destination_concurrency_negative_feedback
       parameter 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
       parameters  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_concur-
       rency_positive_feedback)
       A  transport-specific  override  for the default_destination_concurrency_positive_feedback
       parameter 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
       parameters 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" com-
       mand  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:
       "_destination_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: "_destination_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
       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:  "_ex-
       tra_recipient_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 parame-
       ters 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 trans-
       port, next-hop destination).  See transport(5) 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  "trans-
       port_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
       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: "_mini-
       mum_delivery_slots").

transport_recipient_limit (default: $default_recipient_limit)
       A transport-specific override  for  the  default_recipient_limit  parameter  value,  where
       transport 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 combination of a master.cf service name and a built-in suffix (in this case: "_recip-
       ient_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
       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: "_recip-
       ient_refill_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
       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: "_recip-
       ient_refill_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  combina-
       tion 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
       transport.

trigger_timeout (default: 10s)
       The  time  limit  for sending a trigger to a Postfix daemon (for example, the pickup(8) or
       qmgr(8) 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) server inserts when a message contains  no  To:
       or  Cc: message header. With Postfix 2.8 and later, the default value is empty. With Post-
       fix 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 re-
       strictions 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_re-
       cipient_domain  fail  due to a temporary error condition. Specify "defer" to defer the re-
       mote SMTP client request immediately. With the default "defer_if_permit" action, the Post-
       fix  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  tempo-
       rary 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_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
       $local_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_do-
       mains, 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_do-
       mains, 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_do-
       mains, 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
       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_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  re-
       sponse includes 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 oppor-
       tunities to reject mail, and defers the client request only if it would otherwise  be  ac-
       cepted.

       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
       include the numeric SMTP reply code or the enhanced status code. By default, the  response
       includes 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 default "defer_if_permit" action, the Postfix SMTP server continues to look for oppor-
       tunities to reject mail, and defers the client request only if it would otherwise  be  ac-
       cepted.

       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)
       command 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-ex-
       istent 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 compat-
       ible with Postfix version 1.1.

       The default value is $virtual_alias_maps so that you can keep all information  about  vir-
       tual  alias domains in one place.  If you have many users, it is better to separate infor-
       mation that changes more frequently (virtual address -> local or remote  address  mapping)
       from information 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 ignored).  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
       recipient.

       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 re-
       mote address.  The table format and lookups are documented in virtual(5). For an  overview
       of Postfix address manipulations see the ADDRESS_REWRITING_README document.

       This feature is available in Postfix 2.0 and later. The default value is backwards compat-
       ible 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
       recursive.

       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
       applied 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 future.

       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) delivery agent to change the delivery  status  code  or
       explanatory  text  of  successful  or  unsuccessful deliveries.  See default_delivery_sta-
       tus_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
       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.

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) mailbox delivery.

       This  parameter is specific to the virtual(8) 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
       specified domain that does not have a specific "user AT domain.tld" entry.

       When a recipient address has an optional address extension (user+foo AT domain.tld), the vir-
       tual(8)  delivery  agent  looks  up  the full address first, and when the lookup fails, it
       looks up the unextended address (user AT domain.tld).

       Note 1: for security reasons, the virtual(8) delivery agent disallows  regular  expression
       substitution of $1 etc. in regular expression lookup tables, because that would open a se-
       curity hole.

       Note 2: for security reasons, the virtual(8) delivery agent will silently ignore  requests
       to  use  the  proxymap(8)  server. Instead it will open the table directly. Before Postfix
       version 2.2, the virtual(8) delivery agent will terminate with a fatal error.

virtual_mailbox_base (default: empty)
       A prefix that the virtual(8) 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 con-
       trol 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) 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) de-
       livery agent.  The SMTP server validates recipient  addresses  with  $virtual_mailbox_maps
       and  rejects  mail for non-existent recipients.  See also the virtual mailbox domain class
       in the ADDRESS_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 compat-
       ible with Postfix version 1.1.

virtual_mailbox_limit (default: 51200000)
       The maximal size in bytes of an individual virtual(8) mailbox or maildir file, or zero (no
       limit).

       This parameter is specific to the virtual(8) 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) mailbox before attempting delivery.  For a list of
       available file locking methods, use the "postconf -l" command.

       This parameter is specific to the virtual(8) 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
       specified domain that does not have a specific "user AT domain.tld" entry.

       The remainder of this text is specific to the virtual(8) delivery agent.  It does not  ap-
       ply when mail is delivered with a different mail delivery program.

       The  virtual(8)  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_mailbox_base is unconditionally prepended to this path.

       When a recipient address has an optional address extension (user+foo AT domain.tld), the vir-
       tual(8)  delivery  agent  looks  up  the full address first, and when the lookup fails, it
       looks up the unextended address (user AT domain.tld).

       Note 1: for security reasons, the virtual(8) delivery agent disallows  regular  expression
       substitution of $1 etc. in regular expression lookup tables, because that would open a se-
       curity hole.

       Note 2: for security reasons, the virtual(8) delivery agent will silently ignore  requests
       to  use  the  proxymap(8)  server. Instead it will open the table directly. Before Postfix
       version 2.2, the virtual(8) 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 ver-
       sion  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) delivery agent  accepts  as  a  result  from
       $virtual_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) 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) table.

       Specify  a string of the form transport:nexthop, where transport is the name of a mail de-
       livery transport defined in master.cf.  The :nexthop destination is optional;  its  syntax
       is documented 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) delivery agent uses while
       writing to the recipient's mailbox.

       This parameter is specific to the virtual(8) 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
       specified domain that does not have a specific "user AT domain.tld" entry.

       When a recipient address has an optional address extension (user+foo AT domain.tld), the vir-
       tual(8) delivery agent looks up the full address first, and  when  the  lookup  fails,  it
       looks up the unextended address (user AT domain.tld).

       Note  1:  for security reasons, the virtual(8) delivery agent disallows regular expression
       substitution of $1 etc. in regular expression lookup tables, because that would open a se-
       curity hole.

       Note  2: for security reasons, the virtual(8) delivery agent will silently ignore requests
       to use the proxymap(8) server. Instead it will open the  table  directly.  Before  Postfix
       version 2.2, the virtual(8) delivery agent will terminate with a fatal error.

SEE ALSO
       postconf(1), Postfix configuration parameter maintenance
       master(5), 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)

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