man > postconf(5)

Che Dong

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 parameters; para‐
meters not specified are left at their default values.

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

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

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

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

• A parameter value may refer to other parameters.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

access_map_defer_code (default: 450)
The numerical Postfix SMTP server response code for an access(5) map "defer" action, includ‐
ing "defer_if_permit" or "defer_if_reject". Prior to Postfix 2.6, the response is hard-coded
as "450".

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

This feature is available in Postfix 2.6 and later.

access_map_reject_code (default: 554)
The numerical Postfix SMTP server response code for an access(5) 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 feature
requires that the database supports the "delete" and "sequence" operators. Specify a zero
interval to disable database cleanup.

After each database cleanup run, the verify(8) 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.

Specify a non-negative 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).

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 corrupted,
the world comes to an end. To recover, delete (NOT: truncate) the file and do "postfix re‐
load".

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

Examples:

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

This feature is available in Postfix 2.1 and later.

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

This feature is available in Postfix 2.1 and later.

address_verify_negative_expire_time (default: 3d)
The time after which a failed probe expires from the address verification cache.

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 d (days).

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.

This feature is available in Postfix 2.1 and later.

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

This feature is available in Postfix 3.1 and later.

address_verify_poll_count (default: normal: 3, overload: 1)
How many times to query the verify(8) service for the completion of an address verification
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 delivery
request for a new address.

Examples:

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

This feature is available in Postfix 2.1 and later.

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

The default polling delay is 3 seconds.

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.

This feature is available in Postfix 2.1 and later.

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

This feature is available in Postfix 2.1 and later.

address_verify_relay_transport (default: $relay_transport)
Overrides the relay_transport parameter setting for address verification probes.

This feature is available in Postfix 2.1 and later.

address_verify_relayhost (default: $relayhost)
Overrides the relayhost parameter setting for address verification probes. This information
can be overruled with the transport(5) table.

This feature is available in Postfix 2.1 and later.

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

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

Examples:

address_verify_sender = <>
address_verify_sender = postmaster@mydomain

This feature is available in Postfix 2.1 and later.

address_verify_sender_dependent_default_transport_maps (default: $sender_dependent_default_trans‐
port_maps)
Overrides the sender_dependent_default_transport_maps parameter setting for address verifica‐
tion probes.

This feature is available in Postfix 2.7 and later.

address_verify_sender_dependent_relayhost_maps (default: $sender_dependent_relayhost_maps)
Overrides the sender_dependent_relayhost_maps parameter setting for address verification
probes.

This feature is available in Postfix 2.3 and later.

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

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

To enable time-dependent probe sender addresses, specify a non-zero time value. 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.

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 "send‐
mail -bi".

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

Examples:

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

alias_maps (default: see postconf -d output)
The alias databases that are used for local(8) delivery. See aliases(5) for syntax details.
Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
be searched in the specified order until a match is found. Note: these lookups are recur‐
sive.

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

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

The local(8) 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 terminology).

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 terminology).

Specify zero or more of: alias, forward or include, in order to allow "/file/name" destina‐
tions 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 command
line. Such software would not be able to distinguish a malicious address from a bona fide
command-line option. Although this can be prevented by inserting a "--" option terminator
into the command line, this is difficult to enforce consistently and globally.

As of Postfix version 2.5, this feature is implemented by trivial-rewrite(8). With earlier
versions this feature was implemented by qmgr(8) and was limited to recipient addresses only.

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

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

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

• The message is received from a network client that matches $lo‐
cal_header_rewrite_clients,

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

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

Example:

allow_percent_hack = no

allow_srv_lookup_fallback (default: no)
When SRV record lookup fails or no SRV record exists, fall back to MX or IP address lookup as
if SRV record lookup was not enabled.

This feature is available in Postfix 3.8 and later.

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

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

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

alternate_config_directories (default: empty)
A list of non-default Postfix configuration directories that may be specified with "-c con‐
fig_directory" on the command line (in the case of sendmail(1), 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 parame‐
ter setting. Earlier Postfix versions always add these headers; this may break DKIM signa‐
tures that cover non-existent headers. The undisclosed_recipients_header parameter setting
determines whether a To: header will be added.

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

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

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

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

anvil_rate_time_unit (default: 60s)
The time unit over which client connection rates and other rates are calculated.

This feature is implemented by the anvil(8) 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.

anvil_status_update_time (default: 600s)
How frequently the anvil(8) connection and rate limiting server logs peak usage information.

This feature is available in Postfix 2.2 and later.

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

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

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

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

• The message is received from a network client that matches $lo‐
cal_header_rewrite_clients,

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

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

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

Note 1: When disabled (Postfix 3.0 and later), users will not be able to send mail to
"user@partialdomainname" but will have to specify full domain names instead.

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

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

• The message is received from a network client that matches $lo‐
cal_header_rewrite_clients,

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

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

application_event_drain_time (default: 100s)
How long the postkick(1) command waits for a request to enter the Postfix daemon process in‐
put buffer before giving up.

This feature is available in Postfix 2.1 and later.

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

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

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

This feature is available in Postfix 2.2 and later.

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

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

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

This feature is available in Postfix 2.2 and later.

authorized_submit_users (default: static:anyone)
List of users who are authorized to submit mail with the sendmail(1) 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 pa‐
rameter to smtpd_authorized_verp_clients and changed the default to none.

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

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

backwards_bounce_logfile_compatibility (default: yes)
Produce additional bounce(8) logfile records that can be read by Postfix versions before 2.0.
The current and more extensible "name = value" format is needed in order to implement more
sophisticated functionality.

This feature is available in Postfix 2.1 and later.

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

This feature is available in Postfix 2.0 and later.

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

This feature is available in Postfix 2.0 and later.

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

Specify, for example, "best_mx_transport = local" to pass the mail from the Postfix SMTP
client to the local(8) 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 interactive
users, the biff service can be a performance drain. Specify "biff = no" in main.cf to dis‐
able.

body_checks (default: empty)
Optional lookup tables for content inspection as specified in the body_checks(5) 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.

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 original)
or as text/rfc822-headers (the headers only). With Postfix version 2.4 and earlier, a mes‐
sage is always returned as message/rfc822 and is truncated when it exceeds the size limit.

Notes:

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

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

bounce_template_file (default: empty)
Pathname of a configuration file with bounce message templates. These override the built-in
templates of delivery status notification (DSN) messages for undeliverable mail, 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 parameters.
The result of $name expansion can be previewed with "postconf -b file_name" before the file
is placed into the Postfix configuration directory.

This feature is available in Postfix 2.3 and later.

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

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

canonical_classes (default: envelope_sender, envelope_recipient, header_sender, header_recipient)
What addresses are subject to canonical_maps address mapping. By default, canonical_maps ad‐
dress mapping is applied to envelope sender and recipient addresses, and to header sender and
header recipient addresses.

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

This feature is available in Postfix 2.2 and later.

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

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

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

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

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

• The message is received from a network client that matches $lo‐
cal_header_rewrite_clients,

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

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

Examples:

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

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

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

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

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

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

cleanup_service_name (default: cleanup)
The name of the cleanup(8) 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 commands. Failure to
change directory causes the delivery to be deferred.

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

$user The recipient's username.

$shell The recipient's login shell pathname.

$home The recipient's home directory.

$recipient
The full recipient address.

$extension
The optional recipient address extension.

$domain
The recipient domain.

$local The entire recipient localpart.

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

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

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 $mail‐
box_command and $command_execution_directory. Characters outside the allowed set are re‐
placed by underscores.

command_time_limit (default: 1000s)
Time limit for delivery to external commands. This limit is used by the local(8) 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 pa‐
rameter as well.

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

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

using backwards-compatible default setting name=value
to [accept a specific client request]

using backwards-compatible default setting name=value
to [enable specific Postfix behavior]

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

# postconf name=value
# postfix reload

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

# postconf compatibility_level=N
# postfix reload

For N specify the number that is logged in your postfix(1) warning message:

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

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

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

This feature is available in Postfix 3.0 and later.

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

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

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

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

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

See also: delay_warning_time.

This feature is available in Postfix 3.0 and later.

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

This feature is available in Postfix 2.3 and later.

connection_cache_service_name (default: scache)
The name of the scache(8) 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:destination.
The transport name specifies the first field of a mail delivery agent definition in mas‐
ter.cf; the syntax of the next-hop destination is described in the manual page of the corre‐
sponding delivery agent. More information about external content filters is in the Postfix
FILTER_README file.

Notes:

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

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

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

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

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

daemon_table_open_error_is_fatal (default: no)
How a Postfix daemon process handles errors while opening lookup tables: gradual degradation
or immediate termination.

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

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

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

This feature is available in Postfix 2.9 and later.

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

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

This feature is available in Postfix 2.5 and later.

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

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

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

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

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

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

Examples:

debug_peer_list = 127.0.0.1
debug_peer_list = example.com

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

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

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

Example:

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

default_database_type (default: see postconf -d output)
The default database type for use in newaliases(1), 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 message
with another.

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

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

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

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

Examples:

default_delivery_slot_cost = 0
default_delivery_slot_cost = 2

default_delivery_slot_discount (default: 50)
The default value for transport-specific _delivery_slot_discount settings.

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

Use transport_delivery_slot_discount to specify a transport-specific override, where trans‐
port is the master.cf name of the message delivery transport.

default_delivery_slot_loan (default: 3)
The default value for transport-specific _delivery_slot_loan settings.

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

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

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

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

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

enhanced-status-code SPACE explanatory-text

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

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

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

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

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

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

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

Notes:

• This feature will NOT override the soft_bounce safety net.

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

This feature is available in Postfix 3.0 and later.

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

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

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

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

default_destination_concurrency_limit (default: 20)
The default maximal number of parallel deliveries to the same destination. This is the de‐
fault limit for delivery via the lmtp(8), pipe(8), smtp(8) and virtual(8) delivery agents.
With a per-destination recipient limit > 1, a destination is a domain, otherwise it is a re‐
cipient.

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

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

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

Specify one of the following forms:

number

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

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

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

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

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

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

Specify one of the following forms:

number

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

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

This feature is available in Postfix 2.5 and later.

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

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

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

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

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

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

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

Use transport_destination_rate_delay to specify a transport-specific override, where trans‐
port is the master.cf name of the message delivery transport.

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

This feature is available in Postfix 2.5 and later.

default_destination_recipient_limit (default: 50)
The default maximal number of recipients per message delivery. This is the default limit for
delivery via the lmtp(8), pipe(8), smtp(8) and virtual(8) delivery agents.

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

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

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

• It changes the meaning of other corresponding per-destination settings in a similar
manner, from settings for delivery to the same domain into settings for delivery to
the same recipient.

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

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

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

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

This feature is available in Postfix 2.7 and later.

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

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

default_privs (default: nobody)
The default rights used by the local(8) delivery agent for delivery to an 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 details.

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

$helo_name
The hostname given in HELO or EHLO command or empty string.

$rbl_class
The denylisted entity type: Client host, Helo command, Sender address, or Recipient
address.

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

$rbl_domain
The RBL domain where $rbl_what is denylisted.

$rbl_reason
The reason why $rbl_what is denylisted, or an empty string.

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

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

$recipient_domain
The recipient domain or empty string.

$recipient_name
The recipient address localpart or <> in case of null address.

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

$sender_domain
The sender domain or empty string.

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

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

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

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

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

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

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

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

default_recipient_refill_delay (default: 5s)
The default per-transport maximum delay between refilling recipients. When not all message
recipients fit into 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 a timely manner even when
$default_recipient_refill_limit is too high for too slow deliveries.

Use transport_recipient_refill_delay to specify a transport-specific override, where trans‐
port is the master.cf name of the message delivery transport.

This feature is available in Postfix 2.4 and later.

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

Use transport_recipient_refill_limit to specify a transport-specific override, where trans‐
port is the master.cf name of the message delivery transport.

This feature is available in Postfix 2.4 and later.

default_transport (default: smtp)
The default mail delivery transport and next-hop destination for destinations that do not
match $mydestination, $inet_interfaces, $proxy_interfaces, $virtual_alias_domains, $vir‐
tual_mailbox_domains, or $relay_domains. This information can be overruled with the
sender_dependent_default_transport_maps parameter and with the transport(5) table.

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

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

Example:

default_transport = uucp:relayhostname

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

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

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

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

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

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

NOTE: the delay is enforced by the queue manager.

This feature is available in Postfix 3.1 and later.

default_verp_delimiters (default: +=)
The two default VERP delimiter characters. These are used when no explicit delimiters are
specified with the SMTP XVERP command or with the "sendmail -XV" command-line option (Postfix
2.2 and earlier: -V). Specify characters that are allowed by the verp_delimiter_filter set‐
ting.

This feature is available in Postfix 1.1 and later.

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

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

defer_service_name (default: defer)
The name of the defer service. This service is implemented by the bounce(8) daemon and main‐
tains a record of failed delivery attempts and generates non-delivery notifications.

This feature is available in Postfix 2.0 and later.

defer_transports (default: empty)
The names of message delivery transports that should not deliver mail unless someone issues
"sendmail -q" or equivalent. Specify zero or more mail delivery transport 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 of 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:

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

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

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

• d = time in message transmission

This feature is available in Postfix 2.3 and later.

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

See also: delay_warning_time, notify_classes.

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

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

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

See also: delay_notice_recipient, notify_classes, confirm_delay_cleared.

deliver_lock_attempts (default: 20)
The maximal number of attempts to acquire an exclusive lock on a mailbox file or bounce(8)
logfile.

deliver_lock_delay (default: 1s)
The time between attempts to acquire an exclusive lock on a mailbox file or bounce(8) log‐
file.

destination_concurrency_feedback_debug (default: no)
Make the queue manager's feedback algorithm verbose for performance analysis purposes.

This feature is available in Postfix 2.5 and later.

detect_8bit_encoding_header (default: yes)
Automatically detect 8BITMIME body content by looking at Content-Transfer-Encoding: message
headers; historically, this behavior was hard-coded to be "always on".

This feature is available in Postfix 2.5 and later.

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

DNS lookups are enabled by default.

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

This feature is available in Postfix 2.0 and later.

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

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

This feature is available in Postfix 2.0 and later.

disable_verp_bounces (default: no)
Disable sending one bounce report per recipient.

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

This feature is available in Postfix 1.1 and later.

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

Example:

disable_vrfy_command = no

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

This feature is available in Postfix 3.1 and later.

dnsblog_reply_delay (default: 0s)
A debugging aid to artificially delay DNS responses.

This feature is available in Postfix 2.8.

dnsblog_service_name (default: dnsblog)
The name of the dnsblog(8) service entry in master.cf. This service performs DNS al‐
low/denylist lookups.

This feature is available in Postfix 2.8 and later.

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

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

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

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

Example:

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

Possible reasons why DNSSEC validation may be unavailable:

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

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

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

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

dont_remove (default: 0)
Don't remove queue files and save them to the "saved" mail queue. This is a debugging aid.
To inspect the envelope information and content of a Postfix queue file, use the postcat(1)
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 ad‐
dress.

This feature is available in Postfix 3.6 and later.

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

empty_address_relayhost_maps_lookup_key (default: <>)
The sender_dependent_relayhost_maps search string that will be used instead of the null
sender address.

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

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

enable_idna2003_compatibility (default: no)
Enable 'transitional' compatibility between IDNA2003 and IDNA2008, when converting UTF-8 do‐
main names to/from the ASCII form that is used for DNS lookups. Specify "yes" for compatibil‐
ity with Postfix <= 3.1 (not recommended). This affects the conversion of domain names that
contain for example the German sz and the Greek zeta. See http://unicode.org/cldr/util‐
ity/idna.jsp for more examples.

This feature is available in Postfix 3.2 and later.

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

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

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

• Existing queue file names are not affected.

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

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

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

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

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

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

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

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

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

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

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

# postfix stop
# postconf enable_long_queue_ids=no
# postsuper

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

This feature is available in Postfix 2.9 and later.

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

The original recipient address is used as follows:

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

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

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

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

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

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

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

This feature is available in Postfix 3.6 and later.

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

error_service_name (default: error)
The name of the error(8) pseudo delivery agent. This service always returns mail as undeliv‐
erable.

This feature is available in Postfix 2.0 and later.

execution_directory_expansion_filter (default: see postconf -d output)
Restrict the characters that the local(8) delivery agent allows in $name expansions of $com‐
mand_execution_directory. Characters outside the allowed set are replaced by underscores.

This feature is available in Postfix 2.2 and later.

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

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

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

Example:

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

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

This feature was removed in Postfix version 2.1.

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

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

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

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

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

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

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

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

fallback_transport (default: empty)
Optional message delivery transport that the local(8) 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_mail‐
box, mail_spool_directory, fallback_transport_maps, fallback_transport and luser_relay.

fallback_transport_maps (default: empty)
Optional lookup tables with per-recipient message delivery transports for recipients that the
local(8) delivery agent could not find in the aliases(5) or UNIX password database.

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

This feature is available in Postfix 2.3 and later.

fast_flush_domains (default: $relay_domains)
Optional list of destinations that are eligible for per-destination logfiles with mail that
is queued to those destinations.

By default, Postfix maintains "fast flush" logfiles only for destinations that the Postfix
SMTP server is willing to relay to (i.e. the default is: "fast_flush_domains = $relay_do‐
mains"; see the relay_domains parameter in the postconf(5) manual).

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

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

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

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

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

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

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

fault_injection_code (default: 0)
Force specific internal tests to fail, to test the handling of errors that are difficult to
reproduce otherwise.

flush_service_name (default: flush)
The name of the flush(8) 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.

forward_expansion_filter (default: see postconf -d output)
Restrict the characters that the local(8) delivery agent allows in $name expansions of $for‐
ward_path. Characters outside the allowed set are replaced by underscores.

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

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

$user The recipient's username.

$shell The recipient's login shell pathname.

$home The recipient's home directory.

$recipient
The full recipient address.

$extension
The optional recipient address extension.

$domain
The recipient domain.

$local The entire recipient localpart.

$recipient_delimiter
The address extension delimiter that was found in the recipient address (Postfix 2.11
and later), or the 'first' delimiter specified with the system-wide recipient address
extension delimiter (Postfix 3.5.22, 3.5.12, 3.7.8, 3.8.3 and later). Historically,
this was always the system-wide recipient address extension delimiter (Postfix 2.10
and earlier).

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

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 Delivered-To:
address while expanding aliases or .forward files.

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

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

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

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

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

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

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

header_address_token_limit (default: 10240)
The maximal number of address tokens are allowed in an address message header. Information
that exceeds the limit is discarded. The limit is enforced by the cleanup(8) server.

header_checks (default: empty)
Optional lookup tables for content inspection of primary non-MIME message headers, as speci‐
fied in the header_checks(5) 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 a
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 Post‐
fix 3.3.

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

Notes:

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

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

• The Postfix sendmail(1) command gets name information from the -F command-line option,
from the NAME environment variable, or from the UNIX password file.

This feature is available in Postfix 3.3 and later.

header_size_limit (default: 102400)
The maximal amount of memory in bytes for storing a message header. If a header is larger,
the excess is discarded. The limit is enforced by the cleanup(8) 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.

Examples:

home_mailbox = Mailbox
home_mailbox = Maildir/

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

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

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

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

ignore_srv_lookup_error (default: no)
When SRV record lookup fails, fall back to MX or IP address lookup as if SRV record lookup
was not enabled.

This feature is available in Postfix 3.8 and later.

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

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.

POSTLOG_SERVICE
Needed to make "maillog_file" work during daemon process initialization.

POSTLOG_HOSTNAME
Needed to make "maillog_file" work during daemon process initialization.

Specify a list of names and/or name=value pairs, separated by whitespace or comma. Specify "{
name=value }" to protect whitespace or comma in environment variable 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 mes‐
sage delivery rate. This feature is turned on by default (it's disabled on SCO UNIX due to an
SCO bug).

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

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

inet_interfaces (default: all)
The local 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 ad‐
dress, the Postfix SMTP client will use this address as the IP source address for outbound
mail. Support for IPv6 is available in Postfix version 2.2 and later.

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

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

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

Examples:

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

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

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

This feature is available in Postfix 2.2 and later.

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

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

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

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

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

Examples:

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

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

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

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

from=<name with spaces 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 address
in different forms by playing games with quotes and backslashes. An attacker should not be
able to use such games to circumvent Postfix access policies.

This feature is available in Postfix 3.5 and later.

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

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

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

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

bounce Inspect the content of delivery status notifications.

notify Inspect the content of postmaster notifications by the smtp(8) and smtpd(8) processes.

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

This feature is available in Postfix 2.3 and later.

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

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

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

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

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

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

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

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

This feature is available in Postfix 3.6 and later.

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

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

This feature is available in Postfix 2.11 and later.

lmtp_address_preference (default: ipv6)
The LMTP-specific version of the smtp_address_preference configuration parameter. See there
for details.

This feature is available in Postfix 2.8 and later.

lmtp_address_verify_target (default: rcpt)
The LMTP-specific version of the smtp_address_verify_target configuration parameter. See
there for details.

This feature is available in Postfix 3.0 and later.

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

lmtp_balance_inet_protocols (default: yes)
The LMTP-specific version of the smtp_balance_inet_protocols configuration parameter. See
there for details.

This feature is available in Postfix 3.3 and later.

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

This feature is available in Postfix 2.3 and later.

lmtp_bind_address6 (default: empty)
The LMTP-specific version of the smtp_bind_address6 configuration parameter. See there for
details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 3.7 and later.

lmtp_body_checks (default: empty)
The LMTP-specific version of the smtp_body_checks configuration parameter. See there for de‐
tails.

This feature is available in Postfix 2.5 and later.

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

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

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

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

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

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

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

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

lmtp_cname_overrides_servername (default: yes)
The LMTP-specific version of the smtp_cname_overrides_servername configuration parameter.
See there for details.

This feature is available in Postfix 2.3 and later.

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

Example:

lmtp_connect_timeout = 30s

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

This feature is available in Postfix 2.3 and later.

lmtp_connection_cache_on_demand (default: yes)
The LMTP-specific version of the smtp_connection_cache_on_demand configuration parameter.
See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_connection_cache_time_limit (default: 2s)
The LMTP-specific version of the smtp_connection_cache_time_limit configuration parameter.
See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_connection_reuse_count_limit (default: 0)
The LMTP-specific version of the smtp_connection_reuse_count_limit configuration parameter.
See there for details.

This feature is available in Postfix 2.11 and later.

lmtp_connection_reuse_time_limit (default: 300s)
The LMTP-specific version of the smtp_connection_reuse_time_limit configuration parameter.
See there for details.

This feature is available in Postfix 2.3 and later.

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

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.

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

lmtp_defer_if_no_mx_address_found (default: no)
The LMTP-specific version of the smtp_defer_if_no_mx_address_found configuration parameter.
See there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 3.0 and later.

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

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

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

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

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

Notes:

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

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

lmtp_dns_reply_filter (default: empty)
Optional filter for Postfix LMTP client DNS lookup results. See smtp_dns_reply_filter for
details including an example.

This feature is available in Postfix 3.0 and later.

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

This feature is available in Postfix 2.8 and later.

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

This feature is available in Postfix 2.11 and later.

lmtp_enforce_tls (default: no)
The LMTP-specific version of the smtp_enforce_tls configuration parameter. See there for de‐
tails.

This feature is available in Postfix 2.3 and later.

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

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

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

This feature is available in Postfix 3.1 and later.

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

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.5 and later.

lmtp_host_lookup (default: dns)
The LMTP-specific version of the smtp_host_lookup configuration parameter. See there for de‐
tails.

This feature is available in Postfix 2.3 and later.

lmtp_lhlo_name (default: $myhostname)
The hostname to send in the LMTP LHLO command.

The default value is the machine hostname. Specify a hostname or [ip.add.re.ss] or
[ip:v6:add:re::ss].

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

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

This feature is available in Postfix 2.3 and later.

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

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

lmtp_line_length_limit (default: 998)
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.

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_min_data_rate (default: 500)
The LMTP-specific version of the smtp_min_data_rate configuration parameter. See there for
details.

This feature is available in Postfix 3.7 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_per_request_deadline (default: no)
The LMTP-specific version of the smtp_per_request_deadline configuration parameter. See there
for details.

This feature is available in Postfix 3.7 and later.

lmtp_pix_workaround_delay_time (default: 10s)
The LMTP-specific version of the smtp_pix_workaround_delay_time configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.4 and later.

lmtp_pix_workaround_threshold_time (default: 500s)
The LMTP-specific version of the smtp_pix_workaround_threshold_time configuration parameter.
See there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.4 and later.

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

lmtp_quote_rfc821_envelope (default: yes)
The LMTP-specific version of the smtp_quote_rfc821_envelope configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

lmtp_randomize_addresses (default: yes)
The LMTP-specific version of the smtp_randomize_addresses configuration parameter. See there
for details.

This feature is available in Postfix 2.3 and later.

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

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

This feature is available in Postfix 2.7 and later.

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

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

This feature is available in Postfix 2.5 and later.

lmtp_sasl_auth_cache_time (default: 90d)
The LMTP-specific version of the smtp_sasl_auth_cache_time configuration parameter. See
there for details.

This feature is available in Postfix 2.5 and later.

lmtp_sasl_auth_enable (default: no)
Enable SASL authentication in the Postfix LMTP client.

lmtp_sasl_auth_soft_bounce (default: yes)
The LMTP-specific version of the smtp_sasl_auth_soft_bounce configuration parameter. See
there for details.

This feature is available in Postfix 2.5 and later.

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

This feature is available in Postfix 2.3 and later.

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

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

This feature is available in Postfix 2.3 and later.

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

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

noplaintext
Disallow authentication methods that use plaintext passwords.

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

nodictionary
Disallow authentication methods that are vulnerable to passive dictionary attacks.

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 para‐
meter. See there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

lmtp_send_dummy_mail_auth (default: no)
The LMTP-specific version of the smtp_send_dummy_mail_auth configuration parameter. See
there for details.

This feature is available in Postfix 2.9 and later.

lmtp_send_xforward_command (default: no)
Send an XFORWARD command to the remote LMTP server when the LMTP LHLO server response an‐
nounces XFORWARD support. This allows an lmtp(8) 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 LMTP server. Before you change the value to yes,
it is best to make sure that your content filter supports this command.

This feature is available in Postfix 2.1 and later.

lmtp_sender_dependent_authentication (default: no)
The LMTP-specific version of the smtp_sender_dependent_authentication configuration parame‐
ter. See there for details.

This feature is available in Postfix 2.3 and later.

lmtp_skip_5xx_greeting (default: yes)
The LMTP-specific version of the smtp_skip_5xx_greeting configuration parameter. See there
for details.

This feature is available in Postfix 2.3 and later.

lmtp_skip_quit_response (default: no)
Wait for the response to the LMTP QUIT command.

lmtp_starttls_timeout (default: 300s)
The LMTP-specific version of the smtp_starttls_timeout configuration parameter. See there
for details.

This feature is available in Postfix 2.3 and later.

lmtp_tcp_port (default: 24)
The default TCP port that the Postfix LMTP client connects to. Specify a symbolic name (see
services(5)) or a numeric port.

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

This feature is available in Postfix 2.3 and later.

lmtp_tls_CApath (default: empty)
The LMTP-specific version of the smtp_tls_CApath configuration parameter. See there for de‐
tails.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.7 and later.

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

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 3.4 and later.

lmtp_tls_ciphers (default: medium)
The LMTP-specific version of the smtp_tls_ciphers configuration parameter. See there for de‐
tails.

This feature is available in Postfix 2.6 and later.

lmtp_tls_connection_reuse (default: no)
The LMTP-specific version of the smtp_tls_connection_reuse configuration parameter. See there
for details.

This feature is available in Postfix 3.4 and later.

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

This feature is available in Postfix 2.3 and later.

lmtp_tls_dkey_file (default: $lmtp_tls_dcert_file)
The LMTP-specific version of the smtp_tls_dkey_file configuration parameter. See there for
details.

This feature is available in Postfix 2.3 and later.

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

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

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

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

lmtp_tls_enforce_peername (default: yes)
The LMTP-specific version of the smtp_tls_enforce_peername configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.5 and later.

lmtp_tls_fingerprint_digest (default: see postconf -d output)
The LMTP-specific version of the smtp_tls_fingerprint_digest configuration parameter. See
there for details.

This feature is available in Postfix 2.5 and later.

lmtp_tls_force_insecure_host_tlsa_lookup (default: no)
The LMTP-specific version of the smtp_tls_force_insecure_host_tlsa_lookup configuration para‐
meter. See there for details.

This feature is available in Postfix 2.11 and later.

lmtp_tls_key_file (default: $lmtp_tls_cert_file)
The LMTP-specific version of the smtp_tls_key_file configuration parameter. See there for
details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_loglevel (default: 0)
The LMTP-specific version of the smtp_tls_loglevel configuration parameter. See there for
details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_ciphers (default: medium)
The LMTP-specific version of the smtp_tls_mandatory_ciphers configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_protocols (default: see postconf -d output)
The LMTP-specific version of the smtp_tls_mandatory_protocols configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_note_starttls_offer (default: no)
The LMTP-specific version of the smtp_tls_note_starttls_offer configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

lmtp_tls_protocols (default: see postconf -d output)
The LMTP-specific version of the smtp_tls_protocols configuration parameter. See there for
details.

This feature is available in Postfix 2.6 and later.

lmtp_tls_scert_verifydepth (default: 9)
The LMTP-specific version of the smtp_tls_scert_verifydepth configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_secure_cert_match (default: nexthop)
The LMTP-specific version of the smtp_tls_secure_cert_match configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 3.4 and later.

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

This feature is available in Postfix 2.3 and later.

lmtp_tls_session_cache_timeout (default: 3600s)
The LMTP-specific version of the smtp_tls_session_cache_timeout configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

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

This feature is available in Postfix 2.11 and later.

lmtp_tls_verify_cert_match (default: hostname)
The LMTP-specific version of the smtp_tls_verify_cert_match configuration parameter. See
there for details.

This feature is available in Postfix 2.3 and later.

lmtp_tls_wrappermode (default: no)
The LMTP-specific version of the smtp_tls_wrappermode configuration parameter. See there for
details.

This feature is available in Postfix 3.0 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.

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 commands. By default, non-Post‐
fix commands are executed directly; commands are given to the default shell (typically,
/bin/sh) only when they contain shell meta characters or shell built-in commands.

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

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

Example:

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

local_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the local(8) delivery agent to change the status code or explanatory text
of successful or unsuccessful deliveries. See default_delivery_status_filter for details.

This feature is available in Postfix 3.0 and later.

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

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

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

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

local_header_rewrite_clients (default: permit_inet_interfaces)
Rewrite or add message headers in mail from these clients, updating incomplete addresses with
the domain name in $myorigin or $mydomain, and adding missing headers.

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

See remote_header_rewrite_domain to optionally rewrite or add message headers in mail from
other clients.

Specify a list of zero or more of the following:

permit_inet_interfaces
Append the domain name in $myorigin or $mydomain when the client IP address matches
$inet_interfaces. This is enabled by default.

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

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

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

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

check_address_map type:table

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

Examples:

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

local_header_rewrite_clients = static:all

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

local_header_rewrite_clients = permit_inet_interfaces

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

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

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

local_login_sender_maps (default: static:*)
A list of lookup tables that are searched by the UNIX login name, and that return a list of
allowed envelope sender patterns separated by space or comma. These sender patterns are en‐
forced by the Postfix postdrop(1) 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 nu‐
merical UID and use that instead.

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

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

* This pattern allows any envelope sender address.

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

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

Examples:

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

/etc/postfix/login_senders:
# Allow both the bare username and the user@domain forms.
/(.+)/ $1 $1 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 local
when its domain matches $mydestination, $inet_interfaces or $proxy_interfaces. Specify @do‐
main as a wild-card for domains that do not have a valid recipient list. Technically, tables
listed with $local_recipient_maps are used as lists: Postfix needs to know only if a lookup
string is found or not, but it does not use the result from table lookup.

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

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

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

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

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

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

• You use the "luser_relay", "mailbox_transport", or "fallback_transport" feature of the
Postfix local(8) 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, main‐
taining a copy of the system password file in the chroot jail is not practical.

Examples:

local_recipient_maps =

local_transport (default: local:$myhostname)
The default mail delivery transport and next-hop destination for final delivery to domains
listed with mydestination, and for [ipaddress] destinations that match $inet_interfaces or
$proxy_interfaces. This information can be overruled with the transport(5) 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.

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 unknown
recipients in domains that match $mydestination, $inet_interfaces or $proxy_interfaces is re‐
turned as undeliverable.

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

$domain
The recipient domain.

$extension
The recipient address extension.

$home The recipient's home directory.

$local The entire recipient address localpart.

$recipient
The full recipient address.

$shell The recipient's login shell.

$user The recipient username.

${name?value}

${name?{value}} (Postfix >= 3.0)
Expands to value when $name is non-empty.

${name:value}

${name:{value}} (Postfix >= 3.0)
Expands to value when $name is empty.

${name?{value1}:{value2}} (Postfix >= 3.0)
Expands to value1 when $name is non-empty, value2 otherwise.

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 spec‐
ify "local_recipient_maps =" (i.e. empty) in the main.cf file, otherwise the Postfix SMTP
server will reject mail for non-UNIX accounts with "User unknown in local recipient table".

Examples:

luser_relay = $user 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. Spec‐
ify the name of an unprivileged user account that does not share a user or group ID with
other accounts, and that owns no other files or processes on the system. In particular,
don't specify nobody or daemon. PLEASE USE A DEDICATED USER ID AND GROUP ID.

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

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

mail_spool_directory (default: see postconf -d output)
The directory where local(8) UNIX-style mailboxes are kept. The default setting depends on
the system type. Specify a name ending in / for maildir-style delivery.

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

Examples:

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

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

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

The following environment variables are exported to the command:

CLIENT_ADDRESS
Remote client network address. Available in Postfix version 2.2 and later.

CLIENT_HELO
Remote client EHLO command parameter. Available in Postfix version 2.2 and later.

CLIENT_HOSTNAME
Remote client hostname. Available in Postfix version 2.2 and later.

CLIENT_PROTOCOL
Remote client protocol. Available in Postfix version 2.2 and later.

DOMAIN The domain part of the recipient address.

EXTENSION
The optional address extension.

HOME The recipient home directory.

LOCAL The recipient address localpart.

LOGNAME
The recipient's username.

ORIGINAL_RECIPIENT
The entire recipient address, before any address rewriting or aliasing.

RECIPIENT
The full recipient address.

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

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

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

SENDER The full sender address.

SHELL The recipient's login shell.

USER The recipient username.

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

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

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

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 de‐
livery. Behavior is as with mailbox_command.

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 avail‐
able file locking methods, use the "postconf -l" command.

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

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

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

mailbox_size_limit (default: 51200000)
The maximal size of any local(8) individual mailbox or maildir file, or zero (no limit). In
fact, this limits the size of any file that is written to upon local delivery, including
files written by external commands that are executed by the local(8) delivery agent. The
value cannot exceed LONG_MAX (typically, a 32-bit or 64-bit signed integer).

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 mailbox
delivery to all local recipients, whether or not they are found in the UNIX passwd database.

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.

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

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

This feature is available in Postfix 2.3 and later.

maillog_file (default: empty)
The name of an optional logfile that is written by the Postfix postlogd(8) 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, separated by
comma or whitespace.

This feature is available in Postfix 3.4 and later.

maillog_file_rotate_suffix (default: %Y%m%d-%H%M%S)
The format of the suffix to append to $maillog_file while rotating the file with "postfix
logrotate". See strftime(3) 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_recipient

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

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

masquerade_domains = foo.example.com example.com

strips "user 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 message
header address rewriting is enabled:

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

• The message is received from a network client that matches $lo‐
cal_header_rewrite_clients,

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

To get the behavior before Postfix version 2.2, specify "local_header_rewrite_clients = sta‐
tic: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.

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 exclude 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 sup‐
port wildcards such as "*" or "all". This is intentional.

Examples:

# With Postfix 2.6..2.10 use '.' instead of '/'.
# Turn on all master(8) listener ports (the default).
master_service_disable =
# Turn off only the main SMTP listener port.
master_service_disable = smtp/inet
# Turn off all TCP/IP listener ports.
master_service_disable = inet
# Turn off all TCP/IP listener ports except "foo".
master_service_disable = !foo/inet, inet

This feature is available in Postfix 2.6 and later.

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

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

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

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

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.

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 compatible
with Postfix < 3.0.

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

This feature is available in Postfix 3.0 and later.

message_reject_characters (default: empty)
The set of characters that Postfix will reject in message content. The usual C-like escape
sequences are recognized: \a \b \f \n \r \t \v \ddd (up to three octal digits) and \\.

Note 1: this feature does not recognize text that requires MIME decoding. It inspects raw
message content, just like header_checks and body_checks.

Note 2: this feature is disabled with "receive_override_options = no_header_body_checks".

Example:

message_reject_characters = \0

This feature is available in Postfix 2.3 and later.

message_size_limit (default: 10240000)
The maximal size in bytes of a message, including envelope information. The value cannot ex‐
ceed LONG_MAX (typically, a 32-bit or 64-bit signed integer).

Note: be careful when making changes. Excessively small values will result in the loss of
non-delivery notifications, when a bounce message size exceeds the local or remote MTA's mes‐
sage size limit.

message_strip_characters (default: empty)
The set of characters that Postfix will remove from message content. The usual C-like escape
sequences are recognized: \a \b \f \n \r \t \v \ddd (up to three octal digits) and \\.

Note 1: this feature does not recognize text that requires MIME decoding. It inspects raw
message content, just like header_checks and body_checks.

Note 2: this feature is disabled with "receive_override_options = no_header_body_checks".

Example:

message_strip_characters = \0

This feature is available in Postfix 2.3 and later.

meta_directory (default: see 'postconf -d' output)
The location of non-executable files that are shared among multiple Postfix instances, such
as postfix-files, dynamicmaps.cf, and the multi-instance template files main.cf.proto and
master.cf.proto. This directory should contain only Postfix-related files. Typically, the
meta_directory parameter has the same default as the config_directory parameter (/etc/postfix
or /usr/local/etc/postfix).

For backwards compatibility with Postfix versions 2.6..2.11, specify "meta_directory = $dae‐
mon_directory" in main.cf before installing or upgrading Postfix, or specify "meta_directory
= /path/name" on the "make makefiles", "make install" or "make upgrade" command line.

This feature is available in Postfix 3.0 and later.

milter_command_timeout (default: 30s)
The time limit for sending an SMTP command to a Milter (mail filter) application, and for re‐
ceiving the response.

This feature is available in Postfix 2.3 and later.

milter_connect_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after completion of an SMTP
connection. See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_connect_timeout (default: 30s)
The time limit for connecting to a Milter (mail filter) application, and for negotiating pro‐
tocol options.

This feature is available in Postfix 2.3 and later.

milter_content_timeout (default: 300s)
The time limit for sending message content to a Milter (mail filter) application, and for re‐
ceiving the response.

This feature is available in Postfix 2.3 and later.

milter_data_macros (default: see postconf -d output)
The macros that are sent to version 4 or higher Milter (mail filter) applications after the
SMTP DATA command. See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_default_action (default: tempfail)
The default action when a Milter (mail filter) response is unavailable (for example, bad
Postfix configuration or Milter failure). Specify one of the following:

accept Proceed as if the mail filter was not present.

reject Reject all further commands in this session with a permanent status code.

tempfail
Reject all further commands in this session with a temporary status code.

quarantine
Like "accept", but freeze the message in the "hold" queue. Available with Postfix 2.6
and later.

This feature is available in Postfix 2.3 and later.

milter_end_of_data_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the message end-of-data.
See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_end_of_header_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the end of the message
header. See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.5 and later.

milter_header_checks (default: empty)
Optional lookup tables for content inspection of message headers that are produced by Milter
applications. See the header_checks(5) manual page available actions. Currently, PREPEND is
not implemented.

The following example sends all mail that is marked as SPAM to a spam handling machine. Note
that matches are case-insensitive by default.

/etc/postfix/main.cf:
milter_header_checks = pcre:/etc/postfix/milter_header_checks

/etc/postfix/milter_header_checks:
/^X-SPAM-FLAG:\s+YES/ FILTER mysmtp:sanitizer.example.com:25

The milter_header_checks mechanism could also be used for allowlisting. For example it could
be used to skip heavy content inspection for DKIM-signed mail from known friendly domains.

This feature is available in Postfix 2.7, and as an optional patch for Postfix 2.6.

milter_helo_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the SMTP HELO or EHLO
command. See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_macro_daemon_name (default: $myhostname)
The {daemon_name} macro value for Milter (mail filter) applications. See MILTER_README for a
list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_macro_defaults (default: empty)
Optional list of name=value pairs that specify default values for arbitrary macros that Post‐
fix may send to Milter applications. These defaults are used when there is no corresponding
information from the message delivery context.

Specify name=value or {name=value} pairs separated by comma or whitespace. Enclose a pair in
"{}" when a value contains comma or whitespace (this form ignores whitespace after the en‐
closing "{", around the "=", and before the enclosing "}").

This feature is available in Postfix 3.1 and later.

milter_macro_v (default: $mail_name $mail_version)
The {v} macro value for Milter (mail filter) applications. See MILTER_README for a list of
available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_mail_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the SMTP MAIL FROM com‐
mand. See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_protocol (default: 6)
The mail filter protocol version and optional protocol extensions for communication with a
Milter application; prior to Postfix 2.6 the default protocol is 2. Postfix sends this ver‐
sion number during the initial protocol handshake. It should match the version number that
is expected by the mail filter application (or by its Milter library).

Protocol versions:

2 Use Sendmail 8 mail filter protocol version 2 (default with Sendmail version 8.11 ..
8.13 and Postfix version 2.3 .. 2.5).

3 Use Sendmail 8 mail filter protocol version 3.

4 Use Sendmail 8 mail filter protocol version 4.

6 Use Sendmail 8 mail filter protocol version 6 (default with Sendmail version 8.14 and
Postfix version 2.6).

Protocol extensions:

no_header_reply
Specify this when the Milter application will not reply for each individual message
header.

This feature is available in Postfix 2.3 and later.

milter_rcpt_macros (default: see postconf -d output)
The macros that are sent to Milter (mail filter) applications after the SMTP RCPT TO command.
See MILTER_README for a list of available macro names and their meanings.

This feature is available in Postfix 2.3 and later.

milter_unknown_command_macros (default: see postconf -d output)
The macros that are sent to version 3 or higher Milter (mail filter) applications after an
unknown SMTP command. See MILTER_README for a list of available macro names and their mean‐
ings.

This feature is available in Postfix 2.3 and later.

mime_boundary_length_limit (default: 2048)
The maximal length of MIME multipart boundary strings. The MIME processor is unable to dis‐
tinguish between boundary strings that do not differ in the first $mime_boundary_length_limit
characters.

This feature is available in Postfix 2.0 and later.

mime_header_checks (default: $header_checks)
Optional lookup tables for content inspection of MIME related message headers, as described
in the header_checks(5) manual page.

This feature is available in Postfix 2.0 and later.

mime_nesting_limit (default: 100)
The maximal recursion level that the MIME processor will handle. Postfix refuses mail that
is nested deeper than the specified limit.

This feature is available in Postfix 2.0 and later.

minimal_backoff_time (default: 300s)
The minimal time between attempts to deliver a deferred message; prior to Postfix 2.4 the de‐
fault value was 1000s.

This parameter also limits the time an unreachable destination is kept in the short-term,
in-memory, destination status cache.

This parameter should be set greater than or equal to $queue_run_delay. See also $maxi‐
mal_backoff_time.

multi_instance_directories (default: empty)
An optional list of non-default Postfix configuration directories; these directories belong
to additional Postfix instances that share the Postfix executable files and documentation
with the default Postfix instance, and that are started, stopped, etc., together with the de‐
fault Postfix instance. Specify a list of pathnames separated by comma or whitespace.

When $multi_instance_directories is empty, the postfix(1) 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_in‐
stance_wrapper parameter. The multi-instance manager in turn executes postfix(1) commands for
the default instance and for all Postfix instances in $multi_instance_directories.

Currently, this parameter setting is ignored except for the default main.cf file.

This feature is available in Postfix 2.6 and later.

multi_instance_enable (default: no)
Allow this Postfix instance to be started, stopped, etc., by a multi-instance manager. By
default, new instances are created in a safe state that prevents them from being started in‐
advertently. This parameter is reserved for the multi-instance manager.

This feature is available in Postfix 2.6 and later.

multi_instance_group (default: empty)
The optional instance group name of this Postfix instance. A group identifies closely-related
Postfix instances that the multi-instance manager can start, stop, etc., as a unit. This pa‐
rameter is reserved for the multi-instance manager.

This feature is available in Postfix 2.6 and later.

multi_instance_name (default: empty)
The optional instance name of this Postfix instance. This name becomes also the default value
for the syslog_name parameter.

This feature is available in Postfix 2.6 and later.

multi_instance_wrapper (default: empty)
The pathname of a multi-instance manager command that the postfix(1) command invokes when the
multi_instance_directories parameter value is non-empty. The pathname may be followed by ini‐
tial command arguments separated by whitespace; shell metacharacters such as quotes are not
supported in this context.

The postfix(1) command invokes the manager command with the postfix(1) non-option command ar‐
guments on the manager command line, and with all installation configuration parameters ex‐
ported into the manager command process environment. The manager command in turn invokes the
postfix(1) 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 $local_re‐
cipient_maps and rejects non-existent recipients. See also the local domain class in the AD‐
DRESS_CLASS_README file.

The default mydestination value specifies names for the local machine only. On a mail domain
gateway, you should also include $mydomain.

The $local_transport delivery method is also selected for mail addressed to
user@[the.net.work.address] of the mail system (the IP addresses specified with the inet_in‐
terfaces and proxy_interfaces parameters).

Warnings:

• Do not specify the names of virtual domains - those domains are specified elsewhere.
See VIRTUAL_README for more information.

• Do not specify the names of domains that this machine is backup MX host for. See STAN‐
DARD_CONFIGURATION_README for how to set up backup MX hosts.

• By default, the Postfix SMTP server rejects mail for recipients not listed with the
local_recipient_maps parameter. See the postconf(5) manual for a description of the
local_recipient_maps and unknown_local_recipient_reject_code parameters.

Examples:

mydestination = $myhostname, localhost.$mydomain $mydomain
mydestination = $myhostname, localhost.$mydomain www.$mydomain, ftp.$mydomain

mydomain (default: see postconf -d output)
The internet domain name of this mail system. The default is to use $myhostname minus the
first component, or "localdomain" (Postfix 2.3 and later). $mydomain is used as a default
value for many other configuration parameters.

Example:

mydomain = domain.tld

myhostname (default: see postconf -d output)
The internet hostname of this mail system. The default is to use the fully-qualified domain
name (FQDN) from gethostname(), or to use the non-FQDN result from gethostname() and append
".$mydomain". $myhostname is used as a default value for many other configuration parame‐
ters.

Example:

myhostname = host.example.com

mynetworks (default: see postconf -d output)
The list of "trusted" remote SMTP clients that have more privileges than "strangers".

In particular, "trusted" SMTP clients are allowed to relay mail through Postfix. See the
smtpd_relay_restrictions parameter description in the postconf(5) manual.

You can specify the list of "trusted" network addresses by hand or you can let Postfix do it
for you (which is the default). See the description of the mynetworks_style parameter for
more information.

If you specify the mynetworks list by hand, Postfix ignores the mynetworks_style setting.

Specify a list of network addresses or network/netmask patterns, separated by commas and/or
whitespace. Continue long lines by starting the next line with whitespace.

The netmask specifies the number of bits in the network part of a host address. You can also
specify "/file/name" or "type:table" patterns. A "/file/name" pattern is replaced by its
contents; a "type:table" lookup table is matched when a table entry matches a lookup string
(the lookup result is ignored).

The list is matched left to right, and the search stops on the first match. Specify "!pat‐
tern" to exclude an address or network block from the list. The form "!/file/name" is sup‐
ported only in Postfix version 2.4 and later.

Note 1: Pattern matching of domain names is controlled by the presence or absence of "mynet‐
works" 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.

Note 3: CIDR ranges cannot be specified in hash tables. Use cidr tables if CIDR ranges are
used.

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 = cidr:/etc/postfix/network_table.cidr

mynetworks_style (default: Postfix >= 3.0: host, Postfix < 3.0: subnet)
The method to generate the default value for the mynetworks parameter. This is the list of
trusted networks for relay access control etc.

• Specify "mynetworks_style = host" when Postfix should "trust" only the local machine.

• Specify "mynetworks_style = subnet" when Postfix should "trust" remote SMTP clients in
the same IP subnetworks as the local machine. On Linux, this works correctly only
with interfaces specified with the "ifconfig" or "ip" command.

• Specify "mynetworks_style = class" when Postfix should "trust" remote SMTP clients in
the same IP class A/B/C networks as the local machine. Caution: this may cause Post‐
fix to "trust" your entire provider's network. Instead, specify an explicit mynet‐
works list by hand, as described with the mynetworks configuration parameter.

myorigin (default: $myhostname)
The domain name that locally-posted mail appears to come from, and that locally posted mail
is delivered to. The default, $myhostname, is adequate for small sites. If you run a domain
with multiple machines, you should (1) change this to $mydomain and (2) set up a domain-wide
alias database that aliases each user to user 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 restriction.

non_smtpd_milters (default: empty)
A list of Milter (mail filter) applications for new mail that does not arrive via the Postfix
smtpd(8) 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 a separator. See the MILTER_README document
for details.

This feature is available in Postfix 2.3 and later.

notify_classes (default: resource, software)
The list of error classes that are reported to the postmaster. These postmaster notifications
do not replace user notifications. 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 (bro‐
ken mail software) reports.

NOTE: postmaster notifications may contain confidential information such as SASL passwords or
message content. It is the system administrator's responsibility to treat such information
with care.

The error classes are:

bounce (also implies 2bounce)
Send the postmaster copies of the headers of bounced mail, and send transcripts of
SMTP sessions when Postfix rejects mail. The notification is sent to the address spec‐
ified with the bounce_notice_recipient configuration parameter (default: postmaster).

2bounce
Send undeliverable bounced mail to the postmaster. The notification is sent to the ad‐
dress specified with the 2bounce_notice_recipient configuration parameter (default:
postmaster).

data Send the postmaster a transcript of the SMTP session with an error because a critical
data file was unavailable. The notification is sent to the address specified with the
error_notice_recipient configuration parameter (default: postmaster).
This feature is available in Postfix 2.9 and later.

delay Send the postmaster copies of the headers of delayed mail (see delay_warning_time).
The notification is sent to the address specified with the delay_notice_recipient con‐
figuration parameter (default: postmaster).

policy Send the postmaster a transcript of the SMTP session when a client request was re‐
jected because of (UCE) policy. The notification is sent to the address specified with
the error_notice_recipient configuration parameter (default: postmaster).

protocol
Send the postmaster a transcript of the SMTP session in case of client or server pro‐
tocol errors. The notification is sent to the address specified with the error_no‐
tice_recipient configuration parameter (default: postmaster).

resource
Inform the postmaster of mail not delivered due to resource problems. The notifica‐
tion is sent to the address specified with the error_notice_recipient configuration
parameter (default: postmaster).

software
Inform the postmaster of mail not delivered due to software problems. The notifica‐
tion is sent to the address specified with the error_notice_recipient configuration
parameter (default: postmaster).

Examples:

notify_classes = bounce, delay, policy, protocol, resource, software
notify_classes = 2bounce, resource, software

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 certificates,
and to compute public key digests for DANE TLSA records. In multi-instance environments,
this parameter is always determined from the configuration of the default Postfix instance.

Example:

/etc/postfix/main.cf:
# NetBSD pkgsrc:
openssl_path = /usr/pkg/bin/openssl
# Local build:
openssl_path = /usr/local/bin/openssl

This feature is available in Postfix 3.1 and later.

owner_request_special (default: yes)
Enable special treatment for owner-listname entries in the aliases(5) file, and don't split
owner-listname and listname-request address localparts when the recipient_delimiter is set to
"-". This feature is useful for mailing lists.

parent_domain_matches_subdomains (default: see postconf -d output)
A list of Postfix features where the pattern "example.com" also matches subdomains of exam‐
ple.com, instead of requiring an explicit ".example.com" pattern. This is planned backwards
compatibility: eventually, all Postfix features are expected to require explicit ".exam‐
ple.com" style patterns when you really want to match subdomains.

The following Postfix feature names are supported.

Postfix version 1.0 and later
debug_peer_list, fast_flush_domains, mynetworks, permit_mx_backup_networks, relay_do‐
mains, transport_maps

Postfix version 1.1 and later
qmqpd_authorized_clients, smtpd_access_maps,

Postfix version 2.8 and later
postscreen_access_list

Postfix version 3.0 and later
smtpd_client_event_limit_exceptions

permit_mx_backup_networks (default: empty)
Restrict the use of the permit_mx_backup SMTP access feature to only domains whose primary MX
hosts match the listed networks. The parameter value syntax is the same as with the mynet‐
works parameter; note, however, that the default value is empty.

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

pickup_service_name (default: pickup)
The name of the pickup(8) 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 explana‐
tory text of successful or unsuccessful deliveries. See default_delivery_status_filter for
details.

This feature is available in Postfix 3.0 and later.

plaintext_reject_code (default: 450)
The numerical Postfix SMTP server response code when a request is rejected by the re‐
ject_plaintext_session restriction.

This feature is available in Postfix 2.3 and later.

postlog_service_name (default: postlog)
The name of the postlogd(8) 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.

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" commands,
that operate on running instances. For these commands, disabled instances are skipped.

This feature is available in Postfix 2.6 and later.

postmulti_start_commands (default: start)
The postfix(1) 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 re‐
verse order.

This feature is available in Postfix 2.6 and later.

postscreen_access_list (default: permit_mynetworks)
Permanent allow/denylist for remote SMTP client IP addresses. postscreen(8) searches this
list immediately after a remote SMTP client connects. Specify a comma- or whitespace-sepa‐
rated list of commands (in upper or lower case) or lookup tables. The search stops upon the
first command that fires for the client IP address.

permit_mynetworks
Allowlist the client and terminate the search if the client IP address matches $mynet‐
works. Do not subject the client to any before/after 220 greeting tests. Pass the
connection immediately to a Postfix SMTP server process.
Pattern matching of domain names is controlled by the presence or absence of "post‐
screen_access_list" in the parent_domain_matches_subdomains parameter value.

type:table
Query the specified lookup table. Each table lookup result is an access list, except
that access lists inside a table cannot specify type:table entries.
To discourage the use of hash, btree, etc. tables, there is no support for substring
matching like smtpd(8). Use CIDR tables instead.

permit
Allowlist the client and terminate the search. Do not subject the client to any be‐
fore/after 220 greeting tests. Pass the connection immediately to a Postfix SMTP
server process.

reject
Denylist the client and terminate the search. Subject the client to the action config‐
ured with the postscreen_denylist_action configuration parameter.

dunno All postscreen(8) access lists implicitly have this command at the end.
When dunno is executed inside a lookup table, return from the lookup table and evalu‐
ate the next command.
When dunno is executed outside a lookup table, terminate the search, and subject the
client to the configured before/after 220 greeting tests.

Example:

/etc/postfix/main.cf:
postscreen_access_list = permit_mynetworks,
cidr:/etc/postfix/postscreen_access.cidr
# Postfix < 3.6 use postscreen_blacklist_action.
postscreen_denylist_action = enforce

/etc/postfix/postscreen_access.cidr:
# Rules are evaluated in the order as specified.
# Denylist 192.168.* except 192.168.0.1.
192.168.0.1 dunno
192.168.0.0/16 reject

This feature is available in Postfix 2.8.

postscreen_allowlist_interfaces (default: static:all)
A list of local postscreen(8) server IP addresses where a non-allowlisted remote SMTP client
can obtain postscreen(8)'s temporary allowlist status. This status is required before the
client can talk to a Postfix SMTP server process. By default, a client can obtain post‐
screen(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 addresses
will never become allowlisted, and will never be allowed to talk to a Postfix SMTP server
process.

Specify a list of network addresses or network/netmask patterns, separated by commas and/or
whitespace. The netmask specifies the number of bits in the network part of a host address.
Continue long lines by starting the next line with whitespace.

You can also specify "/file/name" or "type:table" patterns. A "/file/name" pattern is re‐
placed by its contents; a "type:table" lookup table is matched when a table entry matches a
lookup string (the lookup result is ignored).

The list is matched left to right, and the search stops on the first match. Specify "!pat‐
tern" to exclude an address or network block from the list.

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

Example:

/etc/postfix/main.cf:
# Don't allowlist connections to the backup IP address.
# Postfix < 3.6 use postscreen_whitelist_interfaces.
postscreen_allowlist_interfaces = !168.100.189.8, static:all

This feature is available in Postfix 3.6 and later.

Available as postscreen_whitelist_interfaces in Postfix 2.9 - 3.5.

postscreen_bare_newline_action (default: ignore)
The action that postscreen(8) takes when a remote SMTP client sends a bare newline character,
that is, a newline not preceded by carriage return. Specify one of the following:

ignore Ignore the failure of this test. Allow other tests to complete. Do not repeat this
test before the result from some other test expires. This option is useful for test‐
ing and collecting statistics without blocking mail permanently.

enforce
Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
and log the helo/sender/recipient information. Repeat this test the next time the
client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time
the client connects.

This feature is available in Postfix 2.8.

postscreen_bare_newline_enable (default: no)
Enable "bare newline" SMTP protocol tests in the postscreen(8) server. These tests are expen‐
sive: a remote SMTP client must disconnect after it passes the test, before it can talk to a
real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_bare_newline_ttl (default: 30d)
The amount of time that postscreen(8) 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.

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 requires
that the cache database supports the "delete" and "sequence" operators. Specify a zero in‐
terval to disable cache cleanup.

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

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 "post‐
screen_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 before
it is removed. This prevents clients from being logged as "NEW" just because their cache en‐
try expired an hour ago. It also prevents the cache from filling up with clients that passed
some deep protocol test once and never came back.

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 post‐
screen(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 post‐
screen_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 com‐
mands.

This feature is available in Postfix 2.8.

postscreen_command_filter (default: $smtpd_command_filter)
A mechanism to transform commands from remote SMTP clients. See smtpd_command_filter for
further details.

This feature is available in Postfix 2.8 and later.

postscreen_command_time_limit (default: normal: 300s, overload: 10s)
The time limit to read an entire command line with postscreen(8)'s built-in SMTP protocol en‐
gine.

This feature is available in Postfix 2.8.

postscreen_denylist_action (default: ignore)
The action that postscreen(8) takes when a remote SMTP client is permanently denylisted with
the postscreen_access_list parameter. Specify one of the following:

ignore (default)
Ignore this result. Allow other tests to complete. Repeat this test the next time
the client connects. This option is useful for testing and collecting statistics
without blocking mail.

enforce
Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
and log the helo/sender/recipient information. Repeat this test the next time the
client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time
the client connects.

This feature is available in Postfix 3.6 and later.

Available as postscreen_blacklist_action in Postfix 2.8 - 3.5.

postscreen_disable_vrfy_command (default: $disable_vrfy_command)
Disable the SMTP VRFY command in the postscreen(8) daemon. See disable_vrfy_command for de‐
tails.

This feature is available in Postfix 2.8.

postscreen_discard_ehlo_keyword_address_maps (default: $smtpd_discard_ehlo_keyword_address_maps)
Lookup tables, indexed by the remote SMTP client address, with case insensitive lists of EHLO
keywords (pipelining, starttls, auth, etc.) that the postscreen(8) server will not send in
the EHLO response to a remote SMTP client. See smtpd_discard_ehlo_keywords for details. The
table is not searched by hostname for robustness reasons.

This feature is available in Postfix 2.8 and later.

postscreen_discard_ehlo_keywords (default: $smtpd_discard_ehlo_keywords)
A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that the post‐
screen(8) server will not send in the EHLO response to a remote SMTP client. See smtpd_dis‐
card_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 post‐
screen_dnsbl_threshold parameters). Specify one of the following:

ignore (default)
Ignore the failure of this test. Allow other tests to complete. Repeat this test the
next time the client connects. This option is useful for testing and collecting sta‐
tistics without blocking mail.

enforce
Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
and log the helo/sender/recipient information. Repeat this test the next time the
client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time
the client connects.

This feature is available in Postfix 2.8.

postscreen_dnsbl_allowlist_threshold (default: 0)
Allow a remote SMTP client to skip "before" and "after 220 greeting" protocol tests, based on
its combined DNSBL score as defined with the postscreen_dnsbl_sites parameter.

Specify a negative value to enable this feature. When a client passes the post‐
screen_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 updated if it was less than post‐
screen_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.

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.

This feature is available in Postfix 3.1.

postscreen_dnsbl_reply_map (default: empty)
A mapping from an 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:, reg‐
exp: 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 patterns with DNS allow/denylist domains, filters and weight factors. When
the list is non-empty, the dnsblog(8) daemon will query these domains with the reversed IP
addresses of remote SMTP clients, and postscreen(8) will update an SMTP client's DNSBL score
with each non-error reply as described below.

Caution: when postscreen rejects mail, its SMTP response contains 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 post‐
screen_dnsbl_threshold, postscreen(8) can drop the connection with the remote SMTP client.

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

• When a pattern specifies no "=filter", postscreen(8) will use any non-error DNSBL
query result. Otherwise, postscreen(8) will use only DNSBL query results that match
the filter. The filter has the form d.d.d.d, where each d is a number, or a pattern
inside [] that contains one or more ";"-separated numbers or number..number ranges.

• When a pattern specifies no "*weight", the weight of the pattern is 1. Otherwise, the
weight must be an integral number. Specify a negative number for allowlisting.

• When a pattern matches one or more DNSBL query results, postscreen(8) adds that pat‐
tern's weight once to the remote SMTP client's DNSBL score.

Examples:

To use example.com as a high-confidence blocklist, and to block mail with example.net and ex‐
ample.org only when both agree:

postscreen_dnsbl_threshold = 2
postscreen_dnsbl_sites = example.com*2, example.net, example.org

To filter only DNSBL replies containing 127.0.0.4:

postscreen_dnsbl_sites = example.com=127.0.0.4

This feature is available in Postfix 2.8.

postscreen_dnsbl_threshold (default: 1)
The inclusive lower bound for blocking a remote SMTP client, based on its combined DNSBL
score as defined with the postscreen_dnsbl_sites parameter.

This feature is available in Postfix 2.8.

postscreen_dnsbl_timeout (default: 10s)
The time limit for DNSBL or DNSWL lookups. This is separate from the timeouts in the dns‐
blog(8) 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 reputa‐
tion test before a client IP address is required to pass that test again.

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 protocol.
See smtpd_forbidden_commands for syntax, and postscreen_non_smtp_command_action for possible
actions.

This feature is available in Postfix 2.8.

postscreen_greet_action (default: ignore)
The action that postscreen(8) 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:

enforce
Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
and log the helo/sender/recipient information. Repeat this test the next time the
client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time
the client connects.

In either case, postscreen(8) 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 disable 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 relatively
short, because a good client can immediately talk to a real Postfix SMTP server.

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 before
its turn, and for DNS blocklist lookup results to arrive (default: up to 2 seconds under
stress, up to 6 seconds otherwise).

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:

enforce
Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
and log the helo/sender/recipient information. Repeat this test the next time the
client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time
the client connects. This action is the same as with the Postfix SMTP server's
smtpd_forbidden_commands feature.

This feature is available in Postfix 2.8.

postscreen_non_smtp_command_enable (default: no)
Enable "non-SMTP command" tests in the postscreen(8) 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, before it can
talk to a real Postfix SMTP server.

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 instead
of sending one command and waiting for the server to respond. Specify one of the following:

enforce
Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply,
and log the helo/sender/recipient information. Repeat this test the next time the
client connects.

drop Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time
the client connects.

This feature is available in Postfix 2.8.

postscreen_pipelining_enable (default: no)
Enable "pipelining" SMTP protocol tests in the postscreen(8) server. These tests are expen‐
sive: a good client must disconnect after it passes the test, before it can talk to a real
Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_pipelining_ttl (default: 30d)
The amount of time that postscreen(8) will use the result from a successful "pipelining" SMTP
protocol test. During this time, the client IP address is excluded from this test. The de‐
fault is long because a good client must disconnect after it passes the test, before it can
talk to a real Postfix SMTP server.

This feature is available in Postfix 2.8.

postscreen_post_queue_limit (default: $default_process_limit)
The number of clients that can be waiting for service from a real Postfix SMTP server
process. When this queue is full, all clients will receive a 421 response.

This feature is available in Postfix 2.8.

postscreen_pre_queue_limit (default: $default_process_limit)
The number of non-allowlisted clients that can be waiting for a decision whether they will
receive service from a real Postfix SMTP server process. When this queue is full, all non-al‐
lowlisted clients will receive a 421 response.

This feature is available in Postfix 2.8.

postscreen_reject_footer (default: $smtpd_reject_footer)
Optional information that is appended after a 4XX or 5XX postscreen(8) 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_enforce_tls.
See smtpd_tls_security_level for details.

This feature is available in Postfix 2.8 and later.

postscreen_upstream_proxy_protocol (default: empty)
The name of the proxy protocol used by an optional before-postscreen proxy agent. When a
proxy agent is used, this protocol conveys local and remote address and port information.
Specify "postscreen_upstream_proxy_protocol = haproxy" to enable the haproxy protocol; ver‐
sion 2 is supported with Postfix 3.5 and later.

This feature is available in Postfix 2.10 and later.

postscreen_upstream_proxy_timeout (default: 5s)
The time limit for the proxy protocol specified with the postscreen_upstream_proxy_protocol
parameter.

This feature is available in Postfix 2.10 and later.

postscreen_use_tls (default: $smtpd_use_tls)
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not require that
clients use TLS encryption.

This feature is available in Postfix 2.8 and later. Preferably, use postscreen_tls_secu‐
rity_level instead.

postscreen_watchdog_timeout (default: 10s)
How much time a postscreen(8) 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.

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 forwarding
mail and when delivering to file (mailbox) and command. Turning off the Delivered-To: header
when forwarding mail is not recommended.

Specify zero or more of forward, file, or command.

Example:

prepend_delivered_header = forward

process_id (read-only)
The process ID of a Postfix command or daemon process.

process_id_directory (default: pid)
The location of Postfix PID files relative to $queue_directory. This is a read-only parame‐
ter.

process_name (read-only)
The process name of a Postfix command or daemon process.

propagate_unmatched_extensions (default: canonical, virtual)
What address lookup tables copy an address extension from the lookup key to the lookup re‐
sult.

For example, with a virtual(5) mapping of "joe AT example.com => joe.user AT example.net", the ad‐
dress "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 lo‐
cal(8) .forward and :include: file lookups, and with smtp(8) generic maps, respectively.

Note: enabling this feature for types other than canonical and virtual is likely to cause
problems when mail is forwarded to other sites, especially with mail that is sent to a mail‐
ing list exploder address.

Examples:

propagate_unmatched_extensions = canonical, virtual, alias,
forward, include
propagate_unmatched_extensions = canonical, virtual

proxy_interfaces (default: empty)
The remote 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 service.

Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Table ref‐
erences that don't begin with proxy: are ignored.

This feature is available in Postfix 2.0 and later.

proxy_write_maps (default: see postconf -d output)
The lookup tables that the proxymap(8) server is allowed to access for the read-write ser‐
vice. Postfix-owned local database files should be stored under the Postfix-owned data_direc‐
tory. Table references that don't begin with proxy: are ignored.

This feature is available in Postfix 2.5 and later.

proxymap_service_name (default: proxymap)
The name of the proxymap read-only table lookup service. This service is normally imple‐
mented by the proxymap(8) daemon.

This feature is available in Postfix 2.6 and later.

proxywrite_service_name (default: proxywrite)
The name of the proxywrite read-write table lookup service. This service is normally imple‐
mented by the proxymap(8) daemon.

This feature is available in Postfix 2.6 and later.

qmgr_clog_warn_time (default: 300s)
The minimal delay between warnings that a specific destination is clogging up the Postfix ac‐
tive queue. Specify 0 to disable.

This feature is enabled with the helpful_warnings parameter.

This feature is available in Postfix 2.0 and later.

qmgr_daemon_timeout (default: 1000s)
How much time a Postfix queue manager process may take to handle a request before it is ter‐
minated by a built-in watchdog timer.

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 communi‐
cation channel. The purpose is to break out of deadlock situations. If the time limit is ex‐
ceeded the software either retries or aborts the operation.

This feature is available in Postfix 2.8 and later.

qmgr_message_active_limit (default: 20000)
The maximal number of messages in the active queue.

qmgr_message_recipient_limit (default: 20000)
The maximal number of recipients held in memory by the Postfix queue manager, and the maximal
size of the short-term, in-memory "dead" destination status cache.

qmgr_message_recipient_minimum (default: 10)
The minimal number of in-memory recipients for any message. This takes priority over any
other in-memory recipient limits (i.e., the global qmgr_message_recipient_limit and the per
transport _recipient_limit) if necessary. The minimum value allowed for this parameter is 1.

qmqpd_authorized_clients (default: empty)
What remote QMQP clients are allowed to connect to the Postfix QMQP server port.

By default, no client is allowed to use the service. This is because the QMQP server will re‐
lay mail to any destination.

Specify a list of client patterns. A list pattern specifies a host name, a domain name, an
internet address, or a network/mask pattern, where the mask specifies the number of bits in
the network part. When a pattern specifies a file name, its contents are substituted for the
file name; when a pattern is a "type:table" table specification, table lookup is used in‐
stead.

Patterns are separated by whitespace and/or commas. In order to reverse the result, precede a
pattern with an exclamation point (!). The form "!/file/name" is supported only in Postfix
version 2.4 and later.

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

Example:

qmqpd_authorized_clients = !192.168.0.1, 192.168.0.0/24

qmqpd_client_port_logging (default: no)
Enable logging of the remote QMQP client port in addition to the hostname and IP address. The
logging format is "host[address]:port".

This feature is available in Postfix 2.5 and later.

qmqpd_error_delay (default: 1s)
How long the Postfix QMQP server will pause before sending a negative reply to the remote
QMQP client. The purpose is to slow down confused or malicious clients.

qmqpd_timeout (default: 300s)
The time limit for sending or receiving information over the network. If a read or write op‐
eration blocks for more than $qmqpd_timeout seconds the Postfix QMQP server gives up and dis‐
connects.

queue_directory (default: see postconf -d output)
The location of the Postfix top-level queue directory. This is the root directory of Postfix
daemon processes that run chrooted.

queue_file_attribute_count_limit (default: 100)
The maximal number of (name=value) attributes that may be stored in a Postfix queue file. The
limit is enforced by the cleanup(8) server.

This feature is available in Postfix 2.0 and later.

queue_minfree (default: 0)
The minimal amount of free space in bytes in the queue file system that is needed to receive
mail. This is currently used by the Postfix SMTP server to decide if it will accept any mail
at all.

By default, the Postfix SMTP server rejects MAIL FROM commands when the amount of free space
is less than 1.5*$message_size_limit (Postfix version 2.1 and later). To specify a higher
minimum free space limit, specify a queue_minfree value that is at least 1.5*$mes‐
sage_size_limit.

With Postfix versions 2.0 and earlier, a queue_minfree value of zero means there is no mini‐
mum required amount of free space.

queue_run_delay (default: 300s)
The time between deferred queue scans by the queue manager; prior to Postfix 2.4 the default
value was 1000s.

This parameter should be set less than or equal to $minimal_backoff_time. See also $maxi‐
mal_backoff_time.

queue_service_name (default: qmgr)
The name of the qmgr(8) service. This service manages the Postfix queue and schedules deliv‐
ery requests.

This feature is available in Postfix 2.0 and later.

rbl_reply_maps (default: empty)
Optional lookup tables with RBL response templates. The tables are indexed by the RBL domain
name. By default, Postfix uses the default template as specified with the default_rbl_reply
configuration parameter. See there for a discussion of the syntax of RBL reply templates.

This feature is available in Postfix 2.0 and later.

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

receive_override_options (default: empty)
Enable or disable recipient validation, built-in content filtering, or address mapping. Typi‐
cally, these are specified in master.cf as command-line arguments for the smtpd(8), 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 forwarded to
the cleanup server.

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

no_address_mappings
Disable canonical address mapping, virtual alias map expansion, address masquerading,
and automatic BCC (blind carbon-copy) recipients. This is typically specified BEFORE
an external content filter.

no_header_body_checks
Disable header/body_checks. This is typically specified AFTER an external content fil‐
ter.

no_milters
Disable Milter (mail filter) applications. This is typically specified AFTER an exter‐
nal content filter.

Note: when the "BEFORE content filter" receive_override_options setting is specified in the
main.cf file, specify the "AFTER content filter" receive_override_options setting in mas‐
ter.cf (and vice versa).

Examples:

receive_override_options =
no_unknown_recipient_checks, no_header_body_checks
receive_override_options = no_address_mappings

This feature is available in Postfix 2.1 and later.

recipient_bcc_maps (default: empty)
Optional BCC (blind carbon-copy) address lookup tables, indexed by envelope recipient ad‐
dress. The BCC address (multiple results are not supported) is added when mail enters from
outside of Postfix.

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

The table search order is as follows:

• Look up the "user+extension AT domain.tld" address including the optional address exten‐
sion.

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

• Look up the "user+extension" address local part when the recipient domain equals $myo‐
rigin, $mydestination, $inet_interfaces or $proxy_interfaces.

• Look up the "user" address local part when the recipient domain equals $myorigin, $my‐
destination, $inet_interfaces or $proxy_interfaces.

• Look up the "@domain.tld" part.

Note: with Postfix 2.2 and earlier the sender will unconditionally be notified when the BCC
address is undeliverable.

Example:

recipient_bcc_maps = hash:/etc/postfix/recipient_bcc

After a change, run "postmap /etc/postfix/recipient_bcc".

This feature is available in Postfix 2.1 and later.

recipient_canonical_classes (default: envelope_recipient, header_recipient)
What addresses are subject to recipient_canonical_maps address mapping. By default, recipi‐
ent_canonical_maps address mapping is applied to envelope recipient addresses, and to header
recipient addresses.

Specify one or more of: envelope_recipient, header_recipient

This feature is available in Postfix 2.2 and later.

recipient_canonical_maps (default: empty)
Optional address mapping lookup tables for envelope and header recipient addresses. The ta‐
ble format and lookups are documented in canonical(5).

Note: $recipient_canonical_maps is processed before $canonical_maps.

Example:

recipient_canonical_maps = hash:/etc/postfix/recipient_canonical

recipient_delimiter (default: empty)
The set of characters that can separate an email address localpart, user name, or a .forward
file name from its extension. For example, with "recipient_delimiter = +", the software
tries user+foo 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 exten‐
sion may then be used to generate an extended .forward file name. This implementation recog‐
nizes one delimiter character and one extension per email address localpart or email address.
With Postfix 2.10 and earlier, the recipient_delimiter specifies a single character.

See canonical(5), local(8), relocated(5) and virtual(5) for the effects of recipient_delim‐
iter on lookups in aliases, canonical, virtual, and relocated maps, and see the propagate_un‐
matched_extensions parameter for propagating an extension from one email address to another.

When used in command_execution_directory, forward_path, or luser_relay, ${recipient_delim‐
iter} is replaced with the actual recipient delimiter that was found in the recipient email
address (Postfix 2.11 and later), or it is replaced with the main.cf recipient_delimiter pa‐
rameter value (Postfix 2.10 and earlier).

The recipient_delimiter is not applied to the mailer-daemon address, the postmaster address,
or the double-bounce address. With the default "owner_request_special = yes" setting, the re‐
cipient_delimiter is also not applied to addresses with the special "owner-" prefix or the
special "-request" suffix.

Examples:

# Handle Postfix-style extensions.
recipient_delimiter = +

# Handle both Postfix and qmail extensions (Postfix 2.11 and later).
recipient_delimiter = +-

# Use .forward for mail without address extension, and for mail with
# an unrecognized address extension.
forward_path = $home/.forward${recipient_delimiter}${extension},
$home/.forward

reject_code (default: 554)
The numerical Postfix SMTP server response code when a remote SMTP client request is rejected
by the "reject" restriction.

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

reject_tempfail_action (default: defer_if_permit)
The Postfix SMTP server's action when a reject-type restriction fails due to a temporary er‐
ror condition. Specify "defer" to defer the remote SMTP client request immediately. With the
default "defer_if_permit" action, the Postfix SMTP server continues to look for opportunities
to reject mail, and defers the client request only if it would otherwise be accepted.

For finer control, see: unverified_recipient_tempfail_action, unverified_sender_tempfail_ac‐
tion, unknown_address_tempfail_action, and unknown_helo_hostname_tempfail_action.

This feature is available in Postfix 2.6 and later.

relay_clientcerts (default: empty)
List of tables with remote SMTP client-certificate fingerprints or public key fingerprints
(Postfix 2.9 and later) for which the Postfix SMTP server will allow access with the per‐
mit_tls_clientcerts feature. The fingerprint digest algorithm is configurable via the
smtpd_tls_fingerprint_digest parameter (hard-coded as md5 prior to Postfix version 2.5).

The default algorithm is sha256 with Postfix >= 3.6 and the compatibility_level set to 3.6 or
higher. With Postfix <= 3.5, the default algorithm is md5. The best-practice algorithm is
now sha256. Recent advances in hash function cryptanalysis have led to md5 and sha1 being
deprecated in favor of sha256. However, as long as there are no known "second pre-image" at‐
tacks against the older algorithms, their use in this context, though not recommended, is
still likely safe.

Postfix lookup tables are in the form of (key, value) pairs. Since we only need the key, the
value can be chosen freely, e.g. the name of the user or host:
D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80 lutzpc.at.home

Example:

relay_clientcerts = hash:/etc/postfix/relay_clientcerts

For more fine-grained control, use check_ccert_access to select an appropriate access(5) pol‐
icy for each client. See RESTRICTION_CLASS_README.

This feature is available with Postfix version 2.2.

relay_destination_concurrency_limit (default: $default_destination_concurrency_limit)
The maximal number of parallel deliveries to the same destination via the relay message de‐
livery transport. This limit is enforced by the queue manager. The message delivery transport
name is the first field in the entry in the master.cf file.

This feature is available in Postfix 2.0 and later.

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

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

This feature is available in Postfix 2.0 and later.

relay_domains (default: Postfix >= 3.0: empty, Postfix < 3.0: $mydestination)
What destination domains (and subdomains thereof) this system will relay mail to. For details
about how the relay_domains value is used, see the description of the permit_auth_destination
and reject_unauth_destination SMTP recipient restrictions.

Domains that match $relay_domains are delivered with the $relay_transport mail delivery
transport. The SMTP server validates recipient addresses with $relay_recipient_maps and re‐
jects non-existent recipients. See also the relay domains address class in the AD‐
DRESS_CLASS_README file.

Note: Postfix will not automatically forward mail for domains that list this system as their
primary or backup MX host. See the permit_mx_backup restriction in the postconf(5) manual
page.

Specify a list of host or domain names, "/file/name" patterns or "type:table" lookup tables,
separated by commas and/or whitespace. Continue long lines by starting the next line with
whitespace. A "/file/name" pattern is replaced by its contents; a "type:table" lookup table
is matched when a (parent) domain appears as lookup key. Specify "!pattern" to exclude a do‐
main from the list. The form "!/file/name" is supported only in Postfix version 2.4 and
later.

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

relay_domains_reject_code (default: 554)
The numerical Postfix SMTP server response code when a client request is rejected by the re‐
ject_unauth_destination recipient restriction.

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

relay_recipient_maps (default: empty)
Optional lookup tables with all valid addresses in the domains that match $relay_domains.
Specify @domain as a wild-card for domains that have no valid recipient list, and become a
source of backscatter mail: Postfix accepts spam for non-existent recipients and then floods
innocent people with undeliverable mail. Technically, tables listed with $relay_recipi‐
ent_maps are used as lists: Postfix needs to know only if a lookup string is found or not,
but it does not use the result from the table lookup.

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

If this parameter is non-empty, then the Postfix SMTP server will reject mail to unknown re‐
lay users. This feature is off by default.

See also the relay domains address class in the ADDRESS_CLASS_README file.

Example:

relay_recipient_maps = hash:/etc/postfix/relay_recipients

This feature is available in Postfix 2.0 and later.

relay_transport (default: relay)
The default mail delivery transport and next-hop destination for remote delivery to domains
listed with $relay_domains. In order of decreasing precedence, the nexthop destination is
taken from $relay_transport, $sender_dependent_relayhost_maps, $relayhost, or from the recip‐
ient domain. This information can be overruled with the transport(5) table.

See also the relay domains address class in the ADDRESS_CLASS_README file.

This feature is available in Postfix 2.0 and later.

relayhost (default: empty)
The next-hop destination(s) for non-local mail; overrides non-local domains in recipient ad‐
dresses. This information is overruled with relay_transport, sender_dependent_default_trans‐
port_maps, default_transport, sender_dependent_relayhost_maps and with the transport(5) ta‐
ble.

On an intranet, specify the organizational domain name. If your internal DNS uses no MX
records, specify the name of the intranet gateway host instead.

In the case of SMTP or LMTP delivery, specify one or more destinations in the form of a do‐
main name, hostname, hostname:port, [hostname]:port, [hostaddress] or [hostaddress]:port,
separated by comma or whitespace. The form [hostname] turns off MX lookups. Multiple desti‐
nations are supported in Postfix 3.5 and later.

If you're connected via UUCP, see the UUCP_README file for useful information.

Examples:

relayhost = $mydomain
relayhost = [gateway.example.com]
relayhost = mail1.example:587, mail2.example:587
relayhost = [an.ip.add.ress]

relocated_maps (default: empty)
Optional lookup tables with new contact information for users or domains that no longer ex‐
ist. The table format and lookups are documented in relocated(5).

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)
Rewrite or add message headers in mail from remote clients if the remote_header_rewrite_do‐
main parameter value is non-empty, updating incomplete addresses with the domain specified in
the remote_header_rewrite_domain parameter, and adding missing headers.

The local_header_rewrite_clients parameter controls what clients Postfix considers local.

Examples:

The safe setting: append "domain.invalid" to incomplete header addresses from remote SMTP
clients, so that those addresses cannot be confused with local addresses.

remote_header_rewrite_domain = domain.invalid

The default, purist, setting: don't rewrite headers from remote clients at all.

remote_header_rewrite_domain =

require_home_directory (default: no)
Require that a local(8) recipient's home directory exists before mail delivery is attempted.
By default this test is disabled. It can be useful for environments that import home direc‐
tories to the mail server (IMPORTING HOME DIRECTORIES IS NOT RECOMMENDED).

reset_owner_alias (default: no)
Reset the local(8) delivery agent's idea of the owner-alias attribute, when delivering mail
to a child alias that does not have its own owner alias.

This feature is available in Postfix 2.8 and later. With older Postfix releases, the behavior
is as if this parameter is set to "yes".

As documented in aliases(5), 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, instead of at‐
tempting to deliver mail to local addresses as soon as they come out of alias expansion.

Writing local addresses from alias expansion to a new queue file allows for robust handling
of temporary delivery errors: errors with one local member have no effect on deliveries to
other members of the list. On the other hand, delivery to local addresses as soon as they
come out of alias expansion is fragile: a temporary error with one local address from alias
expansion will cause the entire alias to be expanded repeatedly until the error goes away, or
until the message expires in the queue. In that case, a problem with one list member results
in multiple message deliveries to other list members.

The default behavior of Postfix 2.8 and later is to keep the owner-alias attribute of the
parent alias, when delivering mail to a child alias that does not have its own owner alias.
Then, local addresses from that child alias will be written to a new queue file, and a tempo‐
rary error with one local address will not affect delivery to other mailing list members.

Unfortunately, older Postfix releases reset the owner-alias attribute when delivering mail to
a child alias that does not have its own owner alias. To be precise, this resets only the de‐
cision to create a new queue file, not the decision to override the envelope sender address.
The local(8) delivery agent then attempts to deliver local addresses as soon as they come out
of child alias expansion. If delivery to any address from child alias expansion fails with a
temporary error condition, the entire mailing list may be expanded repeatedly until the mail
expires in the queue, resulting in multiple deliveries of the same message to mailing list
members.

resolve_dequoted_address (default: yes)
Resolve a recipient address safely instead of correctly, by looking inside quotes.

By default, the Postfix address resolver does not quote the address localpart as per RFC 822,
so that additional @ or % or ! operators remain visible. This behavior is safe but it is
also technically incorrect.

If you specify "resolve_dequoted_address = no", then the Postfix resolver will not know about
additional @ etc. operators in the address localpart. This opens opportunities for obscure
mail relay attacks with user@domain@domain addresses when Postfix provides backup MX service
for Sendmail systems.

resolve_null_domain (default: no)
Resolve an address that ends in the "@" null domain as if the local hostname were specified,
instead of rejecting the address as invalid.

This feature is available in Postfix 2.1 and later. Earlier versions always resolve the null
domain as the local hostname.

The Postfix SMTP server uses this feature to reject mail from or to addresses that end in the
"@" null domain, and from addresses that rewrite into a form that ends in the "@" null do‐
main.

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

This feature is available in Postfix 2.3 and later.

respectful_logging (default: see 'postconf -d' output)
Avoid logging that implies white is better than black. Instead use 'allowlist', 'denylist',
and variations of those words.

This feature is available in Postfix 3.6 and later.

rewrite_service_name (default: rewrite)
The name of the address rewriting service. This service rewrites addresses to standard form
and resolves them to a (delivery method, next-hop host, recipient) triple.

This feature is available in Postfix 2.0 and later.

sample_directory (default: /etc/postfix)
The name of the directory with example Postfix configuration files. Starting with Postfix
2.1, these files have been replaced with the postconf(5) manual page.

send_cyrus_sasl_authzid (default: no)
When authenticating to a remote SMTP or LMTP server with the default setting "no", send no
SASL authoriZation ID (authzid); send only the SASL authentiCation ID (authcid) plus the au‐
thcid's password.

The non-default setting "yes" enables the behavior of older Postfix versions. These always
send a SASL authzid that is equal to the SASL authcid, but this causes interoperability prob‐
lems with some SMTP servers.

This feature is available in Postfix 2.4.4 and later.

sender_based_routing (default: no)
This parameter should not be used. It was replaced by sender_dependent_relayhost_maps in
Postfix version 2.3.

sender_bcc_maps (default: empty)
Optional BCC (blind carbon-copy) address lookup tables, indexed by envelope 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:

• Look up the "user+extension AT domain.tld" address including the optional address exten‐
sion.

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

• Look up the "user+extension" address local part when the sender domain equals $myori‐
gin, $mydestination, $inet_interfaces or $proxy_interfaces.

• Look up the "user" address local part when the sender domain equals $myorigin, $mydes‐
tination, $inet_interfaces or $proxy_interfaces.

• Look up the "@domain.tld" part.

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

Example:

sender_bcc_maps = hash:/etc/postfix/sender_bcc

After a change, run "postmap /etc/postfix/sender_bcc".

This feature is available in Postfix 2.1 and later.

sender_canonical_classes (default: envelope_sender, header_sender)
What addresses are subject to sender_canonical_maps address mapping. By default,
sender_canonical_maps address mapping is applied to envelope sender addresses, and to header
sender addresses.

Specify one or more of: envelope_sender, header_sender

This feature is available in Postfix 2.2 and later.

sender_canonical_maps (default: empty)
Optional address mapping lookup tables for envelope and header sender addresses. The table
format and lookups are documented in canonical(5).

Example: you want to rewrite the SENDER address "user AT ugly.example" to "user AT pretty.example",
while still being able to send mail to the RECIPIENT address "user AT ugly.example".

Note: $sender_canonical_maps is processed before $canonical_maps.

Example:

sender_canonical_maps = hash:/etc/postfix/sender_canonical

sender_dependent_default_transport_maps (default: empty)
A sender-dependent override for the global default_transport parameter setting. The tables
are searched by the envelope sender address and @domain. A lookup result of DUNNO terminates
the search without overriding the global default_transport parameter setting. This informa‐
tion is overruled with the transport(5) table.

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

Note: this overrides default_transport, not transport_maps, and therefore the expected syntax
is that of default_transport, not the syntax of transport_maps. Specifically, this does not
support the transport_maps syntax for null transport, null nexthop, or null email addresses.

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

This feature is available in Postfix 2.7 and later.

sender_dependent_relayhost_maps (default: empty)
A sender-dependent override for the global relayhost parameter setting. The tables are
searched by the envelope sender address and @domain. A lookup result of DUNNO terminates the
search without overriding the global relayhost parameter setting (Postfix 2.6 and later).
This information is overruled with relay_transport, sender_dependent_default_transport_maps,
default_transport and with the transport(5) table.

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

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

This feature is available in Postfix 2.3 and later.

sendmail_fix_line_endings (default: always)
Controls how the Postfix sendmail command converts email message line endings from <CR><LF>
into UNIX format (<LF>).

always Always convert message lines ending in <CR><LF>. This setting is the default with
Postfix 2.9 and later.

strict Convert message lines ending in <CR><LF> only if the first input line ends in
<CR><LF>. This setting is backwards-compatible with Postfix 2.8 and earlier.

never Never convert message lines ending in <CR><LF>. This setting exists for completeness
only.

This feature is available in Postfix 2.9 and later.

sendmail_path (default: see postconf -d output)
A Sendmail compatibility feature that specifies the location of the Postfix sendmail(1) com‐
mand. This command can be used to submit mail into the Postfix queue.

service_name (read-only)
The master.cf service name of a Postfix daemon process. This can be used to distinguish the
logging from different services that use the same program name.

Example master.cf entries:

# Distinguish inbound MTA logging from submission and smtps logging.
smtp inet n - n - - smtpd
submission inet n - n - - smtpd
-o syslog_name=postfix/$service_name
smtps inet n - n - - smtpd
-o syslog_name=postfix/$service_name

# Distinguish outbound MTA logging from inbound relay logging.
smtp unix - - n - - smtp
relay unix - - n - - smtp
-o syslog_name=postfix/$service_name

service_throttle_time (default: 60s)
How long the Postfix master(8) waits before forking a server that appears to be malfunction‐
ing.

setgid_group (default: postdrop)
The group ownership of set-gid Postfix commands and of group-writable Postfix directories.
When this parameter value is changed you need to re-run "postfix set-permissions" (with Post‐
fix version 2.0 and earlier: "/etc/postfix/post-install set-permissions".

shlib_directory (default: see 'postconf -d' output)
The location of Postfix dynamically-linked libraries (libpostfix-*.so), and the default loca‐
tion of Postfix database plugins (postfix-*.so) that have a relative pathname in the dynam‐
icmaps.cf file. The shlib_directory parameter defaults to "no" when Postfix dynami‐
cally-linked libraries and database plugins are disabled at compile time, otherwise it typi‐
cally defaults to /usr/lib/postfix or /usr/local/lib/postfix.

Notes:

• The directory specified with shlib_directory should contain only Postfix-related
files. Postfix dynamically-linked libraries and database plugins should not be in‐
stalled in a "public" system directory such as /usr/lib or /usr/local/lib. Linking
Postfix dynamically-linked library files or database plugins into non-Postfix programs
is not supported. Postfix dynamically-linked libraries and database plugins implement
a Postfix-internal API that changes without maintaining compatibility.

• You can change the shlib_directory value after Postfix is built. However, you may have
to run ldconfig or equivalent to prevent Postfix programs from failing because the
libpostfix-*.so files are not found. No ldconfig command is needed if you keep the
libpostfix-*.so files in the compiled-in default $shlib_directory location.

This feature is available in Postfix 3.0 and later.

show_user_unknown_table_name (default: yes)
Display the name of the recipient table in the "User unknown" responses. The extra detail
makes troubleshooting easier but also reveals information that is nobody else's business.

This feature is available in Postfix 2.0 and later.

showq_service_name (default: showq)
The name of the showq(8) service. This service produces mail queue status reports.

This feature is available in Postfix 2.0 and later.

smtp_address_preference (default: any)
The address type ("ipv6", "ipv4" or "any") that the Postfix SMTP client will try first, when
a destination has IPv6 and IPv4 addresses with equal MX preference. This feature has no ef‐
fect unless the inet_protocols setting enables both IPv4 and IPv6.

Postfix SMTP client address preference has evolved. With Postfix 2.8 the default is "ipv6";
earlier implementations are hard-coded to prefer IPv6 over IPv4.

Notes for mail delivery between sites that have both IPv4 and IPv6 connectivity:

• The setting "smtp_address_preference = ipv6" is unsafe. It can fail to deliver mail
when there is an outage that affects IPv6, while the destination is still reachable
over IPv4.

• The setting "smtp_address_preference = any" is safe. With this, mail will eventually
be delivered even if there is an outage that affects IPv6 or IPv4, as long as it does
not affect both.

This feature is available in Postfix 2.8 and later.

smtp_address_verify_target (default: rcpt)
In the context of email address verification, the SMTP protocol stage that determines whether
an email address is deliverable. Specify one of "rcpt" or "data". The latter is needed with
remote SMTP servers that reject recipients after the DATA command. Use transport_maps to ap‐
ply this feature selectively:

/etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport:
smtp-domain-that-verifies-after-data smtp-data-target:
lmtp-domain-that-verifies-after-data lmtp-data-target:

/etc/postfix/master.cf:
smtp-data-target unix - - n - - smtp
-o smtp_address_verify_target=data
lmtp-data-target unix - - n - - lmtp
-o lmtp_address_verify_target=data

Unselective use of the "data" target does no harm, but will result in unnecessary "lost con‐
nection after DATA" events at remote SMTP/LMTP servers.

This feature is available in Postfix 3.0 and later.

smtp_always_send_ehlo (default: yes)
Always send EHLO at the start of an SMTP session.

With "smtp_always_send_ehlo = no", the Postfix SMTP client sends EHLO only when the word
"ESMTP" appears in the server greeting banner (example: 220 spike.porcupine.org ESMTP Post‐
fix).

smtp_balance_inet_protocols (default: yes)
When a remote destination resolves to a combination of IPv4 and IPv6 addresses, ensure that
the Postfix SMTP client can try both address types before it runs into the smtp_mx_ad‐
dress_limit.

This avoids an interoperability problem when a destination resolves to primarily IPv6 ad‐
dresses, the smtp_address_limit feature eliminates most or all IPv4 addresses, and the desti‐
nation is not reachable over IPv6.

This feature is available in Postfix 3.3 and later.

smtp_bind_address (default: empty)
An optional numerical network address that the Postfix SMTP client should bind to when making
an IPv4 connection.

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

/etc/postfix/master.cf:
smtp ... smtp -o smtp_bind_address=11.22.33.44

See smtp_bind_address_enforce for how Postfix should handle errors (Postfix 3.7 and later).

Note 1: when inet_interfaces specifies no more than one IPv4 address, and that address is a
non-loopback address, it is automatically used as the smtp_bind_address. This supports vir‐
tual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces docu‐
mentation for more detail.

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

smtp_bind_address6 (default: empty)
An optional numerical network address that the Postfix SMTP client should bind to when making
an IPv6 connection.

This feature is available in Postfix 2.2 and later.

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

/etc/postfix/master.cf:
smtp ... smtp -o smtp_bind_address6=1:2:3:4:5:6:7:8

See smtp_bind_address_enforce for how Postfix should handle errors (Postfix 3.7 and later).

Note 1: when inet_interfaces specifies no more than one IPv6 address, and that address is a
non-loopback address, it is automatically used as the smtp_bind_address6. This supports vir‐
tual IP hosting, but can be a problem on multi-homed firewalls. See the inet_interfaces docu‐
mentation for more detail.

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

smtp_bind_address_enforce (default: no)
Defer delivery when the Postfix SMTP client cannot apply the smtp_bind_address or
smtp_bind_address6 setting. By default, the Postfix SMTP client will continue delivery after
logging a warning.

This feature is available in Postfix 3.7 and later.

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 decisions, or
TLS certificate verification. The value "no" hardens Postfix smtp_tls_per_site hostname-based
policies against false hostname information in DNS CNAME records, and makes SASL password
file lookups more predictable. This is the default setting as of Postfix 2.3.

When DNS CNAME records are validated with secure DNS lookups (smtp_dns_support_level =
dnssec), they are always allowed to override the above servername (Postfix 2.11 and later).

This feature is available in Postfix 2.2.9 and later.

smtp_connect_timeout (default: 30s)
The Postfix SMTP client time limit for completing a TCP connection, or zero (use the operat‐
ing system built-in time limit).

When no connection can be made within the deadline, the Postfix SMTP client tries the next
address on the mail exchanger list. Specify 0 to disable the time limit (i.e. use whatever
timeout is implemented by the operating system).

smtp_connection_cache_destinations (default: empty)
Permanently enable SMTP connection caching for the specified destinations. With SMTP connec‐
tion caching, a connection is not closed immediately after completion of a mail transaction.
Instead, the connection is kept open for up to $smtp_connection_cache_time_limit seconds.
This allows connections to be reused for other deliveries, and can improve mail delivery per‐
formance.

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

• if mail is sent without a relay host: a domain name (the right-hand side of an email
address, without the [] around a numeric IP address),

• if mail is sent via a relay host: a relay host name (without [] or non-default TCP
port), as specified in main.cf or in the transport map,

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

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

• a "type:table" with domain names and/or relay host names on the left-hand side. The
right-hand side result from "type:table" lookups is ignored.

This feature is available in Postfix 2.2 and later.

smtp_connection_cache_on_demand (default: yes)
Temporarily enable SMTP connection caching while a destination has a high volume of mail in
the active queue. With SMTP connection caching, a connection is not closed immediately after
completion of a mail transaction. Instead, the connection is kept open for up to $smtp_con‐
nection_cache_time_limit seconds. This allows connections to be reused for other deliveries,
and can improve mail delivery performance.

This feature is available in Postfix 2.2 and later.

smtp_connection_cache_time_limit (default: 2s)
When SMTP connection caching is enabled, the amount of time that an unused SMTP client socket
is kept open before it is closed. Do not specify larger values without permission from the
remote sites.

This feature is available in Postfix 2.2 and later.

smtp_connection_reuse_count_limit (default: 0)
When SMTP connection caching is enabled, the number of times that an SMTP session may be
reused before it is closed, or zero (no limit). With a reuse count limit of N, a connection
is used up to N+1 times.

NOTE: This feature is unsafe. When a high-volume destination has multiple inbound MTAs, then
the slowest inbound MTA will attract the most connections to that destination. This limita‐
tion does not exist with the smtp_connection_reuse_time_limit feature.

This feature is available in Postfix 2.11.

smtp_connection_reuse_time_limit (default: 300s)
The amount of time during which Postfix will use an SMTP connection repeatedly. The timer
starts when the connection is initiated (i.e. it includes the connect, greeting and helo la‐
tency, in addition to the latencies of subsequent mail delivery transactions).

This feature addresses a performance stability problem with remote SMTP servers. This problem
is not specific to Postfix: it can happen when any MTA sends large amounts of SMTP email to a
site that has multiple MX hosts.

The problem starts when one of a set of MX hosts becomes slower than the rest. Even though
SMTP clients connect to fast and slow MX hosts with equal probability, the slow MX host ends
up with more simultaneous inbound connections than the faster MX hosts, because the slow MX
host needs more time to serve each client request.

The slow MX host becomes a connection attractor. If one MX host becomes N times slower than
the rest, it dominates mail delivery latency unless there are more than N fast MX hosts to
counter the effect. And if the number of MX hosts is smaller than N, the mail delivery la‐
tency becomes effectively that of the slowest MX host divided by the total number of MX
hosts.

The solution uses connection caching in a way that differs from Postfix version 2.2. By lim‐
iting the amount of time during which a connection can be used repeatedly (instead of limit‐
ing the number of deliveries over that connection), Postfix not only restores fairness in the
distribution of simultaneous connections across a set of MX hosts, it also favors deliveries
over connections that perform well, which is exactly what we want.

The default reuse time limit, 300s, is comparable to the various smtp transaction timeouts
which are fair estimates of maximum excess latency for a slow delivery. Note that hosts may
accept thousands of messages over a single connection within the default connection reuse
time limit. This number is much larger than the default Postfix version 2.2 limit of 10 mes‐
sages per cached connection. It may prove necessary to lower the limit to avoid interoper‐
ability issues with MTAs that exhibit bugs when many messages are delivered via a single con‐
nection. A lower reuse time limit risks losing the benefit of connection reuse when the av‐
erage connection and mail delivery latency exceeds the reuse time limit.

This feature is available in Postfix 2.3 and later.

smtp_data_done_timeout (default: 600s)
The Postfix SMTP client time limit for sending the SMTP ".", and for receiving the remote
SMTP server response.

When no response is received within the deadline, a warning is logged that the mail may be
delivered multiple times.

smtp_data_init_timeout (default: 120s)
The Postfix SMTP client time limit for sending the SMTP DATA command, and for receiving the
remote SMTP server response.

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

smtp_data_xfer_timeout (default: 180s)
The Postfix SMTP client time limit for sending the SMTP message content. When the connection
makes no progress for more than $smtp_data_xfer_timeout seconds the Postfix SMTP client ter‐
minates the transfer.

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

smtp_defer_if_no_mx_address_found (default: no)
Defer mail delivery when no MX record resolves to an IP address.

The default (no) is to return the mail as undeliverable. With older Postfix versions the de‐
fault was to keep trying to deliver the mail until someone fixed the MX record or until the
mail was too old.

Note: the Postfix SMTP client always ignores MX records with equal or worse preference than
the local MTA itself.

This feature is available in Postfix 2.1 and later.

smtp_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the smtp(8) delivery agent to change the delivery status code or explana‐
tory text of successful or unsuccessful deliveries. See default_delivery_status_filter for
details.

NOTE: This feature modifies Postfix SMTP client error or non-error messages that may or may
not be derived from remote SMTP server responses. In contrast, the smtp_reply_filter feature
modifies remote SMTP server responses only.

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

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

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

smtp_discard_ehlo_keyword_address_maps (default: empty)
Lookup tables, indexed by the remote SMTP server address, with case insensitive lists of EHLO
keywords (pipelining, starttls, auth, etc.) that the Postfix SMTP client will ignore in the
EHLO response from a remote SMTP server. See smtp_discard_ehlo_keywords for details. The ta‐
ble is not indexed by hostname for consistency with smtpd_discard_ehlo_keyword_address_maps.

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

This feature is available in Postfix 2.2 and later.

smtp_discard_ehlo_keywords (default: empty)
A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
SMTP client will ignore in the EHLO response from a remote SMTP server.

This feature is available in Postfix 2.2 and later.

Notes:

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

• Use the smtp_discard_ehlo_keyword_address_maps feature to discard EHLO keywords selec‐
tively.

smtp_dns_reply_filter (default: empty)
Optional filter for Postfix SMTP client DNS lookup results. Specify zero or more lookup ta‐
bles. The lookup tables are searched in the given order for a match with the DNS lookup re‐
sult, converted to the following form:

name ttl class type preference value

The class field is always "IN", the preference field exists only for MX records, the names of
hosts, domains, etc. end in ".", and those names are in ASCII form (xn--mumble form in the
case of UTF8 names).

When a match is found, the table lookup result specifies an action. By default, the table
query and the action name are case-insensitive. Currently, only the IGNORE action is imple‐
mented.

Notes:

• Postfix DNS reply filters have no effect on implicit DNS lookups through nsswitch.conf
or equivalent mechanisms.

• The Postfix SMTP/LMTP client uses smtp_dns_reply_filter and lmtp_dns_reply_filter only
to discover a remote SMTP or LMTP service (record types MX, A, AAAA, and TLSA). These
lookups are also made to implement the features reject_unverified_sender and re‐
ject_unverified_recipient.

• The Postfix SMTP/LMTP client defers mail delivery when a filter removes all lookup re‐
sults from a successful query.

• Postfix SMTP server uses smtpd_dns_reply_filter only to look up MX, A, AAAA, and TXT
records to implement the features reject_unknown_helo_hostname, reject_un‐
known_sender_domain, reject_unknown_recipient_domain, reject_rbl_*, and re‐
ject_rhsbl_*.

• The Postfix SMTP server logs a warning or defers mail delivery when a filter removes
all lookup results from a successful query.

Example: ignore Google AAAA records in Postfix SMTP client DNS lookups, because Google some‐
times hard-rejects mail from IPv6 clients with valid PTR etc. records.

/etc/postfix/main.cf:
smtp_dns_reply_filter = pcre:/etc/postfix/smtp_dns_reply_filter

/etc/postfix/smtp_dns_reply_filter:
# /domain ttl IN AAAA address/ action, all case-insensitive.
# Note: the domain name ends in ".".
/^\S+\.google\.com\.\s+\S+\s+\S+\s+AAAA\s+/ IGNORE

This feature is available in Postfix 3.0 and later.

smtp_dns_resolver_options (default: empty)
DNS Resolver options for the Postfix SMTP client. Specify zero or more of the following op‐
tions, separated by comma or whitespace. Option names are case-sensitive. Some options refer
to domain names that are specified in the file /etc/resolv.conf or equivalent.

res_defnames
Append the current domain name to single-component names (those that do not contain a
"." character). This can produce incorrect results, and is the hard-coded behavior
prior to Postfix 2.8.

res_dnsrch
Search for host names in the current domain and in parent domains. This can produce
incorrect results and is therefore not recommended.

This feature is available in Postfix 2.8 and later.

smtp_dns_support_level (default: empty)
Level of DNS support in the Postfix SMTP client. With "smtp_dns_support_level" left at its
empty default value, the legacy "disable_dns_lookups" parameter controls whether DNS is en‐
abled in the Postfix SMTP client, otherwise the legacy parameter is ignored.

Specify one of the following:

disabled
Disable DNS lookups. No MX lookups are performed and hostname to address lookups are
unconditionally "native". This setting is not appropriate for hosts that deliver mail
to the public Internet. Some obsolete how-to documents recommend disabling DNS
lookups in some configurations with content_filters. This is no longer required and
strongly discouraged.

enabled
Enable DNS lookups. Nexthop destination domains not enclosed in "[]" will be subject
to MX lookups. If "dns" and "native" are included in the "smtp_host_lookup" parameter
value, DNS will be queried first to resolve MX-host A records, followed by "native"
lookups if no answer is found in DNS.

dnssec Enable DNSSEC lookups. The "dnssec" setting differs from the "enabled" setting above
in the following ways:

• Any MX lookups will set RES_USE_DNSSEC and RES_USE_EDNS0 to request DNSSEC-validated
responses. If the MX response is DNSSEC-validated the corresponding hostnames are con‐
sidered validated.

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

• Temporary failures in DNSSEC-enabled hostname-to-address resolution block any "native"
lookups. Additional "native" lookups only happen when DNSSEC lookups hard-fail (NO‐
DATA or NXDOMAIN).

The Postfix SMTP client considers non-MX "[nexthop]" and "[nexthop]:port" destinations equiv‐
alent to statically-validated MX records of the form "nexthop. IN MX 0 nexthop." Therefore,
with "dnssec" support turned on, validated hostname-to-address lookups apply to the nexthop
domain of any "[nexthop]" or "[nexthop]:port" destination. This is also true for LMTP
"inet:host" and "inet:host:port" destinations, as LMTP hostnames are never subject to MX
lookups.

The "dnssec" setting is recommended only if you plan to use the dane or dane-only TLS secu‐
rity level, otherwise enabling DNSSEC support in Postfix offers no additional security.
Postfix DNSSEC support relies on an upstream recursive nameserver that validates DNSSEC sig‐
natures. Such a DNS server will always filter out forged DNS responses, even when Postfix
itself is not configured to use DNSSEC.

When using Postfix DANE support the "smtp_host_lookup" parameter should include "dns", as
DANE is not applicable to hosts resolved via "native" lookups.

As mentioned above, Postfix is not a validating stub resolver; it relies on the system's con‐
figured DNSSEC-validating recursive nameserver to perform all DNSSEC validation. Since this
nameserver's DNSSEC-validated responses will be fully trusted, it is strongly recommended
that the MTA host have a local DNSSEC-validating recursive caching nameserver listening on a
loopback address, and be configured to use only this nameserver for all lookups. Otherwise,
Postfix may remain subject to man-in-the-middle attacks that forge responses from the recur‐
sive nameserver

DNSSEC support requires a version of Postfix compiled against a reasonably-modern DNS re‐
solver(3) library that implements the RES_USE_DNSSEC and RES_USE_EDNS0 resolver options.

This feature is available in Postfix 2.11 and later.

smtp_enforce_tls (default: no)
Enforcement mode: require that remote SMTP servers use TLS encryption, and never send mail in
the clear. This also requires that the remote SMTP server hostname matches the information
in the remote server certificate, and that the remote SMTP server certificate was issued by a
CA that is trusted by the Postfix SMTP client. If the certificate doesn't verify or the host‐
name doesn't match, delivery is deferred and mail stays in the queue.

The server hostname is matched against all names provided as dNSNames in the SubjectAlterna‐
tiveName. If no dNSNames are specified, the CommonName is checked. The behavior may be
changed with the smtp_tls_enforce_peername option.

This option is useful only if you are definitely sure that you will only connect to servers
that support RFC 2487 _and_ that provide valid server certificates. Typical use is for
clients that send all their email to a dedicated mailhub.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use
smtp_tls_security_level instead.

smtp_fallback_relay (default: $fallback_relay)
Optional list of relay destinations that will be used when an SMTP destination is not found,
or when delivery fails due to a non-permanent error. With Postfix 2.2 and earlier this para‐
meter is called fallback_relay.

By default, smtp_fallback_relay is empty, mail is returned to the sender when a destination
is not found, and delivery is deferred after it fails due to a non-permanent error.

With bulk email deliveries, it can be beneficial to run the fallback relay MTA on the same
host, so that it can reuse the sender IP address. This speeds up deliveries that are delayed
by IP-based reputation systems (greylist, etc.).

To prevent mailer loops between MX hosts and fall-back hosts, Postfix version 2.2 and later
will not use the fallback relays for destinations that it is MX host for (assuming DNS lookup
is turned on).

smtp_generic_maps (default: empty)
Optional lookup tables that perform address rewriting in the Postfix SMTP client, typically
to transform a locally valid address into a globally valid address when sending mail across
the Internet. This is needed when the local machine does not have its own Internet domain
name, but uses something like localdomain.local instead.

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

The table format and lookups are documented in generic(5); 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.

smtp_host_lookup (default: dns)
What mechanisms the Postfix SMTP client uses to look up a host's IP address. This parameter
is ignored when DNS lookups are disabled (see: disable_dns_lookups and smtp_dns_sup‐
port_level). The "dns" mechanism is always tried before "native" if both are listed.

Specify one of the following:

dns Hosts can be found in the DNS (preferred).

native Use the native naming service only (nsswitch.conf, or equivalent mechanism).

dns, native
Use the native service for hosts not found in the DNS.

This feature is available in Postfix 2.1 and later.

smtp_line_length_limit (default: 998)
The maximal length of message header and body lines that Postfix will send via SMTP. This
limit does not include the <CR><LF> at the end of each line. Longer lines are broken by in‐
serting "<CR><LF><SPACE>", to minimize the damage to MIME formatted mail. Specify zero to
disable this limit.

The Postfix limit of 998 characters not including <CR><LF> is consistent with the SMTP limit
of 1000 characters including <CR><LF>. The Postfix limit was 990 with Postfix 2.8 and ear‐
lier.

smtp_mail_timeout (default: 300s)
The Postfix SMTP client time limit for sending the MAIL FROM command, and for receiving the
remote SMTP server response.

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 destination
are not available.

This feature is available in Postfix 2.5 and later.

smtp_min_data_rate (default: 500)
The minimum plaintext data transfer rate in bytes/second for DATA requests, when deadlines
are enabled with smtp_per_request_deadline. After a write operation transfers N plaintext
message bytes (possibly after TLS encryption), and after the DATA request deadline is decre‐
mented by the elapsed time of that write operation, the DATA request deadline is incremented
by N/smtp_min_data_rate seconds. However, the deadline will never be incremented beyond the
time limit specified with smtp_data_xfer_timeout.

This feature is available in Postfix 3.7 and later.

smtp_mx_address_limit (default: 5)
The maximal number of MX (mail exchanger) IP addresses that can result from Postfix SMTP
client mail exchanger lookups, or zero (no limit). Prior to Postfix version 2.3, this limit
was disabled by default.

This feature is available in Postfix 2.1 and later.

smtp_mx_session_limit (default: 2)
The maximal number of SMTP sessions per delivery request before the Postfix SMTP client gives
up or delivers to a fall-back relay host, or zero (no limit). This restriction ignores ses‐
sions that fail to complete the SMTP initial handshake (Postfix version 2.2 and earlier) or
that fail to complete the EHLO and TLS handshake (Postfix version 2.3 and later).

This feature is available in Postfix 2.1 and later.

smtp_nested_header_checks (default: empty)
Restricted nested_header_checks(5) tables for the Postfix SMTP client. These tables are
searched while mail is being delivered. Actions that change the delivery time or destination
are not available.

This feature is available in Postfix 2.5 and later.

smtp_never_send_ehlo (default: no)
Never send EHLO at the start of an SMTP session. See also the smtp_always_send_ehlo parame‐
ter.

smtp_per_record_deadline (default: no)
Change the behavior of the smtp_*_timeout time limits, from a time limit per read or write
system call, to a time limit to send or receive a complete record (an SMTP command line, SMTP
response line, SMTP message content line, or TLS protocol message). This limits the impact
from hostile peers that trickle data one byte at a time.

Note: when per-record deadlines are enabled, a short timeout may cause problems with TLS over
very slow network connections. The reasons are that a TLS protocol message can be up to 16
kbytes long (with TLSv1), and that an entire TLS protocol message must be sent or received
within the per-record deadline.

This feature is available in Postfix 2.9-3.6. With older Postfix releases, the behavior is as
if this parameter is set to "no". Postfix 3.7 and later use smtp_per_request_deadline.

smtp_per_request_deadline (default: no)
Change the behavior of the smtp_*_timeout time limits, from a time limit per plaintext or TLS
read or write call, to a combined time limit for sending a complete SMTP request and for re‐
ceiving a complete SMTP response. The deadline limits only the time spent waiting for plain‐
text or TLS read or write calls, not time spent elsewhere. The per-request deadline limits
the impact from hostile peers that trickle data one byte at a time.

See smtp_min_data_rate for how the per-request deadline is managed during the DATA phase.

Note: when per-request deadlines are enabled, a short time limit may cause problems with TLS
over very slow network connections. The reason is that a TLS protocol message can be up to 16
kbytes long (with TLSv1), and that an entire TLS protocol message must be transferred within
the per-request deadline.

This feature is available in Postfix 3.7 and later. A weaker feature, called
smtp_per_record_deadline, is available with Postfix 2.9-3.6.

This feature is available in Postfix 3.7 and later.

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 too short a time makes this workaround ineffective when sending large messages over
slow network connections.

smtp_pix_workaround_maps (default: empty)
Lookup tables, indexed by the remote SMTP server address, with per-destination workarounds
for CISCO PIX firewall bugs. The table is not indexed by hostname for consistency with
smtp_discard_ehlo_keyword_address_maps.

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

This feature is available in Postfix 2.4 and later.

smtp_pix_workaround_threshold_time (default: 500s)
How long a message must be queued before the Postfix SMTP client turns on the PIX firewall
"<CR><LF>.<CR><LF>" bug workaround for delivery through firewalls with "smtp fixup" mode
turned on.

By default, the workaround is turned off for mail that is queued for less than 500 seconds.
In other words, the workaround is normally turned off for the first delivery attempt.

Specify 0 to enable the PIX firewall "<CR><LF>.<CR><LF>" bug workaround upon the first deliv‐
ery attempt.

smtp_pix_workarounds (default: disable_esmtp, delay_dotcrlf)
A list that specifies zero or more workarounds for CISCO PIX firewall bugs. These workarounds
are implemented by the Postfix SMTP client. Workaround names are separated by comma or space,
and are case insensitive. This parameter setting can be overruled with per-destination
smtp_pix_workaround_maps settings.

delay_dotcrlf
Insert a delay before sending ".<CR><LF>" after the end of the message content. The
delay is subject to the smtp_pix_workaround_delay_time and smtp_pix_workaround_thresh‐
old_time parameter settings.

disable_esmtp
Disable all extended SMTP commands: send HELO instead of EHLO.

This feature is available in Postfix 2.4 and later. The default settings are backwards com‐
patible with earlier Postfix versions.

smtp_quit_timeout (default: 300s)
The Postfix SMTP client time limit for sending the QUIT command, and for receiving the remote
SMTP server response.

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.

smtp_reply_filter (default: empty)
A mechanism to transform replies from remote SMTP servers one line at a time. This is a
last-resort tool to work around server replies that break interoperability with the Postfix
SMTP client. Other uses involve fault injection to test Postfix's handling of invalid re‐
sponses.

Notes:

• In the case of a multi-line reply, the Postfix SMTP client uses the final reply line's
numerical SMTP reply code and enhanced status code.

• The numerical SMTP reply code (XYZ) takes precedence over the enhanced status code
(X.Y.Z). When the enhanced status code initial digit differs from the SMTP reply code
initial digit, or when no enhanced status code is present, the Postfix SMTP client
uses a generic enhanced status code (X.0.0) instead.

Specify the name of a "type:table" lookup table. The search string is a single SMTP reply
line as received from the remote SMTP server, except that the trailing <CR><LF> are removed.
When the lookup succeeds, the result replaces the single SMTP reply line.

Examples:

/etc/postfix/main.cf:
smtp_reply_filter = pcre:/etc/postfix/reply_filter

/etc/postfix/reply_filter:
# Transform garbage into "250-filler..." so that it looks like
# one line from a multi-line reply. It does not matter what we
# substitute here as long it has the right syntax. The Postfix
# SMTP client will use the final line's numerical SMTP reply
# code and enhanced status code.
!/^([2-5][0-9][0-9]($|[- ]))/ 250-filler for garbage

This feature is available in Postfix 2.7.

smtp_rset_timeout (default: 20s)
The Postfix SMTP client time limit for sending the RSET command, and for receiving the remote
SMTP server response. The SMTP client sends RSET in order to finish a recipient address
probe, or to verify that a cached session is still usable.

This feature is available in Postfix 2.1 and later.

smtp_sasl_auth_cache_name (default: empty)
An optional table to prevent repeated SASL authentication failures with the same remote SMTP
server hostname, username and password. Each table (key, value) pair contains a server name,
a username and password, and the full server response. This information is stored when a re‐
mote SMTP server rejects an authentication attempt with a 535 reply code. As long as the
smtp_sasl_password_maps information does not change, and as long as the
smtp_sasl_auth_cache_name information does not expire (see smtp_sasl_auth_cache_time) the
Postfix SMTP client avoids SASL authentication attempts with the same server, username and
password, and instead bounces or defers mail as controlled with the
smtp_sasl_auth_soft_bounce configuration parameter.

Use a per-destination delivery concurrency of 1 (for example, "smtp_destination_concur‐
rency_limit = 1", "relay_destination_concurrency_limit = 1", etc.), otherwise multiple deliv‐
ery agents may experience a login failure at the same time.

The table must be accessed via the proxywrite service, i.e. the map name must start with
"proxy:". The table should be stored under the directory specified with the data_directory
parameter.

This feature uses cryptographic hashing to protect plain-text passwords, and requires that
Postfix is compiled with TLS support.

Example:

smtp_sasl_auth_cache_name = proxy:btree:/var/lib/postfix/sasl_auth_cache

This feature is available in Postfix 2.5 and later.

smtp_sasl_auth_cache_time (default: 90d)
The maximal age of an smtp_sasl_auth_cache_name entry before it is removed.

This feature is available in Postfix 2.5 and later.

smtp_sasl_auth_enable (default: no)
Enable SASL authentication in the Postfix SMTP client. By default, the Postfix SMTP client
uses no authentication.

Example:

smtp_sasl_auth_enable = yes

smtp_sasl_auth_soft_bounce (default: yes)
When a remote SMTP server rejects a SASL authentication request with a 535 reply code, defer
mail delivery instead of returning mail as undeliverable. The latter behavior was hard-coded
prior to Postfix version 2.5.

Note: the setting "yes" overrides the global soft_bounce parameter, but the setting "no" does
not.

Example:

# Default as of Postfix 2.5
smtp_sasl_auth_soft_bounce = yes
# The old hard-coded default
smtp_sasl_auth_soft_bounce = no

This feature is available in Postfix 2.5 and later.

smtp_sasl_mechanism_filter (default: empty)
If non-empty, a Postfix SMTP client filter for the remote SMTP server's list of offered SASL
mechanisms. Different client and server implementations may support different mechanism
lists; by default, the Postfix SMTP client will use the intersection of the two.
smtp_sasl_mechanism_filter specifies an optional third mechanism list to intersect with.

Specify mechanism names, "/file/name" patterns or "type:table" lookup tables. The right-hand
side result from "type:table" lookups is ignored. Specify "!pattern" to exclude a mechanism
name from the list. The form "!/file/name" is supported only in Postfix version 2.4 and
later.

This feature is available in Postfix 2.2 and later.

Examples:

smtp_sasl_mechanism_filter = plain, login
smtp_sasl_mechanism_filter = /etc/postfix/smtp_mechs
smtp_sasl_mechanism_filter = !gssapi, !login, static:rest

smtp_sasl_password_maps (default: empty)
Optional Postfix SMTP client lookup tables with one username:password entry per sender, re‐
mote hostname or next-hop domain. Per-sender lookup is done only when sender-dependent au‐
thentication is enabled. If no username:password entry is found, then the Postfix SMTP
client will not attempt to authenticate to the remote host.

The Postfix SMTP client opens the lookup table before going to chroot jail, so you can leave
the password file in /etc/postfix.

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

smtp_sasl_path (default: empty)
Implementation-specific information that the Postfix SMTP client passes through to the SASL
plug-in implementation that is selected with smtp_sasl_type. Typically this specifies the
name of a configuration file or rendezvous point.

This feature is available in Postfix 2.3 and later.

smtp_sasl_security_options (default: noplaintext, noanonymous)
Postfix SMTP client SASL security options; as of Postfix 2.3 the list of available features
depends on the SASL client implementation that is selected with smtp_sasl_type.

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

Specify zero or more of the following:

noplaintext
Disallow methods that use plaintext passwords.

noactive
Disallow methods subject to active (non-dictionary) attack.

nodictionary
Disallow methods subject to passive (dictionary) attack.

noanonymous
Disallow methods that allow anonymous authentication.

mutual_auth
Only allow methods that provide mutual authentication (not available with SASL version
1).

Example:

smtp_sasl_security_options = noplaintext

smtp_sasl_tls_security_options (default: $smtp_sasl_security_options)
The SASL authentication security options that the Postfix SMTP client uses for TLS encrypted
SMTP sessions.

This feature is available in Postfix 2.2 and later.

smtp_sasl_tls_verified_security_options (default: $smtp_sasl_tls_security_options)
The SASL authentication security options that the Postfix SMTP client uses for TLS encrypted
SMTP sessions with a verified server certificate.

When mail is sent to the public MX host for the recipient's domain, server certificates are
by default optional, and delivery proceeds even if certificate verification fails. For deliv‐
ery via a submission service that requires SASL authentication, it may be appropriate to send
plaintext passwords only when the connection to the server is strongly encrypted and the
server identity is verified.

The smtp_sasl_tls_verified_security_options parameter makes it possible to only enable plain‐
text mechanisms when a secure connection to the server is available. Submission servers sub‐
ject to this policy must either have verifiable certificates or offer suitable non-plaintext
SASL mechanisms.

This feature is available in Postfix 2.6 and later.

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

This feature is available in Postfix 2.3 and later.

smtp_send_dummy_mail_auth (default: no)
Whether or not to append the "AUTH=<>" option to the MAIL FROM command in SASL-authenticated
SMTP sessions. The default is not to send this, to avoid problems with broken remote SMTP
servers. Before Postfix 2.9 the behavior is as if "smtp_send_dummy_mail_auth = yes".

This feature is available in Postfix 2.9 and later.

smtp_send_xforward_command (default: no)
Send the non-standard XFORWARD command when the Postfix SMTP server EHLO response announces
XFORWARD support.

This allows a Postfix SMTP delivery agent, used for injecting mail into a content filter, to
forward the name, address, protocol and HELO name of the original client to the content fil‐
ter and downstream queuing SMTP server. This can produce more useful logging than local‐
host[127.0.0.1] etc.

This feature is available in Postfix 2.1 and later.

smtp_sender_dependent_authentication (default: no)
Enable sender-dependent authentication in the Postfix SMTP client; this is available only
with SASL authentication, and disables SMTP connection caching to ensure that mail from dif‐
ferent senders will use the appropriate credentials.

This feature is available in Postfix 2.3 and later.

smtp_skip_4xx_greeting (default: yes)
Skip SMTP servers that greet with a 4XX status code (go away, try again later).

By default, the Postfix SMTP client moves on the next mail exchanger. Specify
"smtp_skip_4xx_greeting = no" if Postfix should defer delivery immediately.

This feature is available in Postfix 2.0 and earlier. Later Postfix versions always skip re‐
mote SMTP servers that greet with a 4XX status code.

smtp_skip_5xx_greeting (default: yes)
Skip remote SMTP servers that greet with a 5XX status code.

By default, the Postfix SMTP client moves on the next mail exchanger. Specify
"smtp_skip_5xx_greeting = no" if Postfix should bounce the mail immediately. Caution: the
latter behavior appears to contradict RFC 2821.

smtp_skip_quit_response (default: yes)
Do not wait for the response to the SMTP QUIT command.

smtp_starttls_timeout (default: 300s)
Time limit for Postfix SMTP client write and read operations during TLS startup and shutdown
handshake procedures.

This feature is available in Postfix 2.2 and later.

smtp_tcp_port (default: smtp)
The default TCP port that the Postfix SMTP client connects to. Specify a symbolic name (see
services(5)) or a numeric port.

smtp_tls_CAfile (default: empty)
A file containing CA certificates of root CAs trusted to sign either remote SMTP server cer‐
tificates or intermediate CA certificates. These are loaded into memory before the smtp(8)
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 cer‐
tificate trust chain, but it is best to include all the required certificates directly in
$smtp_tls_cert_file (or, Postfix >= 3.4 $smtp_tls_chain_files).

Specify "smtp_tls_CAfile = /path/to/system_CA_file" to use ONLY the system-supplied default
Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending the system-supplied
default CAs and trusting third-party certificates.

Example:

smtp_tls_CAfile = /etc/postfix/CAcert.pem

This feature is available in Postfix 2.2 and later.

smtp_tls_CApath (default: empty)
Directory with PEM format Certification Authority certificates that the Postfix SMTP client
uses to verify a remote SMTP server certificate. Don't forget to create the necessary "hash"
links with, for example, "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs".

To use this option in chroot mode, this directory (or a copy) must be inside the chroot jail.

Specify "smtp_tls_CApath = /path/to/system_CA_directory" to use ONLY the system-supplied de‐
fault Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending the system-supplied
default CAs and trusting third-party certificates.

Example:

smtp_tls_CApath = /etc/postfix/certs

This feature is available in Postfix 2.2 and later.

smtp_tls_block_early_mail_reply (default: no)
Try to detect a mail hijacking attack based on a TLS protocol vulnerability (CVE-2009-3555),
where an attacker prepends malicious HELO, MAIL, RCPT, DATA commands to a Postfix SMTP client
TLS session. The attack would succeed with non-Postfix SMTP servers that reply to the mali‐
cious HELO, MAIL, RCPT, DATA commands after negotiating the Postfix SMTP client TLS session.

This feature is available in Postfix 2.7.

smtp_tls_cert_file (default: empty)
File with the Postfix SMTP client RSA certificate in PEM format. This file may also contain
the Postfix SMTP client private RSA key, and these may be the same as the Postfix SMTP server
RSA certificate and key file. With Postfix >= 3.4 the preferred way to configure client keys
and certificates is via the "smtp_tls_chain_files" parameter.

Do not configure client certificates unless you must present client TLS certificates to one
or more servers. Client certificates are not usually needed, and can cause problems in con‐
figurations that work well without them. The recommended setting is to let the defaults
stand:

smtp_tls_cert_file =
smtp_tls_key_file =
smtp_tls_eccert_file =
smtp_tls_eckey_file =
# Obsolete DSA parameters
smtp_tls_dcert_file =
smtp_tls_dkey_file =
# Postfix >= 3.4 interface
smtp_tls_chain_files =

The best way to use the default settings is to comment out the above parameters in main.cf if
present.

To enable remote SMTP servers to verify the Postfix SMTP client certificate, the issuing CA
certificates must be made available to the server. You should include the required certifi‐
cates in the client certificate file, the client certificate first, then the issuing CA(s)
(bottom-up order).

Example: the certificate for "client.example.com" was issued by "intermediate CA" which it‐
self has a certificate issued by "root CA". As the "root" super-user create the client.pem
file with:

# umask 077
# cat client_key.pem client_cert.pem intermediate_CA.pem > chain.pem

If you also want to verify remote SMTP server certificates issued by these CAs, you can add
the CA certificates to the smtp_tls_CAfile, in which case it is not necessary to have them in
the smtp_tls_cert_file, smtp_tls_dcert_file (obsolete) or smtp_tls_eccert_file.

A certificate supplied here must be usable as an SSL client certificate and hence pass the
"openssl verify -purpose sslclient ..." test.

Example:

smtp_tls_cert_file = /etc/postfix/chain.pem

This feature is available in Postfix 2.2 and later.

smtp_tls_chain_files (default: empty)
List of one or more PEM files, each holding one or more private keys directly followed by a
corresponding certificate chain. The file names are separated by commas and/or whitespace.
This parameter obsoletes the legacy algorithm-specific key and certificate file settings.
When this parameter is non-empty, the legacy parameters are ignored, and a warning is logged
if any are also non-empty.

With the proliferation of multiple private key algorithms-which, as of OpenSSL 1.1.1, include
DSA (obsolete), RSA, ECDSA, Ed25519 and Ed448-it is increasingly impractical to use separate
parameters to configure the key and certificate chain for each algorithm. Therefore, Postfix
now supports storing multiple keys and corresponding certificate chains in a single file or
in a set of files.

Each key must appear immediately before the corresponding certificate, optionally followed by
additional issuer certificates that complete the certificate chain for that key. When multi‐
ple files are specified, they are equivalent to a single file that is concatenated from those
files in the given order. Thus, while a key must always precede its certificate and issuer
chain, it can be in a separate file, so long as that file is listed immediately before the
file that holds the corresponding certificate chain. Once all the files are concatenated,
the sequence of PEM objects must be: key1, cert1, [chain1], key2, cert2, [chain2], ..., keyN,
certN, [chainN].

Storing the private key in the same file as the corresponding certificate is more reliable.
With the key and certificate in separate files, there is a chance that during key rollover a
Postfix process might load a private key and certificate from separate files that don't
match. Various operational errors may even result in a persistent broken configuration in
which the certificate does not match the private key.

The file or files must contain at most one key of each type. If, for example, two or more
RSA keys and corresponding chains are listed, depending on the version of OpenSSL either only
the last one will be used or a configuration error may be detected. Note that while
"Ed25519" and "Ed448" are considered separate algorithms, the various ECDSA curves (typically
one of prime256v1, secp384r1 or secp521r1) are considered as different parameters of a single
"ECDSA" algorithm, so it is not presently possible to configure keys for more than one ECDSA
curve.

Example (separate files for each key and corresponding certificate chain):

/etc/postfix/main.cf:
smtp_tls_chain_files =
${config_directory}/ed25519.pem,
${config_directory}/ed448.pem,
${config_directory}/rsa.pem

/etc/postfix/ed25519.pem:
-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
...
nC0egv51YPDWxEHom4QA
-----END CERTIFICATE-----

/etc/postfix/ed448.pem:
-----BEGIN PRIVATE KEY-----
MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
...
pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
-----END CERTIFICATE-----

/etc/postfix/rsa.pem:
-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
...
ahQkZ3+krcaJvDSMgvu0tDc=
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
...
Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
-----END CERTIFICATE-----

Example (all keys and certificates in a single file):

/etc/postfix/main.cf:
smtp_tls_chain_files = ${config_directory}/chains.pem

/etc/postfix/chains.pem:
-----BEGIN PRIVATE KEY-----
MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
...
nC0egv51YPDWxEHom4QA
-----END CERTIFICATE-----
-----BEGIN PRIVATE KEY-----
MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
...
pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
-----END CERTIFICATE-----
-----BEGIN PRIVATE KEY-----
MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
...
ahQkZ3+krcaJvDSMgvu0tDc=
-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
...
Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
-----END CERTIFICATE-----

This feature is available in Postfix 3.4 and later.

smtp_tls_cipherlist (default: empty)
Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS cipher list. As this feature
applies to all TLS security levels, it is easy to create interoperability problems by choos‐
ing a non-default cipher list. Do not use a non-default TLS cipher list on hosts that deliver
email to the public Internet: you will be unable to send email to servers that only support
the ciphers you exclude. Using a restricted cipher list may be more appropriate for an inter‐
nal MTA, where one can exert some control over the TLS software and settings of the peer
servers.

Note: do not use "" quotes around the parameter value.

This feature is available in Postfix version 2.2. It is not used with Postfix 2.3 and later;
use smtp_tls_mandatory_ciphers instead.

smtp_tls_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP client will use with opportunistic TLS en‐
cryption. Cipher types listed in smtp_tls_exclude_ciphers are excluded from the base defini‐
tion of the selected cipher grade. The default value is "medium" for Postfix releases after
the middle of 2015, "export" for older releases.

When TLS is mandatory the cipher grade is chosen via the smtp_tls_mandatory_ciphers configu‐
ration parameter, see there for syntax details. See smtp_tls_policy_maps for information on
how to configure ciphers on a per-destination basis.

This feature is available in Postfix 2.6 and later. With earlier Postfix releases only the
smtp_tls_mandatory_ciphers parameter is implemented, and opportunistic TLS always uses "ex‐
port" or better (i.e. all) ciphers.

smtp_tls_connection_reuse (default: no)
Try to make multiple deliveries per TLS-encrypted connection. This uses the tlsproxy(8) ser‐
vice 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 security
level is dane, but the MX record was found via an "insecure" MX lookup. The choices are:

may The TLSA records will be ignored and TLS will be optional. If the MX host does not
appear to support STARTTLS, or the STARTTLS handshake fails, mail may be sent in the
clear.

encrypt
The TLSA records will signal a requirement to use TLS. While TLS encryption will be
required, authentication will not be performed.

dane The TLSA records will be used just as with "secure" MX records. TLS encryption will
be required, and, if at least one of the TLSA records is "usable", authentication will
be required. When authentication succeeds, it will be logged only as "Trusted", not
"Verified", because the MX host name could have been forged.
The default setting for Postfix >= 3.6 is "dane" with "smtp_tls_security_level =
dane", otherwise "may". This behavior was backported to Postfix versions 3.5.9,
3.4.19, 3.3.16. 3.2.21. With earlier Postfix versions the default setting was always
"dane".

Though with "insecure" MX records an active attacker can compromise SMTP transport security
by returning forged MX records, such attacks are "tamper-evident" since any forged MX host‐
names will be recorded in the mail logs. Attackers who place a high value on staying hidden
may be deterred from forging MX records.

This feature is available in Postfix 3.1 and later. The may policy is backwards-compatible
with earlier Postfix versions.

smtp_tls_dcert_file (default: empty)
File with the Postfix SMTP client DSA certificate in PEM format. This file may also contain
the Postfix SMTP client private DSA key. The DSA algorithm is obsolete and should not be
used.

See the discussion under smtp_tls_cert_file for more details.

Example:

smtp_tls_dcert_file = /etc/postfix/client-dsa.pem

This feature is available in Postfix 2.2 and later.

smtp_tls_dkey_file (default: $smtp_tls_dcert_file)
File with the Postfix SMTP client DSA private key in PEM format. This file may be combined
with the Postfix SMTP client DSA certificate file specified with $smtp_tls_dcert_file. The
DSA algorithm is obsolete and should not be used.

The private key must be accessible without a pass-phrase, i.e. it must not be encrypted. File
permissions should grant read-only access to the system superuser account ("root"), and no
access to anyone else.

This feature is available in Postfix 2.2 and later.

smtp_tls_eccert_file (default: empty)
File with the Postfix SMTP client ECDSA certificate in PEM format. This file may also con‐
tain the Postfix SMTP client ECDSA private key. With Postfix >= 3.4 the preferred way to
configure client keys and certificates is via the "smtp_tls_chain_files" parameter.

See the discussion under smtp_tls_cert_file for more details.

Example:

smtp_tls_eccert_file = /etc/postfix/ecdsa-ccert.pem

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

smtp_tls_eckey_file (default: $smtp_tls_eccert_file)
File with the Postfix SMTP client ECDSA private key in PEM format. This file may be combined
with the Postfix SMTP client ECDSA certificate file specified with $smtp_tls_eccert_file.
With Postfix >= 3.4 the preferred way to configure client keys and certificates is via the
"smtp_tls_chain_files" parameter.

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

smtp_tls_enforce_peername (default: yes)
With mandatory TLS encryption, require that the remote SMTP server hostname matches the in‐
formation in the remote SMTP server certificate. As of RFC 2487 the requirements for host‐
name checking for MTA clients are not specified.

This option can be set to "no" to disable strict peer name checking. This setting has no ef‐
fect on sessions that are controlled via the smtp_tls_per_site table.

Disabling the hostname verification can make sense in a closed environment where special CAs
are created. If not used carefully, this option opens the danger of a "man-in-the-middle"
attack (the CommonName of this attacker will be logged).

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use
smtp_tls_security_level instead.

smtp_tls_exclude_ciphers (default: empty)
List of ciphers or cipher types to exclude from the Postfix SMTP client cipher list at all
TLS security levels. This is not an OpenSSL cipherlist, it is a simple list separated by
whitespace and/or commas. The elements are a single cipher, or one or more "+" separated ci‐
pher properties, in which case only ciphers matching all the properties are excluded.

Examples (some of these will cause problems):

smtp_tls_exclude_ciphers = aNULL
smtp_tls_exclude_ciphers = MD5, DES
smtp_tls_exclude_ciphers = DES+MD5
smtp_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
smtp_tls_exclude_ciphers = kEDH+aRSA

The first setting disables anonymous ciphers. The next setting disables ciphers that use the
MD5 digest algorithm or the (single) DES encryption algorithm. The next setting disables ci‐
phers that use MD5 and DES together. The next setting disables the two ciphers "AES256-SHA"
and "DES-CBC3-MD5". The last setting disables ciphers that use "EDH" key exchange with RSA
authentication.

This feature is available in Postfix 2.3 and later.

smtp_tls_fingerprint_cert_match (default: empty)
List of acceptable remote SMTP server certificate fingerprints for the "fingerprint" TLS se‐
curity level (smtp_tls_security_level = fingerprint). At this security level, Certification
Authorities are not used, and certificate expiration times are ignored. Instead, server cer‐
tificates are verified directly via their certificate fingerprint or public key fingerprint
(Postfix 2.9 and later). The fingerprint is a message digest of the server certificate (or
public key). The digest algorithm is selected via the smtp_tls_fingerprint_digest parameter.

The colons between each pair of nibbles in the fingerprint value are optional (Postfix >=
3.6). These were required in earlier Postfix releases.

When an smtp_tls_policy_maps table entry specifies the "fingerprint" security level, any
"match" attributes in that entry specify the list of valid fingerprints for the corresponding
destination. Multiple fingerprints can be combined with a "|" delimiter in a single match at‐
tribute, or multiple match attributes can be employed.

Example: Certificate fingerprint verification with internal mailhub. Two matching finger‐
prints are listed. The relayhost may be multiple physical hosts behind a load-balancer, each
with its own private/public key and self-signed certificate. Alternatively, a single relay‐
host may be in the process of switching from one set of private/public keys to another, and
both keys are trusted just prior to the transition.

relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_fingerprint_digest = sha256
smtp_tls_fingerprint_cert_match =
cd:fc:d8:db:f8:c4:82:96:6c:...:28:71:e8:f5:8d:a5:0d:9b:d4:a6
dd:5c:ef:f5:c3:bc:64:25:36:...:99:36:06:ce:40:ef:de:2e:ad:a4

Example: Certificate fingerprint verification with selected destinations. As in the example
above, we show two matching fingerprints:

/etc/postfix/main.cf:
smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
smtp_tls_fingerprint_digest = sha256

/etc/postfix/tls_policy:
example.com fingerprint
match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76
match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f

This feature is available in Postfix 2.5 and later.

smtp_tls_fingerprint_digest (default: see postconf -d output)
The message digest algorithm used to construct remote SMTP server certificate fingerprints.
At the "fingerprint" TLS security level (smtp_tls_security_level = fingerprint), the server
certificate is verified by directly matching its certificate fingerprint or its public key
fingerprint (Postfix 2.9 and later). The fingerprint is the message digest of the server cer‐
tificate (or its public key) using the selected algorithm. With a digest algorithm resistant
to "second pre-image" attacks, it is not feasible to create a new public key and a matching
certificate (or public/private key-pair) that has the same fingerprint.

The default algorithm is sha256 with Postfix >= 3.6 and the compatibility_level set to 3.6 or
higher. With Postfix <= 3.5, the default algorithm is md5.

The best-practice algorithm is now sha256. Recent advances in hash function cryptanalysis
have led to md5 and sha1 being deprecated in favor of sha256. However, as long as there are
no known "second pre-image" attacks against the older algorithms, their use in this context,
though not recommended, is still likely safe.

While additional digest algorithms are often available with OpenSSL's libcrypto, only those
used by libssl in SSL cipher suites are available to Postfix. You'll likely find support for
md5, sha1, sha256 and sha512.

To find the fingerprint of a specific certificate file, with a specific digest algorithm,
run:

$ openssl x509 -noout -fingerprint -digest -in certfile.pem

The text to the right of the "=" sign is the desired fingerprint. For example:

$ openssl x509 -noout -fingerprint -sha256 -in cert.pem
SHA256 Fingerprint=D4:6A:AB:19:24:...:BB:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

To extract the public key fingerprint from an X.509 certificate, you need to extract the pub‐
lic key from the certificate and compute the appropriate digest of its DER (ASN.1) encoding.
With OpenSSL the "-pubkey" option of the "x509" command extracts the public key always in
"PEM" format. We pipe the result to another OpenSSL command that converts the key to DER and
then to the "dgst" command to compute the fingerprint.

The actual command to transform the key to DER format depends on the version of OpenSSL used.
As of OpenSSL 1.0.0, the "pkey" command supports all key types.

# OpenSSL >= 1.0 with SHA-256 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
openssl pkey -pubin -outform DER |
openssl dgst -sha256 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:...:fc:09:1a:61:98:b5:bc:7c:60:58

The Postfix SMTP server and client log the peer (leaf) certificate fingerprint and the public
key fingerprint when the TLS loglevel is 2 or higher.

This feature is available in Postfix 2.5 and later.

smtp_tls_force_insecure_host_tlsa_lookup (default: no)
Lookup the associated DANE TLSA RRset even when a hostname is not an alias and its address
records lie in an unsigned zone. This is unlikely to ever yield DNSSEC validated results,
since child zones of unsigned zones are also unsigned in the absence of DLV or locally con‐
figured non-root trust-anchors. We anticipate that such mechanisms will not be used for just
the "_tcp" subdomain of a host. Suppressing the TLSA RRset lookup reduces latency and avoids
potential interoperability problems with nameservers for unsigned zones that are not prepared
to handle the new TLSA RRset.

This feature is available in Postfix 2.11.

smtp_tls_key_file (default: $smtp_tls_cert_file)
File with the Postfix SMTP client RSA private key in PEM format. This file may be combined
with the Postfix SMTP client RSA certificate file specified with $smtp_tls_cert_file. With
Postfix >= 3.4 the preferred way to configure client keys and certificates is via the
"smtp_tls_chain_files" parameter.

Example:

smtp_tls_key_file = $smtp_tls_cert_file

This feature is available in Postfix 2.2 and later.

smtp_tls_loglevel (default: 0)
Enable additional Postfix SMTP client logging of TLS activity. Each logging level also in‐
cludes the information that is logged at a lower logging level.

0 Disable logging of TLS activity.

1 Log only a summary message on TLS handshake completion - no logging of remote SMTP
server certificate trust-chain verification errors if server certificate verification
is not required. With Postfix 2.8 and earlier, log the summary message and uncondi‐
tionally log trust-chain verification errors.

2 Also log levels during TLS negotiation.

3 Also log the hexadecimal and ASCII dump of the TLS negotiation process.

4 Also log the hexadecimal and ASCII dump of complete transmission after STARTTLS.

Do not use "smtp_tls_loglevel = 2" or higher except in case of problems. Use of loglevel 4 is
strongly discouraged.

This feature is available in Postfix 2.2 and later.

smtp_tls_mandatory_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP client will use with mandatory TLS encryp‐
tion. The default value "medium" is suitable for most destinations with which you may want
to enforce TLS, and is beyond the reach of today's cryptanalytic methods. See smtp_tls_pol‐
icy_maps for information on how to configure ciphers on a per-destination basis.

The following cipher grades are supported:

high Enable only "HIGH" grade OpenSSL ciphers. This setting may be appropriate when all
mandatory TLS destinations (e.g. when all mail is routed to a suitably capable relay‐
host) support at least one "HIGH" grade cipher. The underlying cipherlist is specified
via the tls_high_cipherlist configuration parameter, which you are strongly encouraged
not to change.

medium Enable "MEDIUM" grade or better OpenSSL ciphers. The underlying cipherlist is speci‐
fied via the tls_medium_cipherlist configuration parameter, which you are strongly en‐
couraged not to change.

null Enable only the "NULL" OpenSSL ciphers, these provide authentication without encryp‐
tion. This setting is only appropriate in the rare case that all servers are prepared
to use NULL ciphers (not normally enabled in TLS servers). A plausible use-case is an
LMTP server listening on a UNIX-domain socket that is configured to support "NULL" ci‐
phers. The underlying cipherlist is specified via the tls_null_cipherlist configura‐
tion parameter, which you are strongly encouraged not to change.

low Enable "LOW" grade or stronger OpenSSL ciphers. In Postfix >= 3.8 this cipher grade
is always identical to "medium". Recent versions of OpenSSL do not support any "LOW"
grade ciphers. In earlier Postfix releases the underlying cipherlist was specified
via the tls_low_cipherlist configuration parameter, which you are strongly encouraged
not to change. This obsolete cipher grade SHOULD NOT be used.

export Enable "EXPORT" grade or stronger OpenSSL ciphers. In Postfix >= 3.8 this cipher
grade is always identical to "medium". Recent versions of OpenSSL do not support any
"EXPORT" grade ciphers. In earlier Postfix releases the underlying cipherlist was
specified via the tls_export_cipherlist configuration parameter, which you are
strongly encouraged not to change. This obsolete cipher grade SHOULD NOT be used.

The underlying cipherlists for grades other than "null" include anonymous ciphers, but these
are automatically filtered out if the Postfix SMTP client is configured to verify server cer‐
tificates. You are very unlikely to need to take any steps to exclude anonymous ciphers,
they are excluded automatically as necessary. If you must exclude anonymous ciphers at the
"may" or "encrypt" security levels, when the Postfix SMTP client does not need or use peer
certificates, set "smtp_tls_exclude_ciphers = aNULL". To exclude anonymous ciphers only when
TLS is enforced, set "smtp_tls_mandatory_exclude_ciphers = aNULL".

This feature is available in Postfix 2.3 and later.

smtp_tls_mandatory_exclude_ciphers (default: empty)
Additional list of ciphers or cipher types to exclude from the Postfix SMTP client cipher
list at mandatory TLS security levels. This list works in addition to the exclusions listed
with smtp_tls_exclude_ciphers (see there for syntax details).

Starting with Postfix 2.6, the mandatory cipher exclusions can be specified on a per-destina‐
tion basis via the TLS policy "exclude" attribute. See smtp_tls_policy_maps for notes and ex‐
amples.

This feature is available in Postfix 2.3 and later.

smtp_tls_mandatory_protocols (default: see postconf -d output)
TLS protocols that the Postfix SMTP client will use with mandatory TLS encryption. In
main.cf the values are separated by whitespace, commas or colons. In the policy table "proto‐
cols" attribute (see smtp_tls_policy_maps) the only valid separator is colon. An empty value
means allow all protocols.

The valid protocol names (see SSL_get_version(3)) are "SSLv2", "SSLv3", "TLSv1", "TLSv1.1",
"TLSv1.2" and "TLSv1.3". Starting with Postfix 3.6, the default value is ">=TLSv1", which
sets TLS 1.0 as the lowest supported TLS protocol version (see below). Older releases use
the "!" exclusion syntax, also described below.

As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set a
lowest acceptable TLS protocol version and/or a highest acceptable TLS protocol version. To
set the lower bound include an element of the form: ">=version" where version is a either one
of the TLS protocol names listed above, or a hexadecimal number corresponding to the desired
TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound, use
"<=version". There must be no whitespace between the ">=" or "<=" symbols and the protocol
name or number.

Hexadecimal protocol numbers make it possible to specify protocol bounds for TLS versions
that are known to OpenSSL, but might not be known to Postfix. They cannot be used with the
legacy exclusion syntax. Leading "0" or "0x" prefixes are supported, but not required.
Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent to "TLSv1". Hexadecimal
versions unknown to OpenSSL will fail to set the upper or lower bound, and a warning will be
logged. Hexadecimal versions should only be used when Postfix is linked with some future
version of OpenSSL that supports TLS 1.4 or later, but Postfix does not yet support a sym‐
bolic name for that protocol version.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtp_tls_mandatory_protocols = >=TLSv1.2, <=0305
# Allow only TLS 1.2 and up:
smtp_tls_mandatory_protocols = >=0x0303

With Postfix < 3.6 there is no support for a minimum or maximum version, and the protocol
range is configured via protocol exclusions. To require at least TLS 1.0, set
"smtp_tls_mandatory_protocols = !SSLv2, !SSLv3". Listing the protocols to include, rather
than the 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 exam‐
ple:

smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1.1

also disables any protocol versions 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 not to disable
TLSv1, except perhaps via the policy table for destinations which you are sure will support
"TLSv1.2".

See the documentation of the smtp_tls_policy_maps parameter and TLS_README for more informa‐
tion about security levels.

Example:
# Preferred syntax with Postfix >= 3.6:
smtp_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
# Legacy syntax:
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

This feature is available in Postfix 2.3 and later.

smtp_tls_note_starttls_offer (default: no)
Log the hostname of a remote SMTP server that offers STARTTLS, when TLS is not already en‐
abled for that server.

The logfile record looks like:

postfix/smtp[pid]: Host offered STARTTLS: [name.of.host]

This feature is available in Postfix 2.2 and later.

smtp_tls_per_site (default: empty)
Optional lookup tables with the Postfix SMTP client TLS usage policy by next-hop destination
and by remote SMTP server hostname. When both lookups succeed, the more specific per-site
policy (NONE, MUST, etc.) overrides the less specific one (MAY), and the more secure per-site
policy (MUST, etc.) overrides the less secure one (NONE). With Postfix 2.3 and later
smtp_tls_per_site is strongly discouraged: use smtp_tls_policy_maps instead.

Use of the bare hostname as the per-site table lookup key is discouraged. Always use the full
destination nexthop (enclosed in [] with a possible ":port" suffix). A recipient domain or
MX-enabled transport next-hop with no port suffix may look like a bare hostname, but is still
a suitable destination.

Specify a next-hop destination or server hostname on the left-hand side; no wildcards are al‐
lowed. The next-hop destination is either the recipient domain, or the destination specified
with a transport(5) table, the relayhost parameter, or the relay_transport parameter. On the
right hand side specify one of the following keywords:

NONE Don't use TLS at all. This overrides a less specific MAY lookup result from the alter‐
nate host or next-hop lookup key, and overrides the global smtp_use_tls, smtp_en‐
force_tls, and smtp_tls_enforce_peername settings.

MAY Try to use TLS if the server announces support, otherwise use an unencrypted connec‐
tion. This has less precedence than a more specific result (including NONE) from the
alternate host or next-hop lookup key, and has less precedence than the more specific
global "smtp_enforce_tls = yes" or "smtp_tls_enforce_peername = yes".

MUST_NOPEERMATCH
Require TLS encryption, but do not require that the remote SMTP server hostname
matches the information in the remote SMTP server certificate, or that the server cer‐
tificate was issued by a trusted CA. This overrides a less secure NONE or a less spe‐
cific MAY lookup result from the alternate host or next-hop lookup key, and overrides
the global smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.

MUST Require TLS encryption, require that the remote SMTP server hostname matches the in‐
formation in the remote SMTP server certificate, and require that the remote SMTP
server certificate was issued by a trusted CA. This overrides a less secure NONE or
MUST_NOPEERMATCH or a less specific MAY lookup result from the alternate host or
next-hop lookup key, and overrides the global smtp_use_tls, smtp_enforce_tls and
smtp_tls_enforce_peername settings.

The above keywords correspond to the "none", "may", "encrypt" and "verify" security levels
for the new smtp_tls_security_level parameter introduced in Postfix 2.3. Starting with Post‐
fix 2.3, and independently of how the policy is specified, the smtp_tls_mandatory_ciphers and
smtp_tls_mandatory_protocols parameters apply when TLS encryption is mandatory. Connections
for which encryption is optional typically enable all "export" grade and better ciphers (see
smtp_tls_ciphers and smtp_tls_protocols).

As long as no secure DNS lookup mechanism is available, false hostnames in MX or CNAME re‐
sponses can change the server hostname that Postfix uses for TLS policy lookup and server
certificate verification. Even with a perfect match between the server hostname and the
server certificate, there is no guarantee that Postfix is connected to the right server. See
TLS_README (Closing a DNS loophole with obsolete per-site TLS policies) for a possible
work-around.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use
smtp_tls_policy_maps instead.

smtp_tls_policy_maps (default: empty)
Optional lookup tables with the Postfix SMTP client TLS security policy by next-hop destina‐
tion; when a non-empty value is specified, this overrides the obsolete smtp_tls_per_site pa‐
rameter. See TLS_README for a more detailed discussion of TLS security levels.

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

The TLS policy table is indexed by the full next-hop destination, which is either the recipi‐
ent domain, or the verbatim next-hop specified in the transport table, $local_transport,
$virtual_transport, $relay_transport or $default_transport. This includes any enclosing
square brackets and any non-default destination server port suffix. The LMTP socket type pre‐
fix (inet: or unix:) is not included in the lookup key.

Only the next-hop domain, or $myhostname with LMTP over UNIX-domain sockets, is used as the
nexthop name for certificate verification. The port and any enclosing square brackets are
used in the table lookup key, but are not used for server name verification.

When the lookup key is a domain name without enclosing square brackets or any :port suffix
(typically the recipient domain), and the full domain is not found in the table, just as with
the transport(5) table, the parent domain starting with a leading "." is matched recursively.
This allows one to specify a security policy for a recipient domain and all its sub-domains.

The lookup result is a security level, followed by an optional list of whitespace and/or
comma separated name=value attributes that override related main.cf settings. The TLS secu‐
rity levels in order of increasing security are:

none No TLS. No additional attributes are supported at this level.

may Opportunistic TLS. Since sending in the clear is acceptable, demanding stronger than
default TLS security merely reduces interoperability. The optional "ciphers", "ex‐
clude", and "protocols" attributes (available for opportunistic TLS with Postfix >=
2.6) and "connection_reuse" attribute (Postfix >= 3.4) override the "smtp_tls_ci‐
phers", "smtp_tls_exclude_ciphers", "smtp_tls_protocols", and "smtp_tls_connec‐
tion_reuse" configuration parameters. In the policy table, multiple ciphers, protocols
or excluded ciphers must be separated by colons, as attribute values may not contain
whitespace or commas. When opportunistic TLS handshakes fail, Postfix retries the con‐
nection 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" attribute
overrides the main.cf smtp_tls_mandatory_protocols parameter, the optional "ciphers"
attribute overrides the main.cf smtp_tls_mandatory_ciphers parameter, the optional
"exclude" attribute (Postfix >= 2.6) overrides the main.cf smtp_tls_mandatory_ex‐
clude_ciphers parameter, and the optional "connection_reuse" attribute (Postfix >=
3.4) overrides the main.cf smtp_tls_connection_reuse parameter. In the policy table,
multiple ciphers, protocols or excluded ciphers must be separated by colons, as at‐
tribute 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 optional "connec‐
tion_reuse" attribute (Postfix >= 3.4) overrides the main.cf smtp_tls_connection_reuse
parameter. When the effective security level used is may, the optional "ciphers",
"exclude", and "protocols" attributes (Postfix >= 2.6) override the "smtp_tls_ci‐
phers", "smtp_tls_exclude_ciphers", and "smtp_tls_protocols" configuration parameters.
When the effective security level used is encrypt, the optional "ciphers", "exclude",
and "protocols" attributes (Postfix >= 2.6) override the "smtp_tls_mandatory_ciphers",
"smtp_tls_mandatory_exclude_ciphers", and "smtp_tls_mandatory_protocols" configuration
parameters.

dane-only
Mandatory DANE TLS. The TLS policy for the destination is obtained via TLSA records
in DNSSEC. If no TLSA records are found, or none are usable, no connection is made to
the server. When usable TLSA records are obtained for the remote SMTP server, the
server certificate must match the TLSA records. RFC 7672 (DANE) TLS authentication
and DNSSEC support is available with Postfix 2.11 and later. The optional "ciphers",
"exclude", and "protocols" attributes (Postfix >= 2.6) override the "smtp_tls_manda‐
tory_ciphers", "smtp_tls_mandatory_exclude_ciphers", and "smtp_tls_mandatory_proto‐
cols" configuration parameters. The optional "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" attribute,
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 fingerprint is selected by the
smtp_tls_fingerprint_digest parameter. Multiple fingerprints can be combined with a
"|" delimiter in a single match attribute, or multiple match attributes can be em‐
ployed. The ":" character is not used as a delimiter as it occurs between each pair of
fingerprint (hexadecimal) digits. The optional "ciphers", "exclude", and "protocols"
attributes (Postfix >= 2.6) override the "smtp_tls_mandatory_ciphers",
"smtp_tls_mandatory_exclude_ciphers", and "smtp_tls_mandatory_protocols" configuration
parameters. The optional "connection_reuse" attribute (Postfix >= 3.4) overrides the
main.cf smtp_tls_connection_reuse parameter.

verify Mandatory TLS verification. At this security level, DNS MX lookups are trusted to be
secure enough, and the name verified in the server certificate is usually obtained in‐
directly via unauthenticated DNS MX lookups. The optional "match" attribute overrides
the main.cf smtp_tls_verify_cert_match parameter. In the policy table, multiple match
patterns and strategies must be separated by colons. In practice explicit control
over matching is more common with the "secure" policy, described below. The optional
"ciphers", "exclude", and "protocols" attributes (Postfix >= 2.6) override the
"smtp_tls_mandatory_ciphers", "smtp_tls_mandatory_exclude_ciphers", and
"smtp_tls_mandatory_protocols" configuration parameters. The optional "connec‐
tion_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 secure
enough for TLS peername verification. Instead, the default name verified in the server
certificate is obtained directly from the next-hop, or is explicitly specified via the
optional "match" attribute which overrides the main.cf smtp_tls_secure_cert_match pa‐
rameter. In the policy table, multiple match patterns and strategies must be separated
by colons. The match attribute is most useful when multiple domains are supported by
a common server: the policy entries for additional domains specify matching rules for
the primary domain certificate. While transport table overrides that route the sec‐
ondary domains to the primary nexthop also allow secure verification, they risk deliv‐
ery to the wrong destination when domains change hands or are re-assigned to new gate‐
ways. With the "match" attribute approach, routing is not perturbed, and mail is de‐
ferred if verification of a new MX host fails. The optional "ciphers", "exclude", and
"protocols" attributes (Postfix >= 2.6) override the "smtp_tls_mandatory_ciphers",
"smtp_tls_mandatory_exclude_ciphers", and "smtp_tls_mandatory_protocols" configuration
parameters. The optional "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 "proto‐
cols" attribute (see smtp_tls_policy_maps) the only valid separator is colon. An empty value
means allow all protocols.

As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set the
lowest acceptable TLS protocol version and/or the highest acceptable TLS protocol version.
To set the lower bound include an element of the form: ">=version" where version is either
one of the TLS protocol names listed above, or a hexadecimal number corresponding to the de‐
sired TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound,
use "<=version". There must be no whitespace between the ">=" or "<=" symbols and the proto‐
col name or number.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtp_tls_protocols = >=TLSv1, <=0305
# Allow only TLS 1.0 and up:
smtp_tls_protocols = >=0x0301

With Postfix < 3.6 there is no support for a minimum or maximum version, and the protocol
range is configured via protocol exclusions. To require at least TLS 1.0, set "smtp_tls_pro‐
tocols = !SSLv2, !SSLv3". Listing the protocols to include, rather than protocols to ex‐
clude, is supported, but not recommended. The exclusion form more accurately matches the un‐
derlying OpenSSL interface.

When using the exclusion syntax, take care to ensure that the range of protocols advertised
by an SSL/TLS client is contiguous. When a protocol version is enabled, disabling any higher
version implicitly disables all versions above that higher version. Thus, for example:

smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1.1
also disables any protocols version higher than TLSv1.1 leaving only "TLSv1" enabled.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this protocol via
"!TLSv1.3" is supported since Postfix 3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and
3.3.2).

Example:
# Preferred syntax with Postfix >= 3.6:
smtp_tls_protocols = >=TLSv1, <=TLSv1.3
# Legacy syntax:
smtp_tls_protocols = !SSLv2, !SSLv3

This feature is available in Postfix 2.6 and later.

smtp_tls_scert_verifydepth (default: 9)
The verification depth for remote SMTP server certificates. A depth of 1 is sufficient if the
issuing CA is listed in a local CA file.

The default verification depth is 9 (the OpenSSL default) for compatibility with earlier
Postfix behavior. Prior to Postfix 2.5, the default value was 5, but the limit was not actu‐
ally enforced. If you have set this to a lower non-default value, certificates with longer
trust chains may now fail to verify. Certificate chains with 1 or 2 CAs are common, deeper
chains are more rare and any number between 5 and 9 should suffice in practice. You can
choose a lower number if, for example, you trust certificates directly signed by an issuing
CA but not any CAs it delegates to.

This feature is available in Postfix 2.2 and later.

smtp_tls_secure_cert_match (default: nexthop, dot-nexthop)
How the Postfix SMTP client verifies the server certificate peername for the "secure" TLS se‐
curity level. In a "secure" TLS policy table ($smtp_tls_policy_maps) entry the optional
"match" attribute overrides this main.cf setting.

This parameter specifies one or more patterns or strategies separated by commas, whitespace
or colons. In the policy table the only valid separator is the colon character.

For a description of the pattern and strategy syntax see the smtp_tls_verify_cert_match para‐
meter. The "hostname" strategy should be avoided in this context, as in the absence of a se‐
cure 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; when no value is specified for smtp_tls_enforce_peername or the
obsolete parameters, the default SMTP TLS security level is none.

Specify one of the following security levels:

none No TLS. TLS will not be used unless enabled for specific destinations via
smtp_tls_policy_maps.

may Opportunistic TLS. Use TLS if this is supported by the remote SMTP server, otherwise
use plaintext. Since sending in the clear is acceptable, demanding stronger than de‐
fault TLS security merely reduces interoperability. The "smtp_tls_ciphers" and
"smtp_tls_protocols" (Postfix >= 2.6) configuration parameters provide control over
the protocols and cipher grade used with opportunistic TLS. With earlier releases the
opportunistic TLS cipher grade is always "export" and no protocols are disabled. When
TLS handshakes fail, the connection is retried with TLS disabled. This allows mail
delivery to sites with non-interoperable TLS implementations.

encrypt
Mandatory TLS encryption. Since a minimum level of security is intended, it is reason‐
able to be specific about sufficiently secure protocol versions and ciphers. At this
security level and higher, the main.cf parameters smtp_tls_mandatory_protocols and
smtp_tls_mandatory_ciphers specify the TLS protocols and minimum cipher grade which
the administrator considers secure enough for mandatory encrypted sessions. This secu‐
rity level is not an appropriate default for systems delivering mail to the Internet.

dane Opportunistic DANE TLS. At this security level, the TLS policy for the destination is
obtained via DNSSEC. For TLSA policy to be in effect, the destination domain's con‐
taining DNS zone must be signed and the Postfix SMTP client's operating system must be
configured to send its DNS queries to a recursive DNS nameserver that is able to vali‐
date the signed records. Each MX host's DNS zone should also be signed, and should
publish DANE TLSA (RFC 7672) records that specify how that MX host's TLS certificate
is to be verified. TLSA records do not preempt the normal SMTP MX host selection al‐
gorithm, if some MX hosts support TLSA and others do not, TLS security will vary from
delivery to delivery. It is up to the domain owner to configure their MX hosts and
their DNS sensibly. To configure the Postfix SMTP client for DNSSEC lookups see the
documentation for the smtp_dns_support_level main.cf parameter. When DNSSEC-validated
TLSA records are not found the effective tls security level is "may". When TLSA
records are found, but are all unusable the effective security level is "encrypt".
For purposes of protocol and cipher selection, the "dane" security level is treated
like a "mandatory" TLS security level, and weak ciphers and protocols are disabled.
Since DANE authenticates server certificates the "aNULL" cipher-suites are transpar‐
ently excluded at this level, no need to configure this manually. RFC 7672 (DANE) TLS
authentication is available with Postfix 2.11 and later.

dane-only
Mandatory DANE TLS. This is just like "dane" above, but DANE TLSA authentication is
required. There is no fallback to "may" or "encrypt" when TLSA records are missing or
unusable. RFC 7672 (DANE) TLS authentication is available with Postfix 2.11 and
later.

fingerprint
Certificate fingerprint verification. At this security level, there are no trusted
Certification Authorities. The certificate trust chain, expiration date, etc., are
not checked. Instead, the smtp_tls_fingerprint_cert_match parameter lists the certifi‐
cate fingerprint or public key fingerprint (Postfix 2.9 and later) of the valid server
certificate. The digest algorithm used to calculate the fingerprint is selected by the
smtp_tls_fingerprint_digest parameter. Available with Postfix 2.5 and later.

verify Mandatory TLS verification. At this security level, DNS MX lookups are trusted to be
secure enough, and the name verified in the server certificate is usually obtained in‐
directly via unauthenticated DNS MX lookups. The smtp_tls_verify_cert_match parameter
controls how the server name is verified. In practice explicit control over matching
is more common at the "secure" level, described below. This security level is not an
appropriate default for systems delivering mail to the Internet.

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 protocols unless it is essential
# to do so (if a security vulnerability is found in the SSL library that
# can be mitigated by disabling a particular protocol or raising the
# cipher grade).
smtp_tls_ciphers = medium
smtp_tls_protocols = >=TLSv1
# Legacy (Postfix < 3.6) syntax:
smtp_tls_protocols = !SSLv2, !SSLv3

# Mandatory (high-grade) TLS encryption.
smtp_tls_security_level = encrypt
smtp_tls_mandatory_ciphers = high

# Authenticated TLS 1.2 or better matching the nexthop domain or a
# subdomain.
smtp_tls_security_level = secure
smtp_tls_mandatory_ciphers = high
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_secure_cert_match = nexthop, dot-nexthop

# Certificate fingerprint verification (Postfix >= 2.5).
# The CA-less "fingerprint" security level only scales to a limited
# number of destinations. As a global default rather than a per-site
# setting, this is practical only when mail for all recipients is sent
# to a central mail hub.
relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_mandatory_ciphers = high
smtp_tls_fingerprint_cert_match =
3D:95:34:51:...:40:99:C0:C1
EC:3B:2D:B0:...:A3:9D:72:F6

This feature is available in Postfix 2.3 and later.

smtp_tls_servername (default: empty)
Optional name to send to the remote SMTP server in the TLS Server Name Indication (SNI) ex‐
tension. The SNI extension is always on when DANE is used to authenticate the server, and in
that case the SNI name sent is the one required by RFC7672 and this parameter is ignored.

Some SMTP servers use the received SNI name to select an appropriate certificate chain to
present to the client. While this may improve interoperability with such servers, it may re‐
duce interoperability with other servers that choose to abort the connection when they don't
have a certificate chain configured for the requested name. Such servers should select a de‐
fault certificate chain and continue the handshake, but some may not. Therefore, absent
DANE, no SNI name is sent by default.

The SNI name must be either a valid DNS hostname, or else one of the special values hostname
or nexthop, which select either the remote hostname or the nexthop domain respectively. DNS
names for SNI must be in A-label (punycode) form. Invalid DNS names log a configuration er‐
ror warning and mail delivery is deferred.

Except when using a relayhost to forward all email, the only sensible non-empty main.cf set‐
ting for this parameter is hostname. Other non-empty values are only practical on a per-des‐
tination basis via the servername attribute of the Postfix TLS policy table. When in doubt,
leave this parameter empty, and configure per-destination SNI as needed.

This feature is available in Postfix 3.4 and later.

smtp_tls_session_cache_database (default: empty)
Name of the file containing the optional Postfix SMTP client TLS session cache. Specify a
database type that supports enumeration, such as btree or sdbm; there is no need to support
concurrent access. The file is created if it does not exist. The smtp(8) 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_ses‐
sion_cache_database, $smtp_tls_session_cache_database (and with Postfix 2.3 and later
$lmtp_tls_session_cache_database), needs to be stored separately. It is not at this time pos‐
sible to store multiple caches in a single database.

Note: dbm databases are not suitable. TLS session objects are too large.

As of version 2.5, Postfix no longer uses root privileges when opening this file. The file
should now be stored under the Postfix-owned data_directory. As a migration aid, an attempt
to open the file under a non-Postfix directory is redirected to the Postfix-owned data_direc‐
tory, and a warning is logged.

Example:

smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_scache

This feature is available in Postfix 2.2 and later.

smtp_tls_session_cache_timeout (default: 3600s)
The expiration time of Postfix SMTP client TLS session cache information. A cache cleanup is
performed periodically every $smtp_tls_session_cache_timeout seconds. As with $smtp_tls_ses‐
sion_cache_database, this parameter is implemented in the tlsmgr(8) daemon and therefore
per-smtp-instance master.cf overrides are not possible.

As of Postfix 2.11 this setting cannot exceed 100 days. If set <= 0, session caching is dis‐
abled. If set to a positive value less than 2 minutes, the minimum value of 2 minutes is
used instead.

This feature is available in Postfix 2.2 and later.

smtp_tls_trust_anchor_file (default: empty)
Zero or more PEM-format files with trust-anchor certificates and/or public keys. If the pa‐
rameter is not empty the root CAs in CAfile and CApath are no longer trusted. Rather, the
Postfix SMTP client will only trust certificate-chains signed by one of the trust-anchors
contained in the chosen files. The specified trust-anchor certificates and public keys are
not subject to expiration, and need not be (self-signed) root CAs. They may, if desired, be
intermediate certificates. Therefore, these certificates also may be found "in the middle" of
the trust chain presented by the remote SMTP server, and any untrusted issuing parent cer‐
tificates will be ignored. Specify a list of pathnames separated by comma or whitespace.

Whether specified in main.cf, or on a per-destination basis, the trust-anchor PEM file must
be accessible to the Postfix SMTP client in the chroot jail if applicable. The trust-anchor
file should contain only certificates and public keys, no private key material, and must be
readable by the non-privileged $mail_owner user. This allows destinations to be bound to a
set of specific CAs or public keys without trusting the same CAs for all destinations.

The main.cf parameter supports single-purpose Postfix installations that send mail to a fixed
set of SMTP peers. At most sites, if trust-anchor files are used at all, they will be speci‐
fied on a per-destination basis via the "tafile" attribute of the "verify" and "secure" lev‐
els in smtp_tls_policy_maps.

The underlying mechanism is in support of RFC 7672 (DANE TLSA), which defines mechanisms for
an SMTP client MTA to securely determine server TLS certificates via DNS.

If you want your trust anchors to be public keys, with OpenSSL you can extract a single PEM
public key from a PEM X.509 file containing a single certificate, as follows:

$ openssl x509 -in cert.pem -out ta-key.pem -noout -pubkey

This feature is available in Postfix 2.11 and later.

smtp_tls_verify_cert_match (default: hostname)
How the Postfix SMTP client verifies the server certificate peername for the "verify" TLS se‐
curity level. In a "verify" TLS policy table ($smtp_tls_policy_maps) entry the optional
"match" attribute overrides this main.cf setting.

This parameter specifies one or more patterns or strategies separated by commas, whitespace
or colons. In the policy table the only valid separator is the colon character.

Patterns specify domain names, or domain name suffixes:

example.com
Match the example.com domain, i.e. one of the names in the server certificate must be
example.com. Upper and lower case distinctions are ignored.

.example.com
Match subdomains of the example.com domain, i.e. match a name in the server certifi‐
cate that consists of a non-zero number of labels followed by a .example.com suffix.
Case distinctions are ignored.

Strategies specify a transformation from the next-hop domain to the expected name in the
server certificate:

nexthop
Match against the next-hop domain, which is either the recipient domain, or the trans‐
port next-hop configured for the domain stripped of any optional socket type prefix,
enclosing square brackets and trailing port. When MX lookups are not suppressed, this
is the original nexthop domain prior to the MX lookup, not the result of the MX
lookup. For LMTP delivery via UNIX-domain sockets, the verified next-hop name is $my‐
hostname. This strategy is suitable for use with the "secure" policy. Case is ig‐
nored.

dot-nexthop
As above, but match server certificate names that are subdomains of the next-hop do‐
main. Case is ignored.

hostname
Match against the hostname of the server, often obtained via an unauthenticated DNS MX
lookup. For LMTP delivery via UNIX-domain sockets, the verified name is $myhostname.
This matches the verification strategy of the "MUST" keyword in the obsolete
smtp_tls_per_site table, and is suitable for use with the "verify" security level.
When the next-hop name is enclosed in square brackets to suppress MX lookups, the
"hostname" strategy is the same as the "nexthop" strategy. Case is ignored.

Sample main.cf setting:

smtp_tls_verify_cert_match = hostname, nexthop, dot-nexthop

Sample policy table override:

example.com verify match=hostname:nexthop
.example.com verify match=example.com:.example.com:hostname

This feature is available in Postfix 2.3 and later.

smtp_tls_wrappermode (default: no)
Request that the Postfix SMTP client connects using the SUBMISSIONS/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, otherwise
send the mail in the clear. Beware: some SMTP servers offer STARTTLS even if it is not con‐
figured. With Postfix < 2.3, if the TLS handshake fails, and no other server is available,
delivery is deferred and mail stays in the queue. If this is a concern for you, use the
smtp_tls_per_site feature instead.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use
smtp_tls_security_level instead.

smtp_xforward_timeout (default: 300s)
The Postfix SMTP client time limit for sending the XFORWARD command, and for receiving the
remote SMTP server response.

This feature is available in Postfix 2.1 and later.

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

By default, no clients are allowed to specify XVERP.

This parameter was renamed with Postfix version 2.1. The default value is backwards compati‐
ble with Postfix version 2.0.

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

smtpd_authorized_xclient_hosts (default: empty)
What remote SMTP clients are allowed to use the XCLIENT feature. This command overrides re‐
mote SMTP client information that is used for access control. Typical use is for SMTP-based
content filters, fetchmail-like programs, or SMTP server access rule testing. See the
XCLIENT_README document for details.

This feature is available in Postfix 2.1 and later.

By default, no clients are allowed to specify XCLIENT.

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

smtpd_authorized_xforward_hosts (default: empty)
What remote SMTP clients are allowed to use the XFORWARD feature. This command forwards in‐
formation that is used to improve logging after SMTP-based content filters. See the XFOR‐
WARD_README document for details.

This feature is available in Postfix 2.1 and later.

By default, no clients are allowed to specify XFORWARD.

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

smtpd_banner (default: $myhostname ESMTP $mail_name)
The text that follows the 220 status code in the SMTP greeting banner. Some people like to
see the mail version advertised. By default, Postfix shows no version.

You MUST specify $myhostname at the start of the text. This is required by the SMTP protocol.

Example:

smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)

smtpd_client_auth_rate_limit (default: 0)
The maximal number of AUTH commands that any client is allowed to send to this service per
time unit, regardless of whether or not Postfix actually accepts those commands. The time
unit is specified with the anvil_rate_time_unit configuration parameter.

By default, there is no limit on the number of AUTH commands that a client may send.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be used to regulate le‐
gitimate mail traffic.

This feature is available in Postfix 3.1 and later.

smtpd_client_connection_count_limit (default: 50)
How many simultaneous connections any client is allowed to make to this service. By default,
the limit is set to half the default process limit value.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be used to regulate le‐
gitimate mail traffic.

This feature is available in Postfix 2.2 and later.

smtpd_client_connection_rate_limit (default: 0)
The maximal number of connection attempts any client is allowed to make to this service per
time unit. The time unit is specified with the anvil_rate_time_unit configuration parameter.

By default, a client can make as many connections per time unit as Postfix can accept.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be used to regulate le‐
gitimate mail traffic.

This feature is available in Postfix 2.2 and later.

Example:

smtpd_client_connection_rate_limit = 1000

smtpd_client_event_limit_exceptions (default: $mynetworks)
Clients that are excluded from smtpd_client_*_count/rate_limit restrictions. See the mynet‐
works parameter description for the parameter value syntax.

By default, clients in trusted networks are excluded. Specify a list of network blocks, host‐
names or .domain names (the initial dot causes the domain to match any name below it).

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

Pattern matching of domain names is controlled by the presence or absence of
"smtpd_client_event_limit_exceptions" in the parent_domain_matches_subdomains parameter value
(Postfix 3.0 and later).

This feature is available in Postfix 2.2 and later.

smtpd_client_ipv4_prefix_length (default: 32)
Aggregate smtpd_client_*_count and smtpd_client_*_rate statistics by IPv4 network blocks with
the specified network prefix. Aggregation uses fewer anvil(8) resources to maintain counters.
By default, aggregation is disabled for IPv4.

This feature is available in Postfix 3.8 and later.

smtpd_client_ipv6_prefix_length (default: 84)
Aggregate smtpd_client_*_count and smtpd_client_*_rate statistics by IPv6 network blocks with
the specified network prefix. Aggregation uses fewer the anvil(8) resources to maintain coun‐
ters. By default, aggregation is enabled for IPv6.

This feature is available in Postfix 3.8 and later.

smtpd_client_message_rate_limit (default: 0)
The maximal number of message delivery requests that any client is allowed to make to this
service per time unit, regardless of whether or not Postfix actually accepts those messages.
The time unit is specified with the anvil_rate_time_unit configuration parameter.

By default, a client can send as many message delivery requests per time unit as Postfix can
accept.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be used to regulate le‐
gitimate mail traffic.

This feature is available in Postfix 2.2 and later.

Example:

smtpd_client_message_rate_limit = 1000

smtpd_client_new_tls_session_rate_limit (default: 0)
The maximal number of new (i.e., uncached) TLS sessions that a remote SMTP client is allowed
to negotiate with this service per time unit. The time unit is specified with the
anvil_rate_time_unit configuration parameter.

By default, a remote SMTP client can negotiate as many new TLS sessions per time unit as
Postfix can accept.

To disable this feature, specify a limit of 0. Otherwise, specify a limit that is at least
the per-client concurrent session limit, or else legitimate client sessions may be rejected.

WARNING: The purpose of this feature is to limit abuse. It must not be used to regulate le‐
gitimate mail traffic.

This feature is available in Postfix 2.3 and later.

Example:

smtpd_client_new_tls_session_rate_limit = 100

smtpd_client_port_logging (default: no)
Enable logging of the remote SMTP client port in addition to the hostname and IP address. The
logging format is "host[address]:port".

This feature is available in Postfix 2.5 and later.

smtpd_client_recipient_rate_limit (default: 0)
The maximal number of recipient addresses that any client is allowed to send to this service
per time unit, regardless of whether or not Postfix actually accepts those recipients. The
time unit is specified with the anvil_rate_time_unit configuration parameter.

By default, a client can send as many recipient addresses per time unit as Postfix can ac‐
cept.

To disable this feature, specify a limit of 0.

WARNING: The purpose of this feature is to limit abuse. It must not be used to regulate le‐
gitimate mail traffic.

This feature is available in Postfix 2.2 and later.

Example:

smtpd_client_recipient_rate_limit = 1000

smtpd_client_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the context of a client connec‐
tion request. See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restric‐
tion lists" for a discussion of evaluation context and time.

The default is to allow all connection requests.

Specify a list of restrictions, separated by commas and/or whitespace. Continue long lines
by starting the next line with whitespace. Restrictions are applied in the order as speci‐
fied; the first restriction that matches wins.

The following restrictions are specific to client hostname or client network address informa‐
tion.

check_ccert_access type:table
By default use the remote SMTP client certificate fingerprint or the public key fin‐
gerprint (Postfix 2.9 and later) as the lookup key for the specified access(5) data‐
base; with Postfix version 2.2, also require that the remote SMTP client certificate
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 cryptanalysis have led to
md5 and sha1 being deprecated in favor of sha256. However, as long as there are no
known "second pre-image" attacks against the older algorithms, their use in this con‐
text, though not recommended, is still likely safe.
Alternatively, check_ccert_access accepts an explicit search order (Postfix 3.5 and
later). The default search order as described above corresponds with:
check_ccert_access { type:table, { search_order = cert_fingerprint, pubkey_fingerprint
} }
The commas are optional.

check_client_access type:table
Search the specified access database for the client hostname, parent domains, client
IP address, or networks obtained by stripping least significant octets. See the ac‐
cess(5) manual page for details.

check_client_a_access type:table
Search the specified access(5) database for the IP addresses for the client hostname,
and execute the corresponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
This feature is available in Postfix 3.0 and later.

check_client_mx_access type:table
Search the specified access(5) 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 hostname,
and execute the corresponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
This feature is available in Postfix 2.7 and later.

check_reverse_client_hostname_access type:table
Search the specified access database for the unverified reverse client hostname, par‐
ent domains, client IP address, or networks obtained by stripping least significant
octets. See the access(5) manual page for details. Note: a result of "OK" is not al‐
lowed for safety reasons. Instead, use DUNNO in order to exclude specific hosts from
denylists. This feature is available in Postfix 2.6 and later.

check_reverse_client_hostname_a_access type:table
Search the specified access(5) 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 reverse
client hostname, and execute the corresponding action. Note: a result of "OK" is not
allowed for safety reasons. Instead, use DUNNO in order to exclude specific hosts
from denylists. This feature is available in Postfix 2.7 and later.

check_sasl_access type:table
Use the remote SMTP client SASL user name as the lookup key for the specified ac‐
cess(5) database. The lookup key has the form "username@domainname" when the
smtpd_sasl_local_domain parameter value is non-empty. Unlike the check_client_access
feature, check_sasl_access does not perform matches of parent domains or IP subnet
ranges. This feature is available with Postfix version 2.11 and later.

permit_inet_interfaces
Permit the request when the client IP address matches $inet_interfaces.

permit_mynetworks
Permit the request when the client IP address matches any network or network address
listed in $mynetworks.

permit_sasl_authenticated
Permit the request when the client is successfully authenticated via the RFC 4954
(AUTH) protocol.

permit_tls_all_clientcerts
Permit the request when the remote SMTP client certificate is verified successfully.
This option must be used only if a special CA issues the certificates and only this CA
is listed as a trusted CA. Otherwise, clients with a third-party certificate would
also be allowed to relay. Specify "tls_append_default_CA = no" when the trusted CA is
specified with smtpd_tls_CAfile or smtpd_tls_CApath, to prevent Postfix from appending
the system-supplied default CAs. This feature requires "smtpd_tls_ask_ccert = yes"
and is available with Postfix version 2.2 and later.

permit_tls_clientcerts
Permit the request when the remote SMTP client certificate fingerprint or public key
fingerprint (Postfix 2.9 and later) is listed in $relay_clientcerts. The fingerprint
digest algorithm is configurable via the smtpd_tls_fingerprint_digest parameter
(hard-coded as md5 prior to Postfix version 2.5). This feature requires
"smtpd_tls_ask_ccert = yes" and is available with Postfix version 2.2 and later.
The default algorithm is sha256 with Postfix >= 3.6 and the compatibility_level set to
3.6 or higher. With Postfix <= 3.5, the default algorithm is md5. The best-practice
algorithm is now sha256. Recent advances in hash function cryptanalysis have led to
md5 and sha1 being deprecated in favor of sha256. However, as long as there are no
known "second pre-image" attacks against the older algorithms, their use in this con‐
text, though not recommended, is still likely safe.

reject_rbl_client rbl_domain=d.d.d.d
Reject the request when the reversed client network address is listed with the A
record "d.d.d.d" under rbl_domain (Postfix version 2.1 and later only). Each "d" is a
number, or a pattern inside "[]" that contains one or more ";"-separated numbers or
number..number ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is specified,
reject the request when the reversed client network address is listed with any A
record under rbl_domain.
The maps_rbl_reject_code parameter specifies the response code for rejected requests
(default: 554), the default_rbl_reply parameter specifies the default server reply,
and the rbl_reply_maps parameter specifies tables with server replies indexed by
rbl_domain. This feature is available in Postfix 2.0 and later.

permit_dnswl_client dnswl_domain=d.d.d.d
Accept the request when the reversed client network address is listed with the A
record "d.d.d.d" under dnswl_domain. Each "d" is a number, or a pattern inside "[]"
that contains one or more ";"-separated numbers or number..number ranges. If no
"=d.d.d.d" is specified, accept the request when the reversed client network address
is listed with any A record under dnswl_domain.
For safety, permit_dnswl_client is silently ignored when it would override re‐
ject_unauth_destination. The result is DEFER_IF_REJECT when allowlist lookup fails.
This feature is available in Postfix 2.8 and later.

reject_rhsbl_client rbl_domain=d.d.d.d
Reject the request when the client hostname is listed with the A record "d.d.d.d" un‐
der rbl_domain (Postfix version 2.1 and later only). Each "d" is a number, or a pat‐
tern inside "[]" that contains one or more ";"-separated numbers or number..number
ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is specified, reject the re‐
quest when the client hostname is listed with any A record under rbl_domain. See the
reject_rbl_client description above for additional RBL related configuration parame‐
ters. This feature is available in Postfix 2.0 and later; with Postfix version 2.8
and later, reject_rhsbl_reverse_client will usually produce better results.

permit_rhswl_client rhswl_domain=d.d.d.d
Accept the request when the client hostname is listed with the A record "d.d.d.d" un‐
der rhswl_domain. Each "d" is a number, or a pattern inside "[]" that contains one or
more ";"-separated numbers or number..number ranges. If no "=d.d.d.d" is specified,
accept the request when the client hostname is listed with any A record under
rhswl_domain.
Caution: client name allowlisting is fragile, since the client name lookup can fail
due to temporary outages. Client name allowlisting should be used only to reduce
false positives in e.g. DNS-based blocklists, and not for making access rule excep‐
tions.
For safety, permit_rhswl_client is silently ignored when it would override re‐
ject_unauth_destination. The result is DEFER_IF_REJECT when allowlist lookup fails.
This feature is available in Postfix 2.8 and later.

reject_rhsbl_reverse_client rbl_domain=d.d.d.d
Reject the request when the unverified reverse client hostname is listed with the A
record "d.d.d.d" under rbl_domain. Each "d" is a number, or a pattern inside "[]"
that contains one or more ";"-separated numbers or number..number ranges. If no
"=d.d.d.d" is specified, reject the request when the unverified reverse client host‐
name is listed with any A record under rbl_domain. See the reject_rbl_client descrip‐
tion above for additional RBL related configuration parameters. This feature is
available in Postfix 2.8 and later.

reject_unknown_client_hostname (with Postfix < 2.3: reject_unknown_client)
Reject the request when 1) the client IP address->name mapping fails, or 2) the
name->address mapping fails, or 3) the name->address mapping does not match the client
IP address.
This is a stronger restriction than the reject_unknown_reverse_client_hostname fea‐
ture, which triggers only under condition 1) above.
The unknown_client_reject_code parameter specifies the response code for rejected re‐
quests (default: 450). The reply is always 450 in case the address->name or name->ad‐
dress lookup failed due to a temporary problem.

reject_unknown_reverse_client_hostname
Reject the request when the client IP address has no address->name mapping.
This is a weaker restriction than the reject_unknown_client_hostname feature, which
requires not only that the address->name and name->address mappings exist, but also
that the two mappings reproduce the client IP address.
The unknown_client_reject_code parameter specifies the response code for rejected re‐
quests (default: 450). The reply is always 450 in case the address->name lookup
failed due to a temporary problem.
This feature is available in Postfix 2.3 and later.

In addition, you can use any of the following generic restrictions. These restrictions are
applicable in any SMTP command context.

check_policy_service servername
Query the specified policy server. See the SMTPD_POLICY_README document for details.
This feature is available in Postfix 2.1 and later.

defer Defer the request. The client is told to try again later. This restriction is useful
at the end of a restriction list, to make the default policy explicit.
The defer_code parameter specifies the SMTP server reply code (default: 450).

defer_if_permit
Defer the request if some later restriction would result in an explicit or implicit
PERMIT action. This is useful when a denylisting feature fails due to a temporary
problem. This feature is available in Postfix version 2.1 and later.

defer_if_reject
Defer the request if some later restriction would result in a REJECT action. This is
useful when an allowlisting feature fails due to a temporary problem. This feature is
available in Postfix version 2.1 and later.

permit Permit the request. This restriction is useful at the end of a restriction list, to
make the default policy explicit.

reject_multi_recipient_bounce
Reject the request when the envelope sender is the null address, and the message has
multiple envelope recipients. This usage has rare but legitimate applications: under
certain conditions, multi-recipient mail that was posted with the DSN option NO‐
TIFY=NEVER may be forwarded with the null sender address.
Note: this restriction can only work reliably when used in smtpd_data_restrictions or
smtpd_end_of_data_restrictions, because the total number of recipients is not known at
an earlier stage of the SMTP conversation. Use at the RCPT stage will only reject the
second etc. recipient.
The multi_recipient_bounce_reject_code parameter specifies the response code for re‐
jected requests (default: 550). This feature is available in Postfix 2.1 and later.

reject_plaintext_session
Reject the request when the connection is not encrypted. This restriction should not
be used before the client has had a chance to negotiate encryption with the AUTH or
STARTTLS commands.
The plaintext_reject_code parameter specifies the response code for rejected requests
(default: 450). This feature is available in Postfix 2.3 and later.

reject_unauth_pipelining
Reject the request when the client sends SMTP commands ahead of time where it is not
allowed, or when the client sends SMTP commands ahead of time without knowing that
Postfix actually supports ESMTP command pipelining. This stops mail from bulk mail
software that improperly uses ESMTP command pipelining in order to speed up deliver‐
ies.
With Postfix 2.6 and later, the SMTP server sets a per-session flag whenever it de‐
tects illegal pipelining, including pipelined HELO or EHLO commands. The re‐
ject_unauth_pipelining feature simply tests whether the flag was set at any point in
time during the session.
With older Postfix versions, reject_unauth_pipelining checks the current status of the
input read queue, and its usage is not recommended in contexts other than
smtpd_data_restrictions.

reject Reject the request. This restriction is useful at the end of a restriction list, to
make the default policy explicit. The reject_code configuration parameter specifies
the response code for rejected requests (default: 554).

sleep seconds
Pause for the specified number of seconds and proceed with the next restriction in the
list, if any. This may stop zombie mail when used as:
/etc/postfix/main.cf:
smtpd_client_restrictions =
sleep 1, reject_unauth_pipelining
smtpd_delay_reject = no
This feature is available in Postfix 2.3.

warn_if_reject
A safety net for testing. When "warn_if_reject" is placed before a reject-type re‐
striction, access table query, or check_policy_service query, this logs a "re‐
ject_warning" message instead of rejecting a request (when a reject-type restriction
fails due to a temporary error, this logs a "reject_warning" message for any implicit
"defer_if_permit" actions that would normally prevent mail from being accepted by some
later access restriction). This feature has no effect on defer_if_reject restrictions.

Other restrictions that are valid in this context:

• SMTP command specific restrictions that are described under the smtpd_helo_restric‐
tions, smtpd_sender_restrictions or smtpd_recipient_restrictions parameters. When
helo, sender or recipient restrictions are listed under smtpd_client_restrictions,
they have effect only with "smtpd_delay_reject = yes", so that $smtpd_client_restric‐
tions is evaluated at the time of the RCPT TO command.

Example:

smtpd_client_restrictions = permit_mynetworks, reject_unknown_client_hostname

smtpd_command_filter (default: empty)
A mechanism to transform commands from remote SMTP clients. This is a last-resort tool to
work around client commands that break interoperability with the Postfix SMTP server. Other
uses involve fault injection to test Postfix's handling of invalid commands.

Specify the name of a "type:table" lookup table. The search string is the SMTP command as re‐
ceived from the remote SMTP client, except that initial whitespace and the trailing <CR><LF>
are removed. The result value is executed by the Postfix SMTP server.

There is no need to use smtpd_command_filter for the following cases:

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

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

• Use "strict_rfc821_envelopes = no" to accept "RCPT TO:<User Name <user AT example.com>>".
Postfix will ignore the "User Name" part and deliver to the <user AT example.com> ad‐
dress.

Examples of problems that can be solved with the smtpd_command_filter feature:

/etc/postfix/main.cf:
smtpd_command_filter = pcre:/etc/postfix/command_filter

/etc/postfix/command_filter:
# Work around clients that send malformed HELO commands.
/^HELO\s*$/ HELO domain.invalid

# Work around clients that send empty lines.
/^\s*$/ NOOP

# Work around clients that send RCPT TO:<'user@domain'>.
# WARNING: do not lose the parameters that follow the address.
/^(RCPT\s+TO:\s*<)'([^[:space:]]+)'(>.*)/ $1$2$3

# Append XVERP to MAIL FROM commands to request VERP-style delivery.
# See VERP_README for more information on how to use Postfix VERP.
/^(MAIL\s+FROM:\s*<listname@example\.com>.*)/ $1 XVERP

# Bounce-never mail sink. Use notify_classes=bounce,resource,software
# to send bounced mail to the postmaster (with message body removed).
/^(RCPT\s+TO:\s*<.*>.*)\s+NOTIFY=\S+(.*)/ $1 NOTIFY=NEVER$2
/^(RCPT\s+TO:.*)/ $1 NOTIFY=NEVER

This feature is available in Postfix 2.7.

smtpd_data_restrictions (default: empty)
Optional access restrictions that the Postfix SMTP server applies in the context of the SMTP
DATA command. See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restric‐
tion lists" for a discussion of evaluation context and time.

This feature is available in Postfix 2.0 and later.

The following restrictions are valid in this context:

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

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

• However, no recipient information is available in the case of multi-recipient mail.
Acting on only one recipient would be misleading, because any decision will affect all
recipients equally. Acting on all recipients would require a possibly very large
amount of memory, and would also be misleading for the reasons mentioned before.

Examples:

smtpd_data_restrictions = reject_unauth_pipelining
smtpd_data_restrictions = reject_multi_recipient_bounce

smtpd_delay_open_until_valid_rcpt (default: yes)
Postpone the start of an SMTP mail transaction until a valid RCPT TO command is received.
Specify "no" to create a mail transaction as soon as the Postfix SMTP server receives a valid
MAIL FROM command.

With sites that reject lots of mail, the default setting reduces the use of disk, CPU and
memory resources. The downside is that rejected recipients are logged with NOQUEUE instead of
a mail transaction ID. This complicates the logfile analysis of multi-recipient mail.

This feature is available in Postfix 2.3 and later.

smtpd_delay_reject (default: yes)
Wait until the RCPT TO command before evaluating $smtpd_client_restrictions, $smtpd_helo_re‐
strictions and $smtpd_sender_restrictions, or wait until the ETRN command before evaluating
$smtpd_client_restrictions and $smtpd_helo_restrictions.

This feature is turned on by default because some clients apparently mis-behave when the
Postfix SMTP server rejects commands before RCPT TO.

The default setting has one major benefit: it allows Postfix to log recipient address infor‐
mation when rejecting a client name/address or sender address, so that it is possible to find
out whose mail is being rejected.

smtpd_discard_ehlo_keyword_address_maps (default: empty)
Lookup tables, indexed by the remote SMTP client address, with case insensitive lists of EHLO
keywords (pipelining, starttls, auth, etc.) that the Postfix SMTP server will not send in the
EHLO response to a remote SMTP client. See smtpd_discard_ehlo_keywords for details. The ta‐
bles are not searched by hostname for robustness reasons.

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

This feature is available in Postfix 2.2 and later.

smtpd_discard_ehlo_keywords (default: empty)
A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
SMTP server will not send in the EHLO response to a remote SMTP client.

This feature is available in Postfix 2.2 and later.

Notes:

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

• Use the smtpd_discard_ehlo_keyword_address_maps feature to discard EHLO keywords se‐
lectively.

smtpd_dns_reply_filter (default: empty)
Optional filter for Postfix SMTP server DNS lookup results. See smtp_dns_reply_filter for
details including an example.

This feature is available in Postfix 3.0 and later.

smtpd_end_of_data_restrictions (default: empty)
Optional access restrictions that the Postfix SMTP server applies in the context of the SMTP
END-OF-DATA command. See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access re‐
striction lists" for a discussion of evaluation context and time.

This feature is available in Postfix 2.2 and later.

See smtpd_data_restrictions for details and limitations.

smtpd_enforce_tls (default: no)
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require that clients use
TLS encryption. According to RFC 2487 this MUST NOT be applied in case of a publicly-refer‐
enced SMTP server. This option is therefore off by default.

Note 1: "smtpd_enforce_tls = yes" implies "smtpd_tls_auth_only = yes".

Note 2: when invoked via "sendmail -bs", Postfix will never offer STARTTLS due to insuffi‐
cient privileges to access the server private key. This is intended behavior.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use
smtpd_tls_security_level instead.

smtpd_error_sleep_time (default: 1s)
With Postfix version 2.1 and later: the SMTP server response delay after a client has made
more than $smtpd_soft_error_limit errors, and fewer than $smtpd_hard_error_limit errors,
without delivering mail.

With Postfix version 2.0 and earlier: the SMTP server delay before sending a reject (4xx or
5xx) response, when the client has made fewer than $smtpd_soft_error_limit errors without de‐
livering mail. When the client has made $smtpd_soft_error_limit or more errors, delay all re‐
sponses with the larger of (number of errors) seconds or $smtpd_error_sleep_time.

smtpd_etrn_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the context of a client ETRN
command. See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restriction
lists" for a discussion of evaluation context and time.

The Postfix ETRN implementation accepts only destinations that are eligible for the Postfix
"fast flush" service. See the ETRN_README file for details.

The following restrictions are specific to the domain name information received with the ETRN
command.

check_etrn_access type:table
Search the specified access database for the ETRN domain name or its parent domains.
See the access(5) manual page for details.

Other restrictions that are valid in this context:

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

• SMTP command specific restrictions described under smtpd_client_restrictions and
smtpd_helo_restrictions.

Example:

smtpd_etrn_restrictions = permit_mynetworks, reject

smtpd_expansion_filter (default: see postconf -d output)
What characters are allowed in $name expansions of RBL reply templates. Characters not in the
allowed set are replaced by "_". Use C like escapes to specify special characters such as
whitespace.

The smtpd_expansion_filter value is not subject to Postfix configuration parameter $name ex‐
pansion.

This feature is available in Postfix 2.0 and later.

smtpd_forbid_bare_newline (default: Postfix < 3.9: no)
Reject or restrict input lines from an SMTP client that end in <LF> instead of the standard
<CR><LF>. Such line endings are commonly allowed with UNIX-based SMTP servers, but they vio‐
late RFC 5321, and allowing such line endings can make a server vulnerable to SMTP smuggling.

Specify one of the following values (case does not matter):

normalize
Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>. Otherwise, allow command
or message content lines ending in the non-standard <LF>, and process them as if the
client sent the standard <CR><LF>.
This maintains compatibility with many legitimate SMTP client applications that send a
mix of standard and non-standard line endings, but will fail to receive email from
client implementations that do not terminate DATA content with the standard
End-of-DATA sequence <CR><LF>.<CR><LF>.
Such clients can be excluded with smtpd_forbid_bare_newline_exclusions.

yes Compatibility alias for normalize.

reject Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>. Reject a command or mes‐
sage content when a line contains bare <LF>, log a "bare <LF> received" error, and re‐
ply with the SMTP status code in $smtpd_forbid_bare_newline_reject_code.
This will reject email from SMTP clients that send any non-standard line endings such
as web applications, netcat, or load balancer health checks.
This will also reject email from services that use BDAT to send MIME text containing a
bare newline (RFC 3030 Section 3 requires canonical MIME format for text message
types, defined in RFC 2045 Sections 2.7 and 2.8).
Such clients can be excluded with smtpd_forbid_bare_newline_exclusions (or, in the
case of BDAT violations, BDAT can be selectively disabled with smtpd_discard_ehlo_key‐
word_address_maps, or globally disabled with smtpd_discard_ehlo_keywords).

no (default)
Do not require the standard End-of-DATA sequence <CR><LF>.<CR><LF>. Always process a
bare <LF> as if the client sent <CR><LF>. This option is fully backwards compatible,
but is not recommended for an Internet-facing SMTP server, because it is vulnerable to
SMTP smuggling.

Recommended settings:

# Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>.
# Otherwise, allow bare <LF> and process it as if the client sent
# <CR><LF>.
#
# This maintains compatibility with many legitimate SMTP client
# applications that send a mix of standard and non-standard line
# endings, but will fail to receive email from client implementations
# that do not terminate DATA content with the standard End-of-DATA
# sequence <CR><LF>.<CR><LF>.
#
# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
# The example below allowlists SMTP clients in trusted networks.
#
smtpd_forbid_bare_newline = normalize
smtpd_forbid_bare_newline_exclusions = $mynetworks

Alternative:

# Reject input lines that contain <LF> and log a "bare <LF> received"
# error. Require that input lines end in <CR><LF>, and require the
# standard End-of-DATA sequence <CR><LF>.<CR><LF>.
#
# This will reject email from SMTP clients that send any non-standard
# line endings such as web applications, netcat, or load balancer
# health checks.
#
# This will also reject email from services that use BDAT to send
# MIME text containing a bare newline (RFC 3030 Section 3 requires
# canonical MIME format for text message types, defined in RFC 2045
# Sections 2.7 and 2.8).
#
# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
# The example below allowlists SMTP clients in trusted networks.
#
smtpd_forbid_bare_newline = reject
smtpd_forbid_bare_newline_exclusions = $mynetworks
#
# Alternatively, in the case of BDAT violations, BDAT can be selectively
# disabled with smtpd_discard_ehlo_keyword_address_maps, or globally
# disabled with smtpd_discard_ehlo_keywords.
#
# smtpd_discard_ehlo_keyword_address_maps = cidr:/path/to/file
# /path/to/file:
# 10.0.0.0/24 chunking, silent-discard
# smtpd_discard_ehlo_keywords = chunking, silent-discard

This feature with settings yes and no is available in Postfix 3.8.4, 3.7.9, 3.6.13, and
3.5.23. Additionally, the settings reject, and normalize are available with Postfix >= 3.9,
3.8.5, 3.7.10, 3.6.14, and 3.5.24.

smtpd_forbid_bare_newline_exclusions (default: $mynetworks)
Exclude the specified clients from smtpd_forbid_bare_newline enforcement. This setting uses
the same syntax and parent-domain matching behavior as mynetworks.

This feature is available in Postfix >= 3.9, 3.8.4, 3.7.9, 3.6.13, and 3.5.23.

smtpd_forbid_bare_newline_reject_code (default: 550)
The numerical Postfix SMTP server response code when rejecting a request with "smtpd_for‐
bid_bare_newline = reject". Specify a 5XX status code (521 to disconnect).

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

smtpd_forbid_unauth_pipelining (default: Postfix >= 3.9: yes)
Disconnect remote SMTP clients that violate RFC 2920 (or 5321) command pipelining con‐
straints. The server replies with "554 5.5.0 Error: SMTP protocol synchronization" and logs
the unexpected remote SMTP client input. Specify "smtpd_forbid_unauth_pipelining = yes" to
enable. This feature is enabled by default with Postfix >= 3.9.

This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6, 3.6.10, and 3.5.20.

smtpd_forbidden_commands (default: CONNECT GET POST regexp:{{/^[^A-Z]/ Bogus}})
List of commands that cause the Postfix SMTP server to immediately terminate the session with
a 221 code. This can be used to disconnect clients that obviously attempt to abuse the sys‐
tem. In addition to the commands listed in this parameter, commands that follow the "Label:"
format of message headers will also cause a disconnect. With Postfix versions 3.6 and ear‐
lier, the default value is "CONNECT GET POST".

This feature is available in Postfix 2.2 and later.

Support for inline regular expressions was added in Postfix version 3.7. See regexp_table(5)
for a description of the syntax and features.

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 reached. 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. Valid values are greater than zero.

smtpd_helo_required (default: no)
Require that a remote SMTP client introduces itself with the HELO or EHLO command before
sending the MAIL command or other commands that require EHLO negotiation.

Example:

smtpd_helo_required = yes

smtpd_helo_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the context of a client HELO
command. See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restriction
lists" for a discussion of evaluation context and time.

The default is to permit everything.

Note: specify "smtpd_helo_required = yes" to fully enforce this restriction (without
"smtpd_helo_required = yes", a client can simply skip smtpd_helo_restrictions by not sending
HELO or EHLO).

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 al‐
lowed for safety reasons. Instead, use DUNNO in order to exclude specific hosts from
denylists. Note 2: specify "smtpd_helo_required = yes" to fully enforce this restric‐
tion (without "smtpd_helo_required = yes", a client can simply skip check_helo_a_ac‐
cess by not sending HELO or EHLO). This feature is available in Postfix 3.0 and
later.

check_helo_mx_access type:table
Search the specified access(5) database for the MX hosts for the HELO or EHLO host‐
name, and execute the corresponding action. If no MX record is found, look up A or
AAAA records, just like the Postfix SMTP client would. Note 1: a result of "OK" is
not allowed for safety reasons. Instead, use DUNNO in order to exclude specific hosts
from denylists. Note 2: specify "smtpd_helo_required = yes" to fully enforce this re‐
striction (without "smtpd_helo_required = yes", a client can simply skip
check_helo_mx_access by not sending HELO or EHLO). This feature is available in Post‐
fix 2.1 and later.

check_helo_ns_access type:table
Search the specified access(5) database for the DNS servers for the HELO or EHLO host‐
name, and execute the corresponding action. Note 1: a result of "OK" is not allowed
for safety reasons. Instead, use DUNNO in order to exclude specific hosts from
denylists. Note 2: specify "smtpd_helo_required = yes" to fully enforce this restric‐
tion (without "smtpd_helo_required = yes", a client can simply skip check_helo_ns_ac‐
cess by not sending HELO or EHLO). This feature is available in Postfix 2.1 and later.

reject_invalid_helo_hostname (with Postfix < 2.3: reject_invalid_hostname)
Reject the request when the HELO or EHLO hostname is malformed. Note: specify
"smtpd_helo_required = yes" to fully enforce this restriction (without "smtpd_helo_re‐
quired = yes", a client can simply skip reject_invalid_helo_hostname by not sending
HELO or EHLO).
The invalid_hostname_reject_code specifies the response code for rejected requests
(default: 501).

reject_non_fqdn_helo_hostname (with Postfix < 2.3: reject_non_fqdn_hostname)
Reject the request when the HELO or EHLO hostname is not in fully-qualified domain or
address literal form, as required by the RFC. Note: specify "smtpd_helo_required =
yes" to fully enforce this restriction (without "smtpd_helo_required = yes", a client
can simply skip reject_non_fqdn_helo_hostname by not sending HELO or EHLO).
The non_fqdn_reject_code parameter specifies the response code for rejected requests
(default: 504).

reject_rhsbl_helo rbl_domain=d.d.d.d
Reject the request when the HELO or EHLO hostname is listed with the A record
"d.d.d.d" under rbl_domain (Postfix version 2.1 and later only). Each "d" is a num‐
ber, or a pattern inside "[]" that contains one or more ";"-separated numbers or num‐
ber..number ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is specified,
reject the request when the HELO or EHLO hostname is listed with any A record under
rbl_domain. See the reject_rbl_client description for additional RBL related configu‐
ration parameters. Note: specify "smtpd_helo_required = yes" to fully enforce this
restriction (without "smtpd_helo_required = yes", a client can simply skip re‐
ject_rhsbl_helo by not sending HELO or EHLO). This feature is available in Postfix 2.0
and later.

reject_unknown_helo_hostname (with Postfix < 2.3: reject_unknown_hostname)
Reject the request when the HELO or EHLO hostname has no DNS A or MX record.
The reply is specified with the unknown_hostname_reject_code parameter (default: 450)
or unknown_helo_hostname_tempfail_action (default: defer_if_permit). See the respec‐
tive parameter descriptions for details.
Note: specify "smtpd_helo_required = yes" to fully enforce this restriction (without
"smtpd_helo_required = yes", a client can simply skip reject_unknown_helo_hostname by
not sending HELO or EHLO).

Other restrictions that are valid in this context:

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

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

• SMTP command specific restrictions described under smtpd_sender_restrictions or
smtpd_recipient_restrictions. When sender or recipient restrictions are listed under
smtpd_helo_restrictions, they have effect only with "smtpd_delay_reject = yes", so
that $smtpd_helo_restrictions is evaluated at the time of the RCPT TO command.

Examples:

smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname
smtpd_helo_restrictions = permit_mynetworks, reject_unknown_helo_hostname

smtpd_history_flush_threshold (default: 100)
The maximal number of lines in the Postfix SMTP server command history before it is flushed
upon receipt of EHLO, RSET, or end of DATA.

smtpd_junk_command_limit (default: normal: 100, overload: 1)
The number of junk commands (NOOP, VRFY, ETRN or RSET) that a remote SMTP client can send be‐
fore the Postfix SMTP server starts to increment the error counter with each junk command.
The junk command count is reset after mail is delivered. See also the smtpd_error_sleep_time
and smtpd_soft_error_limit configuration parameters. Normally the default limit is 100, but
it changes under overload to just 1. With Postfix 2.5 and earlier, the SMTP server always al‐
lows up to 100 junk commands by default.

smtpd_log_access_permit_actions (default: empty)
Enable logging of the named "permit" actions in SMTP server access lists (by default, the
SMTP server logs "reject" actions but not "permit" actions). This feature does not affect
conditional actions such as "defer_if_permit".

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

Examples:

/etc/postfix/main.cf:
# Log all "permit" actions.
smtpd_log_access_permit_actions = static:all

/etc/postfix/main.cf:
# Log "permit_dnswl_client" only.
smtpd_log_access_permit_actions = permit_dnswl_client

This feature is available in Postfix 2.10 and later.

smtpd_milter_maps (default: empty)
Lookup tables with Milter settings per remote SMTP client IP address. The lookup result
overrides the smtpd_milters setting, and has the same syntax.

Note: lookup tables cannot return empty responses. Specify a lookup result of DISABLE (case
does not matter) to indicate that Milter support should be disabled.

Example to disable Milters for local clients:

/etc/postfix/main.cf:
smtpd_milter_maps = cidr:/etc/postfix/smtpd_milter_map
smtpd_milters = inet:host:port, { inet:host:port, ... }, ...

/etc/postfix/smtpd_milter_map:
# Disable Milters for local clients.
127.0.0.0/8 DISABLE
192.168.0.0/16 DISABLE
::/64 DISABLE
2001:db8::/32 DISABLE

This feature is available in Postfix 3.2 and later.

smtpd_milters (default: empty)
A list of Milter (mail filter) applications for new mail that arrives via the Postfix
smtpd(8) server. Specify space or comma as separator. See the MILTER_README document for de‐
tails.

This feature is available in Postfix 2.3 and later.

smtpd_min_data_rate (default: 500)
The minimum plaintext data transfer rate in bytes/second for DATA and BDAT requests, when
deadlines are enabled with smtpd_per_request_deadline. After a read operation transfers N
plaintext message bytes (possibly after TLS decryption), and after the DATA or BDAT request
deadline is decremented by the elapsed time of that read operation, the DATA or BDAT request
deadline is incremented by N/smtpd_min_data_rate seconds. However, the deadline will never be
incremented beyond the time limit specified with smtpd_timeout.

This feature is available in Postfix 3.7 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 verified,
or when name lookup is disabled. Turning off name lookup reduces delays due to DNS lookup
and increases the maximal inbound delivery rate.

This feature is available in Postfix 2.3 and later.

smtpd_per_record_deadline (default: normal: no, overload: yes)
Change the behavior of the smtpd_timeout and smtpd_starttls_timeout time limits, from a time
limit per read or write system call, to a time limit to send or receive a complete record (an
SMTP command line, SMTP response line, SMTP message content line, or TLS protocol message).
This limits the impact from hostile peers that trickle data one byte at a time.

This feature is available in Postfix 2.9-3.6. With older Postfix releases, the behavior is as
if this parameter is set to "no". Postfix 3.7 and later use smtpd_per_request_deadline.

smtpd_per_request_deadline (default: normal: no, overload: yes)
Change the behavior of the smtpd_timeout and smtpd_starttls_timeout time limits, from a time
limit per plaintext or TLS read or write call, to a combined time limit for receiving a com‐
plete SMTP request and for sending a complete SMTP response. The deadline limits only the
time spent waiting for plaintext or TLS read or write calls, not time spent elsewhere. The
per-request deadline limits the impact from hostile peers that trickle data one byte at a
time.

See smtpd_min_data_rate for how the per-request deadline is managed during the DATA and BDAT
phase.

This feature is available in Postfix 3.7 and later. A weaker feature, called
smtpd_per_record_deadline, is available with Postfix 2.9-3.6. With older Postfix releases,
the behavior is as if this parameter is set to "no".

This feature is available in Postfix 3.7 and later.

smtpd_policy_service_default_action (default: 451 4.3.5 Server configuration problem)
The default action when an SMTPD policy service request fails. Specify "DUNNO" to behave as
if the failed SMTPD policy service request was not sent, and to continue processing other
access restrictions, if any.

Limitations:

• This parameter may specify any value that would be a valid SMTPD policy server re‐
sponse (or access(5) 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 setting.

• If the specified action invokes another check_policy_service request, that request
will have the built-in default action.

This feature is available in Postfix 3.0 and later.

smtpd_policy_service_max_idle (default: 300s)
The time after which an idle SMTPD policy service connection is closed.

This feature is available in Postfix 2.1 and later.

smtpd_policy_service_max_ttl (default: 1000s)
The time after which an active SMTPD policy service connection is closed.

This feature is available in Postfix 2.1 and later.

smtpd_policy_service_policy_context (default: empty)
Optional information that the Postfix SMTP server specifies in the "policy_context" attribute
of a policy service request (originally, to share the same service endpoint among multiple
check_policy_service clients).

This feature is available in Postfix 3.1 and later.

smtpd_policy_service_request_limit (default: 0)
The maximal number of requests per SMTPD policy service connection, or zero (no limit). Once
a connection reaches this limit, the connection is closed and the next request will be sent
over a new connection. This is a workaround to avoid error-recovery delays with policy
servers that cannot maintain a persistent connection.

This feature is available in Postfix 3.0 and later.

smtpd_policy_service_retry_delay (default: 1s)
The delay between attempts to resend a failed SMTPD policy service request. Specify a value
greater than zero.

This feature is available in Postfix 3.0 and later.

smtpd_policy_service_timeout (default: 100s)
The time limit for connecting to, writing to, or receiving from a delegated SMTPD policy
server.

This feature is available in Postfix 2.1 and later.

smtpd_policy_service_try_limit (default: 2)
The maximal number of attempts to send an SMTPD policy service request before giving up.
Specify a value greater than zero.

This feature is available in Postfix 3.0 and later.

smtpd_proxy_ehlo (default: $myhostname)
How the Postfix SMTP server announces itself to the proxy filter. By default, the Postfix
hostname is used.

This feature is available in Postfix 2.1 and later.

smtpd_proxy_filter (default: empty)
The hostname and TCP port of the mail filtering proxy server. The proxy receives all mail
from the Postfix SMTP server, and is supposed to give the result to another Postfix SMTP
server process.

Specify "host:port" or "inet:host:port" for a TCP endpoint, or "unix:pathname" for a UNIX-do‐
main endpoint. The host can be specified as an IP address or as a symbolic name; no MX
lookups are done. When no "host" or "host:" is specified, the local machine is assumed.
Pathname interpretation is relative to the Postfix queue directory.

This feature is available in Postfix 2.1 and later.

The "inet:" and "unix:" prefixes are available in Postfix 2.3 and later.

smtpd_proxy_options (default: empty)
List of options that control how the Postfix SMTP server communicates with a before-queue
content filter. Specify zero or more of the following, separated by comma or whitespace.

speed_adjust
Do not connect to a before-queue content filter until an entire message has been re‐
ceived. This reduces the number of simultaneous before-queue content filter processes.

NOTE 1: A filter must not selectively reject recipients of a multi-recipient message. Re‐
jecting all recipients is OK, as is accepting all recipients.

NOTE 2: This feature increases the minimum amount of free queue space by $message_size_limit.
The extra space is needed to save the message to a temporary file.

This feature is available in Postfix 2.7 and later.

smtpd_proxy_timeout (default: 100s)
The time limit for connecting to a proxy filter and for sending or receiving information.
When a connection fails the client gets a generic error message while more detailed informa‐
tion is logged to the maillog file.

This feature is available in Postfix 2.1 and later.

smtpd_recipient_limit (default: 1000)
The maximal number of recipients that the Postfix SMTP server accepts per message delivery
request.

smtpd_recipient_overshoot_limit (default: 1000)
The number of recipients that a remote SMTP client can send in excess of the limit specified
with $smtpd_recipient_limit, before the Postfix SMTP server increments the per-session error
count for each excess recipient.

smtpd_recipient_restrictions (default: see postconf -d output)
Optional restrictions that the Postfix SMTP server applies in the context of a client RCPT TO
command, after smtpd_relay_restrictions. See SMTPD_ACCESS_README, section "Delayed evalua‐
tion of SMTP access restriction lists" for a discussion of evaluation context and time.

With Postfix versions before 2.10, the rules for relay permission and spam blocking were com‐
bined under smtpd_recipient_restrictions, resulting in error-prone configuration. As of
Postfix 2.10, relay permission rules are preferably implemented with smtpd_relay_restric‐
tions, so that a permissive spam blocking policy under smtpd_recipient_restrictions will no
longer result in a permissive mail relay policy.

For backwards compatibility, sites that migrate from Postfix versions before 2.10 can set
smtpd_relay_restrictions to the empty value, and use smtpd_recipient_restrictions exactly as
before.

IMPORTANT: Either the smtpd_relay_restrictions or the smtpd_recipient_restrictions parameter
must specify at least one of the following restrictions. Otherwise Postfix will refuse to re‐
ceive mail:

reject, reject_unauth_destination

defer, defer_if_permit, defer_unauth_destination

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, par‐
ent 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 domain,
and execute the corresponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
This feature is available in Postfix 3.0 and later.

check_recipient_mx_access type:table
Search the specified access(5) 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:

• Postfix is a mail forwarder: the resolved RCPT TO domain matches $relay_domains or a
subdomain thereof, and the address contains no sender-specified routing (user@else‐
where@domain),

• Postfix is the final destination: the resolved RCPT TO domain matches $mydestination,
$inet_interfaces, $proxy_interfaces, $virtual_alias_domains, or $virtual_mailbox_do‐
mains, and the address contains no sender-specified routing (user@elsewhere@domain).

permit_mx_backup
Permit the request when the local mail system is a backup MX for the RCPT TO domain,
or when the domain is an authorized destination (see permit_auth_destination for defi‐
nition).

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

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

• Safety: as of Postfix version 2.3, permit_mx_backup no longer accepts the address when
the local mail system is a primary MX for the recipient domain. Exception: per‐
mit_mx_backup accepts the address when it specifies an authorized destination (see
permit_auth_destination for definition).

• Limitation: mail may be rejected in case of a temporary DNS lookup problem with Post‐
fix prior to version 2.0.

reject_non_fqdn_recipient
Reject the request when the RCPT TO address specifies a domain that is not in
fully-qualified domain form, as required by the RFC.
The non_fqdn_reject_code parameter specifies the response code for rejected requests
(default: 504).

reject_rhsbl_recipient rbl_domain=d.d.d.d
Reject the request when the RCPT TO domain is listed with the A record "d.d.d.d" under
rbl_domain (Postfix version 2.1 and later only). Each "d" is a number, or a pattern
inside "[]" that contains one or more ";"-separated numbers or number..number ranges
(Postfix version 2.8 and later). If no "=d.d.d.d" is specified, reject the request
when the RCPT TO domain is listed with any A record under rbl_domain.
The maps_rbl_reject_code parameter specifies the response code for rejected requests
(default: 554); the default_rbl_reply parameter specifies the default server reply;
and the rbl_reply_maps parameter specifies tables with server replies indexed by
rbl_domain. This feature is available in Postfix version 2.0 and later.

reject_unauth_destination
Reject the request unless one of the following is true:

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

• Postfix is the final destination: the resolved RCPT TO domain matches $mydestination,
$inet_interfaces, $proxy_interfaces, $virtual_alias_domains, or $virtual_mailbox_do‐
mains, and contains no sender-specified routing (user@elsewhere@domain).
The relay_domains_reject_code parameter specifies the response code for rejected re‐
quests (default: 554).

defer_unauth_destination
Reject the same requests as reject_unauth_destination, with a non-permanent error
code. This feature is available in Postfix 2.10 and later.

reject_unknown_recipient_domain
Reject the request when Postfix is not final destination for the recipient domain, and
the RCPT TO domain has 1) no DNS MX and no DNS A record or 2) a malformed MX record
such as a record with a zero-length MX hostname (Postfix version 2.3 and later).
The reply is specified with the unknown_address_reject_code parameter (default: 450),
unknown_address_tempfail_action (default: defer_if_permit), or 556 (nullmx, Postfix
3.0 and later). See the respective parameter descriptions for details.

reject_unlisted_recipient (with Postfix version 2.0: check_recipient_maps)
Reject the request when the RCPT TO address is not listed in the list of valid recipi‐
ents for its domain class. See the smtpd_reject_unlisted_recipient parameter descrip‐
tion for details. This feature is available in Postfix 2.1 and later.

reject_unverified_recipient
Reject the request when mail to the RCPT TO address is known to bounce, or when the
recipient address destination is not reachable. Address verification information is
managed by the verify(8) 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 it to 550 when you are confi‐
dent that it is safe to do so).
The unverified_recipient_defer_code parameter specifies the numerical response code
when an address probe failed due to a temporary problem (default: 450).
The unverified_recipient_tempfail_action parameter specifies the action after address
probe failure due to a temporary problem (default: defer_if_permit).
This feature breaks for aliased addresses with "enable_original_recipient = no" (Post‐
fix <= 3.2).
This feature is available in Postfix 2.1 and later.

Other restrictions that are valid in this context:

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

• SMTP command specific restrictions described under smtpd_client_restrictions,
smtpd_helo_restrictions and smtpd_sender_restrictions.

Example:

# The Postfix before 2.10 default mail relay policy. Later Postfix
# versions implement this preferably with smtpd_relay_restrictions.
smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

smtpd_reject_footer (default: empty)
Optional information that is appended after each Postfix SMTP server 4XX or 5XX response.

The following example uses "\c" at the start of the template (supported in Postfix 2.10 and
later) to suppress the line break between the reply text and the footer text. With earlier
Postfix versions, the footer text always begins on a new line, and the "\c" is output liter‐
ally.

/etc/postfix/main.cf:
smtpd_reject_footer = \c. For assistance, call 800-555-0101.
Please provide the following information in your problem report:
time ($localtime), client ($client_address) and server
($server_name).

Server response:

550-5.5.1 <user@example> Recipient address rejected: User
unknown. For assistance, call 800-555-0101. Please provide the
following information in your problem report: time (Jan 4 15:42:00),
client (192.168.1.248) and server (mail1.example.com).

Note: the above text is meant to make it easier to find the Postfix logfile records for a
failed SMTP session. The text itself is not logged to the Postfix SMTP server's maillog file.

Be sure to keep the text as short as possible. Long text may be truncated before it is logged
to the remote SMTP client's maillog file, or before it is returned to the sender in a deliv‐
ery status notification.

The template text is not subject to Postfix configuration parameter $name expansion. Instead,
this feature supports a limited number of $name attributes in the footer text. These attrib‐
utes are replaced with their current value for the SMTP session.

Note: specify $$name in footer text that is looked up from regexp: or pcre:-based smtpd_re‐
ject_footer_maps, otherwise the Postfix server will not use the footer text and will log a
warning instead.

client_address
The Client IP address that is logged in the maillog file.

client_port
The client TCP port that is logged in the maillog file.

localtime
The server local time (Mmm dd hh:mm:ss) that is logged in the maillog file.

server_name
The server's myhostname value. This attribute is made available for sites with multi‐
ple MTAs (perhaps behind a load-balancer), where the server name can help the server
support team to quickly find the right log files.

Notes:

• NOT SUPPORTED are other attributes such as sender, recipient, or main.cf parameters.

• For safety reasons, text that does not match $smtpd_expansion_filter is censored.

This feature supports the two-character sequence \n as a request for a line break in the
footer text. Postfix automatically inserts after each line break the three-digit SMTP reply
code (and optional enhanced status code) from the original Postfix reject message.

To work around mail software that mis-handles multi-line replies, specify the two-character
sequence \c at the start of the template. This suppresses the line break between the reply
text and the footer text (Postfix 2.10 and later).

This feature is available in Postfix 2.8 and later.

smtpd_reject_footer_maps (default: empty)
Lookup tables, indexed by the complete Postfix SMTP server 4xx or 5xx response, with reject
footer templates. See smtpd_reject_footer for details.

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

This feature is available in Postfix 3.4 and later.

smtpd_reject_unlisted_recipient (default: yes)
Request that the Postfix SMTP server rejects mail for unknown recipient addresses, even when
no explicit reject_unlisted_recipient access restriction is specified. This prevents the
Postfix queue from filling up with undeliverable MAILER-DAEMON messages.

An address is considered "unknown" when 1) it does not match a virtual(5) alias or canoni‐
cal(5) mapping, and 2) the address is not valid for its address class. For a definition of
class-based address validation, see ADDRESS_CLASS_README.

This feature is available in Postfix 2.1 and later.

smtpd_reject_unlisted_sender (default: no)
Request that the Postfix SMTP server rejects mail from unknown sender addresses, even when no
explicit reject_unlisted_sender access restriction is specified. This can slow down an explo‐
sion of forged mail from worms or viruses.

This feature is available in Postfix 2.1 and later.

smtpd_relay_before_recipient_restrictions (default: see postconf -d output)
Evaluate smtpd_relay_restrictions before smtpd_recipient_restrictions. Historically,
smtpd_relay_restrictions was evaluated after smtpd_recipient_restrictions, contradicting doc‐
umented behavior.

Background: the smtpd_relay_restrictions feature is primarily designed to enforce a mail re‐
laying policy, while smtpd_recipient_restrictions is primarily designed to enforce spam
blocking policy. Both are evaluated while replying to the RCPT TO command, and both support
the same features.

This feature is available in Postfix 3.6 and later.

smtpd_relay_restrictions (default: permit_mynetworks, permit_sasl_authenticated, defer_unauth_desti‐
nation)
Access restrictions for mail relay control that the Postfix SMTP server applies in the con‐
text of the RCPT TO command, before smtpd_recipient_restrictions. See SMTPD_ACCESS_README,
section "Delayed evaluation of SMTP access restriction lists" for a discussion of evaluation
context and time.

For backwards compatibility, sites that migrate from Postfix versions before 2.10 can set
smtpd_relay_restrictions to the empty value, and use smtpd_recipient_restrictions exactly as
before.

By default, the Postfix SMTP server accepts:

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

• Mail from clients who are SASL authenticated, or:

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

• Mail to local destinations that match $inet_interfaces or $proxy_interfaces, $mydesti‐
nation, $virtual_alias_domains, or $virtual_mailbox_domains.

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) table.

One major application is for implementing per-recipient UCE control. See the RESTRIC‐
TION_CLASS_README document for other examples.

smtpd_sasl_application_name (default: smtpd)
The application name that the Postfix SMTP server uses for SASL server initialization. This
controls the name of the SASL configuration file. The default value is smtpd, corresponding
to a SASL configuration file named smtpd.conf.

This feature is available in Postfix 2.1 and 2.2. With Postfix 2.3 it was renamed to
smtpd_sasl_path.

smtpd_sasl_auth_enable (default: no)
Enable SASL authentication in the Postfix SMTP server. By default, the Postfix SMTP server
does not use authentication.

If a remote SMTP client is authenticated, the permit_sasl_authenticated access restriction
can be used to permit relay access, like this:

# With Postfix 2.10 and later, the mail relay policy is
# preferably specified under smtpd_relay_restrictions.
smtpd_relay_restrictions =
permit_mynetworks, permit_sasl_authenticated, ...

# With Postfix before 2.10, the relay policy can be
# specified only under smtpd_recipient_restrictions.
smtpd_recipient_restrictions =
permit_mynetworks, permit_sasl_authenticated, ...

To reject all SMTP connections from unauthenticated clients, specify "smtpd_delay_reject =
yes" (which is the default) and use:

smtpd_client_restrictions = permit_sasl_authenticated, reject

See the SASL_README file for SASL configuration and operation details.

smtpd_sasl_authenticated_header (default: no)
Report the SASL authenticated user name in the smtpd(8) Received message header.

This feature is available in Postfix 2.3 and later.

smtpd_sasl_exceptions_networks (default: empty)
What remote SMTP clients the Postfix SMTP server will not offer AUTH support to.

Some clients (Netscape 4 at least) have a bug that causes them to require a login and pass‐
word whenever AUTH is offered, whether it's necessary or not. To work around this, specify,
for example, $mynetworks to prevent Postfix from offering AUTH to local clients.

Specify a list of network/netmask patterns, separated by commas and/or whitespace. The mask
specifies the number of bits in the network part of a host address. You can also 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). Continue long lines by starting the next line with whitespace. Specify
"!pattern" to exclude an address or network block from the list. The form "!/file/name" is
supported only in Postfix version 2.4 and later.

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

Example:

smtpd_sasl_exceptions_networks = $mynetworks

This feature is available in Postfix 2.1 and later.

smtpd_sasl_local_domain (default: empty)
The name of the Postfix SMTP server's local SASL authentication realm.

By default, the local authentication realm name is the null string.

Examples:

smtpd_sasl_local_domain = $mydomain
smtpd_sasl_local_domain = $myhostname

smtpd_sasl_mechanism_filter (default: !external, static:rest)
If non-empty, a filter for the SASL mechanism names that the Postfix SMTP server will an‐
nounce in the EHLO response. By default, the Postfix SMTP server will not announce the EXTER‐
NAL mechanism, because Postfix support for that is not implemented.

Specify mechanism names, "/file/name" patterns, or "type:table" lookup tables, separated by
comma or whitespace. The right-hand side result from "type:table" lookups is ignored. Specify
"!pattern" to exclude a mechanism name from the list.

Examples:

smtpd_sasl_mechanism_filter = !external, !gssapi, static:rest
smtpd_sasl_mechanism_filter = login, plain
smtpd_sasl_mechanism_filter = /etc/postfix/smtpd_mechs

This feature is available in Postfix 3.6 and later.

smtpd_sasl_path (default: smtpd)
Implementation-specific information that the Postfix SMTP server passes through to the SASL
plug-in implementation that is selected with smtpd_sasl_type. Typically this specifies the
name of a configuration file or rendezvous point.

This feature is available in Postfix 2.3 and later. In earlier releases it was called
smtpd_sasl_application_name.

smtpd_sasl_response_limit (default: 12288)
The maximum length of a SASL client's response to a server challenge. When the client's
"initial response" is longer than the normal limit for SMTP commands, the client must omit
its initial response, and wait for an empty server challenge; it can then send what would
have been its "initial response" as a response to the empty server challenge. RFC4954 re‐
quires the server to accept client responses up to at least 12288 octets of base64-encoded
text. The default value is therefore also the minimum value accepted for this parameter.

This feature is available in Postfix 3.4 and later. Prior versions use "line_length_limit",
which may need to be raised to accommodate larger client responses, as may be needed with
GSSAPI authentication of Windows AD users who are members of many groups.

smtpd_sasl_security_options (default: noanonymous)
Postfix SMTP server SASL security options; as of Postfix 2.3 the list of available features
depends on the SASL server implementation that is selected with smtpd_sasl_type.

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

Restrict what authentication mechanisms the Postfix SMTP server will offer to the client.
The list of available authentication mechanisms is system dependent.

Specify zero or more of the following:

noplaintext
Disallow methods that use plaintext passwords.

noactive
Disallow methods subject to active (non-dictionary) attack.

nodictionary
Disallow methods subject to passive (dictionary) attack.

noanonymous
Disallow methods that allow anonymous authentication.

forward_secrecy
Only allow methods that support forward secrecy (Dovecot only).

mutual_auth
Only allow methods that provide mutual authentication (not available with Cyrus SASL
version 1).

By default, the Postfix SMTP server accepts plaintext passwords but not anonymous logins.

Warning: it appears that clients try authentication methods in the order as advertised by the
server (e.g., PLAIN ANONYMOUS CRAM-MD5) which means that if you disable plaintext passwords,
clients will log in anonymously, even when they should be able to use CRAM-MD5. So, if you
disable plaintext logins, disable anonymous logins too. Postfix treats anonymous login as no
authentication.

Example:

smtpd_sasl_security_options = noanonymous, noplaintext

smtpd_sasl_service (default: smtp)
The service name that is passed to the SASL plug-in that is selected with smtpd_sasl_type and
smtpd_sasl_path.

This feature is available in Postfix 2.11 and later. Prior versions behave as if "smtp" is
specified.

smtpd_sasl_tls_security_options (default: $smtpd_sasl_security_options)
The SASL authentication security options that the Postfix SMTP server uses for TLS encrypted
SMTP sessions.

This feature is available in Postfix 2.2 and later.

smtpd_sasl_type (default: cyrus)
The SASL plug-in type that the Postfix SMTP server should use for authentication. The avail‐
able types are listed with the "postconf -a" command.

This feature is available in Postfix 2.3 and later.

smtpd_sender_login_maps (default: empty)
Optional lookup table with the SASL login names that own the sender (MAIL FROM) addresses.

Specify zero or more "type:name" lookup tables, separated by whitespace or comma. Tables will
be searched in the specified order until a match is found. With lookups from indexed files
such as DB or DBM, or from networked tables such as NIS, LDAP or SQL, the following search
operations are done with a sender address of user@domain:

1) user@domain
This table lookup is always done and has the highest precedence.

2) user
This table lookup is done only when the domain part of the sender address matches $my‐
origin, $mydestination, $inet_interfaces or $proxy_interfaces.

3) @domain
This table lookup is done last and has the lowest precedence.

In all cases the result of table lookup must be either "not found" or a list of SASL login
names separated by comma and/or whitespace.

smtpd_sender_restrictions (default: empty)
Optional restrictions that the Postfix SMTP server applies in the context of a client MAIL
FROM command. See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access restric‐
tion lists" for a discussion of evaluation context and time.

The default is to permit everything.

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 do‐
mains, 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 domain,
and execute the corresponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
This feature is available in Postfix 3.0 and later.

check_sender_mx_access type:table
Search the specified access(5) 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 domain,
and execute the corresponding action. Note: a result of "OK" is not allowed for
safety reasons. Instead, use DUNNO in order to exclude specific hosts from denylists.
This feature is available in Postfix 2.1 and later.

reject_authenticated_sender_login_mismatch
Reject the request when the client is authenticated with SASL, but either the MAIL
FROM address is not listed in $smtpd_sender_login_maps, or the SASL login name is not
an owner for that address.
This prevents an authenticated client from using a MAIL FROM address that they do not
explicitly own.
This feature is available in Postfix version 2.1 and later.

reject_known_sender_login_mismatch
When the client is authenticated with SASL, reject the request when the MAIL FROM ad‐
dress is listed in $smtpd_sender_login_maps, but the SASL login name is not an owner
for that address.
When the client is not authenticated with SASL, reject the request when SASL is en‐
abled, and the MAIL FROM address is listed in $smtpd_sender_login_maps.
This protects any MAIL FROM address that is listed in $smtpd_sender_login_maps, while
still allowing a client to use any unlisted MAIL FROM address.
This feature is available in Postfix version 2.11 and later.

reject_non_fqdn_sender
Reject the request when the MAIL FROM address specifies a domain that is not in
fully-qualified domain form as required by the RFC.
The non_fqdn_reject_code parameter specifies the response code for rejected requests
(default: 504).

reject_rhsbl_sender rbl_domain=d.d.d.d
Reject the request when the MAIL FROM domain is listed with the A record "d.d.d.d" un‐
der rbl_domain (Postfix version 2.1 and later only). Each "d" is a number, or a pat‐
tern inside "[]" that contains one or more ";"-separated numbers or number..number
ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is specified, reject the re‐
quest when the MAIL FROM domain is listed with any A record under rbl_domain.
The maps_rbl_reject_code parameter specifies the response code for rejected requests
(default: 554); the default_rbl_reply parameter specifies the default server reply;
and the rbl_reply_maps parameter specifies tables with server replies indexed by
rbl_domain. This feature is available in Postfix 2.0 and later.

reject_sender_login_mismatch
As of Postfix 2.1, this is an alias for "reject_authenticated_sender_login_mismatch,
reject_unauthenticated_sender_login_mismatch".

reject_unauthenticated_sender_login_mismatch
Reject the request when SASL is enabled, the MAIL FROM address is listed in
$smtpd_sender_login_maps, but the client is not authenticated with SASL.
With SASL enabled, this prevents an unauthenticated client from using any MAIL FROM
address that is listed in $smtpd_sender_login_maps.
This feature is available in Postfix version 2.1 and later.

reject_unknown_sender_domain
Reject the request when Postfix is not the final destination for the sender address,
and the MAIL FROM domain has 1) no DNS MX and no DNS A record, or 2) a malformed MX
record such as a record with a zero-length MX hostname (Postfix version 2.3 and
later).
The reply is specified with the unknown_address_reject_code parameter (default: 450),
unknown_address_tempfail_action (default: defer_if_permit), or 550 (nullmx, Postfix
3.0 and later). See the respective parameter descriptions for details.

reject_unlisted_sender
Reject the request when the MAIL FROM address is not listed in the list of valid re‐
cipients for its domain class. See the smtpd_reject_unlisted_sender parameter descrip‐
tion for details. This feature is available in Postfix 2.1 and later.

reject_unverified_sender
Reject the request when mail to the MAIL FROM address is known to bounce, or when the
sender address destination is not reachable. Address verification information is man‐
aged by the verify(8) server; see the ADDRESS_VERIFICATION_README file for details.
The unverified_sender_reject_code parameter specifies the numerical response code when
an address is known to bounce (default: 450, change into 550 when you are confident
that it is safe to do so).
The unverified_sender_defer_code specifies the numerical response code when an address
probe failed due to a temporary problem (default: 450).
The unverified_sender_tempfail_action parameter specifies the action after address
probe failure due to a temporary problem (default: defer_if_permit).
This feature breaks for aliased addresses with "enable_original_recipient = no" (Post‐
fix <= 3.2).
This feature is available in Postfix 2.1 and later.

Other restrictions that are valid in this context:

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

• SMTP command specific restrictions described under smtpd_client_restrictions and
smtpd_helo_restrictions.

• SMTP command specific restrictions described under smtpd_recipient_restrictions. When
recipient restrictions are listed under smtpd_sender_restrictions, they have effect
only with "smtpd_delay_reject = yes", so that $smtpd_sender_restrictions is evaluated
at the time of the RCPT TO command.

Examples:

smtpd_sender_restrictions = reject_unknown_sender_domain
smtpd_sender_restrictions = reject_unknown_sender_domain,
check_sender_access hash:/etc/postfix/access

smtpd_service_name (default: smtpd)
The internal service that postscreen(8) hands off allowed connections to. In a future version
there may be different classes of SMTP service.

This feature is available in Postfix 2.8.

smtpd_soft_error_limit (default: 10)
The number of errors a remote SMTP client is allowed to make without delivering mail before
the Postfix SMTP server slows down all its responses.

• With Postfix version 2.1 and later, when the error count is > $smtpd_soft_error_limit,
the Postfix SMTP server delays all responses by $smtpd_error_sleep_time.

• With Postfix versions 2.0 and earlier, when the error count is > $smtpd_soft_er‐
ror_limit, the Postfix SMTP server delays all responses by the larger of (number of
errors) seconds or $smtpd_error_sleep_time.

• With Postfix versions 2.0 and earlier, when the error count is <= $smtpd_soft_er‐
ror_limit, the Postfix SMTP server delays 4XX and 5XX responses by $smtpd_er‐
ror_sleep_time.

smtpd_starttls_timeout (default: see postconf -d output)
The time limit for Postfix SMTP server write and read operations during TLS startup and shut‐
down handshake procedures. The current default value is stress-dependent. Before Postfix ver‐
sion 2.8, it was fixed at 300s.

This feature is available in Postfix 2.2 and later.

smtpd_timeout (default: normal: 300s, overload: 10s)
When the Postfix SMTP server wants to send an SMTP server response, how long the Postfix SMTP
server will wait for an underlying network write operation to complete; and when the Postfix
SMTP server Postfix wants to receive an SMTP client request, how long the Postfix SMTP server
will wait for an underlying network read operation to complete. See the smtpd_per_re‐
quest_deadline for how this time limit may be enforced (with Postfix 2.9-3.6 see
smtpd_per_record_deadline).

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 default.

Note: if you set SMTP time limits to very large values you may have to update the global
ipc_timeout parameter.

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 us‐
ing smtpd_tls_CApath instead, but note that the latter directory must be present in the ch‐
root 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 di‐
rectly in the server certificate file.

Specify "smtpd_tls_CAfile = /path/to/system_CA_file" to use ONLY the system-supplied default
Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending the system-supplied
default CAs and trusting third-party certificates.

By default (see smtpd_tls_ask_ccert), client certificates are not requested, and
smtpd_tls_CAfile should remain empty. If you do make use of client certificates, the distin‐
guished names (DNs) of the Certification Authorities listed in smtpd_tls_CAfile are sent to
the remote SMTP client in the client certificate request message. MUAs with multiple client
certificates may use the list of preferred Certification Authorities to select the correct
client certificate. You may want to put your "preferred" CA or CAs in this file, and install
other trusted CAs in $smtpd_tls_CApath.

Example:

smtpd_tls_CAfile = /etc/postfix/CAcert.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_CApath (default: empty)
A directory containing (PEM format) CA certificates of root CAs trusted to sign either remote
SMTP client certificates or intermediate CA certificates. Do not forget to create the neces‐
sary "hash" links with, for example, "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs". To use
smtpd_tls_CApath in chroot mode, this directory (or a copy) must be inside the chroot jail.

Specify "smtpd_tls_CApath = /path/to/system_CA_directory" to use ONLY the system-supplied de‐
fault Certification Authority certificates.

Specify "tls_append_default_CA = no" to prevent Postfix from appending the system-supplied
default CAs and trusting third-party certificates.

By default (see smtpd_tls_ask_ccert), client certificates are not requested, and
smtpd_tls_CApath should remain empty. In contrast to smtpd_tls_CAfile, DNs of Certification
Authorities installed in $smtpd_tls_CApath are not included in the client certificate request
message. MUAs with multiple client certificates may use the list of preferred Certification
Authorities to select the correct client certificate. You may want to put your "preferred"
CA or CAs in $smtpd_tls_CAfile, and install the remaining trusted CAs in $smtpd_tls_CApath.

Example:

smtpd_tls_CApath = /etc/postfix/certs

This feature is available in Postfix 2.2 and later.

smtpd_tls_always_issue_session_ids (default: yes)
Force the Postfix SMTP server to issue a TLS session id, even when TLS session caching is
turned off (smtpd_tls_session_cache_database is empty). This behavior is compatible with
Postfix < 2.3.

With Postfix 2.3 and later the Postfix SMTP server can disable session id generation when TLS
session caching is turned off. This keeps remote SMTP clients from caching sessions that al‐
most certainly cannot be re-used.

By default, the Postfix SMTP server always generates TLS session ids. This works around a
known defect in mail client applications such as MS Outlook, and may also prevent interoper‐
ability issues with other MTAs.

Example:

smtpd_tls_always_issue_session_ids = no

This feature is available in Postfix 2.3 and later.

smtpd_tls_ask_ccert (default: no)
Ask a remote SMTP client for a client certificate. This information is needed for certificate
based mail relaying with, for example, the permit_tls_clientcerts feature.

Some clients such as Netscape will either complain if no certificate is available (for the
list of CAs in $smtpd_tls_CAfile) or will offer multiple client certificates to choose from.
This may be annoying, so this option is "off" by default.

This feature is available in Postfix 2.2 and later.

smtpd_tls_auth_only (default: no)
When TLS encryption is optional in the Postfix SMTP server, do not announce or accept SASL
authentication over unencrypted connections.

This feature is available in Postfix 2.2 and later.

smtpd_tls_ccert_verifydepth (default: 9)
The verification depth for remote SMTP client certificates. A depth of 1 is sufficient if the
issuing CA is listed in a local CA file.

This feature is available in Postfix 2.2 and later.

smtpd_tls_cert_file (default: empty)
File with the Postfix SMTP server RSA certificate in PEM format. This file may also contain
the Postfix SMTP server private RSA key. With Postfix >= 3.4 the preferred way to configure
server keys and certificates is via the "smtpd_tls_chain_files" parameter.

Public Internet MX hosts without certificates signed by a "reputable" CA must generate, and
be prepared to present to most clients, a self-signed or private-CA signed certificate. The
client will not be able to authenticate the server, but unless it is running Postfix 2.3 or
similar software, it will still insist on a server certificate.

For servers that are not public Internet MX hosts, Postfix supports configurations with no
certificates. This entails the use of just the anonymous TLS ciphers, which are not supported
by typical SMTP clients. Since some clients may not fall back to plain text after a TLS hand‐
shake failure, a certificate-less Postfix SMTP server will be unable to receive email from
some TLS-enabled clients. To avoid accidental configurations with no certificates, Postfix
enables certificate-less operation only when the administrator explicitly sets
"smtpd_tls_cert_file = none". This ensures that new Postfix SMTP server configurations will
not accidentally enable TLS without certificates.

Note that server certificates are not optional in TLS 1.3. To run without certificates you'd
have to disable the TLS 1.3 protocol by including '!TLSv1.3' in "smtpd_tls_protocols" and
perhaps also "smtpd_tls_mandatory_protocols". It is simpler instead to just configure a cer‐
tificate chain. Certificate-less operation is not recommended.

Both RSA and DSA certificates are supported. When both types are present, the cipher used
determines which certificate will be presented to the client. For Netscape and OpenSSL
clients without special cipher choices the RSA certificate is preferred.

To enable a remote SMTP client to verify the Postfix SMTP server certificate, the issuing CA
certificates must be made available to the client. You should include the required certifi‐
cates in the server certificate file, the server certificate first, then the issuing CA(s)
(bottom-up order).

Example: the certificate for "server.example.com" was issued by "intermediate CA" which it‐
self has a certificate of "root CA". Create the server.pem file with "cat server_cert.pem
intermediate_CA.pem root_CA.pem > server.pem".

If you also want to verify client certificates issued by these CAs, you can add the CA cer‐
tificates to the smtpd_tls_CAfile, in which case it is not necessary to have them in the
smtpd_tls_cert_file, smtpd_tls_dcert_file (obsolete) or smtpd_tls_eccert_file.

A certificate supplied here must be usable as an SSL server certificate and hence pass the
"openssl verify -purpose sslserver ..." test.

Example:

smtpd_tls_cert_file = /etc/postfix/server.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_chain_files (default: empty)
List of one or more PEM files, each holding one or more private keys directly followed by a
corresponding certificate chain. The file names are separated by commas and/or whitespace.
This parameter obsoletes the legacy algorithm-specific key and certificate file settings.
When this parameter is non-empty, the legacy parameters are ignored, and a warning is logged
if any are also non-empty.

RSA is still the most widely supported algorithm. Presently (late 2018), ECDSA support is
common, but not yet universal, and Ed25519 and Ed448 support is mostly absent. Therefore, an
RSA key should generally be configured, along with any additional keys for the other algo‐
rithms when desired.

Example (separate files for each key and corresponding certificate chain):

/etc/postfix/main.cf:
smtpd_tls_chain_files =
${config_directory}/ed25519.pem,
${config_directory}/ed448.pem,
${config_directory}/rsa.pem

Example (all keys and certificates in a single file):

/etc/postfix/main.cf:
smtpd_tls_chain_files = ${config_directory}/chains.pem

This feature is available in Postfix 3.4 and later.

smtpd_tls_cipherlist (default: empty)
Obsolete Postfix < 2.3 control for the Postfix SMTP server TLS cipher list. It is easy to
create interoperability problems by choosing a non-default cipher list. Do not use a non-de‐
fault TLS cipherlist for MX hosts on the public Internet. Clients that begin the TLS hand‐
shake, but are unable to agree on a common cipher, may not be able to send any email to the
SMTP server. Using a restricted cipher list may be more appropriate for a dedicated MSA or an
internal mailhub, where one can exert some control over the TLS software and settings of the
connecting clients.

Note: do not use "" quotes around the parameter value.

This feature is available with Postfix version 2.2. It is not used with Postfix 2.3 and
later; use smtpd_tls_mandatory_ciphers instead.

smtpd_tls_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP server will use with opportunistic TLS en‐
cryption. Cipher types listed in smtpd_tls_exclude_ciphers are excluded from the base defini‐
tion of the selected cipher grade. The default value is "medium" for Postfix releases after
the middle of 2015, "export" for older releases.

When TLS is mandatory the cipher grade is chosen via the smtpd_tls_mandatory_ciphers configu‐
ration parameter, see there for syntax details.

This feature is available in Postfix 2.6 and later. With earlier Postfix releases only the
smtpd_tls_mandatory_ciphers parameter is implemented, and opportunistic TLS always uses "ex‐
port" or better (i.e. all) ciphers.

smtpd_tls_dcert_file (default: empty)
File with the Postfix SMTP server DSA certificate in PEM format. This file may also contain
the Postfix SMTP server private DSA key. The DSA algorithm is obsolete and should not be
used.

See the discussion under smtpd_tls_cert_file for more details.

Example:

smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_dh1024_param_file (default: empty)
File with DH parameters that the Postfix SMTP server should use with non-export EDH ciphers.

With Postfix >= 3.7, built with OpenSSL version is 3.0.0 or later, if the parameter value is
either empty or "auto", then the DH parameter selection is delegated to the OpenSSL library,
which selects appropriate parameters based on the TLS handshake. This choice is likely to be
the most interoperable with SMTP clients using various TLS libraries, and custom local para‐
meters are no longer recommended when using Postfix >= 3.7 built against OpenSSL 3.0.0.

The best-practice choice of parameters uses a 2048-bit prime. This is fine, despite the his‐
torical "1024" in the parameter name. Do not be tempted to use much larger values, perfor‐
mance degrades quickly, and you may also cease to interoperate with some mainstream SMTP
clients. As of Postfix 3.1, the compiled-in default prime is 2048-bits, and it is not
strictly necessary, though perhaps somewhat beneficial to generate custom DH parameters.

Instead of using the exact same parameter sets as distributed with other TLS packages, it is
more secure to generate your own set of parameters with something like the following com‐
mands:

openssl dhparam -out /etc/postfix/dh2048.pem 2048
openssl dhparam -out /etc/postfix/dh1024.pem 1024
# As of Postfix 3.6, export-grade 512-bit DH parameters are no longer
# supported or needed.
openssl dhparam -out /etc/postfix/dh512.pem 512

It is safe to share the same DH parameters between multiple Postfix instances. If you pre‐
fer, you can generate separate parameters for each instance.

If you want to take maximal advantage of ciphers that offer forward secrecy see the Getting
started section of FORWARD_SECRECY_README. The full document conveniently presents all in‐
formation about Postfix "perfect" forward secrecy support in one place: what forward secrecy
is, how to tweak settings, and what you can expect to see when Postfix uses ciphers with for‐
ward secrecy.

Example:

smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem

This feature is available in Postfix 2.2 and later.

smtpd_tls_dh512_param_file (default: empty)
File with DH parameters that the Postfix SMTP server should use with export-grade EDH ci‐
phers. The default SMTP server cipher grade is "medium" with Postfix releases after the mid‐
dle of 2015, and as a result export-grade cipher suites are by default not used.

With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no longer supported, and this
parameter is silently ignored.

See also the discussion under the smtpd_tls_dh1024_param_file configuration parameter.

Example:

smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem

This feature is available in Postfix 2.2 and later, but is ignored in Postfix 3.6 and later.

smtpd_tls_dkey_file (default: $smtpd_tls_dcert_file)
File with the Postfix SMTP server DSA private key in PEM format. This file may be combined
with the Postfix SMTP server DSA certificate file specified with $smtpd_tls_dcert_file. The
DSA algorithm is obsolete and should not be used.

This feature is available in Postfix 2.2 and later.

smtpd_tls_eccert_file (default: empty)
File with the Postfix SMTP server ECDSA certificate in PEM format. This file may also con‐
tain the Postfix SMTP server private ECDSA key. With Postfix >= 3.4 the preferred way to
configure server keys and certificates is via the "smtpd_tls_chain_files" parameter.

See the discussion under smtpd_tls_cert_file for more details.

Example:

smtpd_tls_eccert_file = /etc/postfix/ecdsa-scert.pem

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

smtpd_tls_eckey_file (default: $smtpd_tls_eccert_file)
File with the Postfix SMTP server ECDSA private key in PEM format. This file may be combined
with the Postfix SMTP server ECDSA certificate file specified with $smtpd_tls_eccert_file.
With Postfix >= 3.4 the preferred way to configure server keys and certificates is via the
"smtpd_tls_chain_files" parameter.

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

smtpd_tls_eecdh_grade (default: see postconf -d output)
The Postfix SMTP server security grade for ephemeral elliptic-curve Diffie-Hellman (EECDH)
key exchange. As of Postfix 3.6, the value of this parameter is always ignored, and Postfix
behaves as though the auto value (described below) was chosen.

The available choices are:

auto Use the most preferred curve that is supported by both the client and the server.
This setting requires Postfix >= 3.2 compiled and linked with OpenSSL >= 1.0.2. This
is the default setting under the above conditions (and the only setting used with
Postfix >= 3.6).

none Don't use EECDH. Ciphers based on EECDH key exchange will be disabled. This is the de‐
fault in Postfix versions 2.6 and 2.7.

strong Use EECDH with approximately 128 bits of security at a reasonable computational cost.
This is the default in Postfix versions 2.8-3.5.

ultra Use EECDH with approximately 192 bits of security at computational cost that is ap‐
proximately twice as high as 128 bit strength ECC.

This feature is available in Postfix 2.6 and later, when it is compiled and linked with
OpenSSL 1.0.0 or later on platforms where EC algorithms have not been disabled by the vendor.

smtpd_tls_exclude_ciphers (default: empty)
List of ciphers or cipher types to exclude from the SMTP server cipher list at all TLS secu‐
rity levels. Excluding valid ciphers can create interoperability problems. DO NOT exclude ci‐
phers unless it is essential to do so. This is not an OpenSSL cipherlist; it is a simple list
separated by whitespace and/or commas. The elements are a single cipher, or one or more "+"
separated cipher properties, in which case only ciphers matching all the properties are ex‐
cluded.

Examples (some of these will cause problems):

smtpd_tls_exclude_ciphers = aNULL
smtpd_tls_exclude_ciphers = MD5, DES
smtpd_tls_exclude_ciphers = DES+MD5
smtpd_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
smtpd_tls_exclude_ciphers = kEDH+aRSA

This feature is available in Postfix 2.3 and later.

smtpd_tls_fingerprint_digest (default: see postconf -d output)
The message digest algorithm to construct remote SMTP client-certificate fingerprints or pub‐
lic key fingerprints (Postfix 2.9 and later) for check_ccert_access and per‐
mit_tls_clientcerts.

The default algorithm is sha256 with Postfix >= 3.6 and the compatibility_level set to 3.6 or
higher. With Postfix <= 3.5, the default algorithm is md5.

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

Example:

$ openssl x509 -in cert.pem -noout -pubkey |
openssl pkey -pubin -outform DER |
openssl dgst -sha256 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58

The Postfix SMTP server and client log the peer (leaf) certificate fingerprint and public key
fingerprint when the TLS loglevel is 2 or higher.

Example: client-certificate access table, with sha256 fingerprints:

/etc/postfix/main.cf:
smtpd_tls_fingerprint_digest = sha256
smtpd_client_restrictions =
check_ccert_access hash:/etc/postfix/access,
reject
/etc/postfix/access:
# Action folded to next line...
AF:88:7C:AD:51:95:6F:36:96:...:01:FB:2E:48:CD:AB:49:25:A2:3B
OK
85:16:78:FD:73:6E:CE:70:E0:...:5F:0D:3C:C8:6D:C4:2C:24:59:E1
permit_auth_destination

This feature is available in Postfix 2.5 and later.

smtpd_tls_key_file (default: $smtpd_tls_cert_file)
File with the Postfix SMTP server RSA private key in PEM format. This file may be combined
with the Postfix SMTP server RSA certificate file specified with $smtpd_tls_cert_file. With
Postfix >= 3.4 the preferred way to configure server keys and certificates is via the
"smtpd_tls_chain_files" parameter.

smtpd_tls_loglevel (default: 0)
Enable additional Postfix SMTP server logging of TLS activity. Each logging level also in‐
cludes the information that is logged at a lower logging level.

0 Disable logging of TLS activity.

1 Log only a summary message on TLS handshake completion - no logging of client cer‐
tificate trust-chain verification errors if client certificate verification is not re‐
quired. With Postfix 2.8 and earlier, log the summary message, peer certificate sum‐
mary information and unconditionally log trust-chain verification errors.

2 Also log levels during TLS negotiation.

3 Also log hexadecimal and ASCII dump of TLS negotiation process.

4 Also log hexadecimal and ASCII dump of complete transmission after STARTTLS.

Do not use "smtpd_tls_loglevel = 2" or higher except in case of problems. Use of loglevel 4
is strongly discouraged.

This feature is available in Postfix 2.2 and later.

smtpd_tls_mandatory_ciphers (default: medium)
The minimum TLS cipher grade that the Postfix SMTP server will use with mandatory TLS encryp‐
tion. The default grade ("medium") is sufficiently strong that any benefit from globally re‐
stricting TLS sessions to a more stringent grade is likely negligible, especially given the
fact that many implementations still do not offer any stronger ("high" grade) ciphers, while
those that do, will always use "high" grade ciphers. So insisting on "high" grade ciphers is
generally counter-productive. Allowing "export" or "low" ciphers is typically not a good
idea, as systems limited to just these are limited to obsolete browsers. No known SMTP
clients fail to support at least one "medium" or "high" grade cipher.

The following cipher grades are supported:

high Enable only "HIGH" grade OpenSSL ciphers. The underlying cipherlist is specified via
the tls_high_cipherlist configuration parameter, which you are strongly encouraged to
not change.

medium Enable "MEDIUM" grade or stronger OpenSSL ciphers. These use 128-bit or longer symmet‐
ric bulk-encryption keys. This is the default minimum strength for mandatory TLS en‐
cryption. The underlying cipherlist is specified via the tls_medium_cipherlist config‐
uration parameter, which you are strongly encouraged not to change.

null Enable only the "NULL" OpenSSL ciphers, these provide authentication without encryp‐
tion. This setting is only appropriate in the rare case that all clients are prepared
to use NULL ciphers (not normally enabled in TLS clients). The underlying cipherlist
is specified via the tls_null_cipherlist configuration parameter, which you are
strongly encouraged not to change.

Cipher types listed in smtpd_tls_mandatory_exclude_ciphers or smtpd_tls_exclude_ciphers are
excluded from the base definition of the selected cipher grade. See smtpd_tls_ciphers for ci‐
pher controls that apply to opportunistic TLS.

The underlying cipherlists for grades other than "null" include anonymous ciphers, but these
are automatically filtered out if the server is configured to ask for remote SMTP client cer‐
tificates. You are very unlikely to need to take any steps to exclude anonymous ciphers,
they are excluded automatically as required. If you must exclude anonymous ciphers even when
Postfix does not need or use peer certificates, set "smtpd_tls_exclude_ciphers = aNULL". To
exclude anonymous ciphers only when TLS is enforced, set "smtpd_tls_mandatory_exclude_ciphers
= aNULL".

This feature is available in Postfix 2.3 and later.

smtpd_tls_mandatory_exclude_ciphers (default: empty)
Additional list of ciphers or cipher types to exclude from the Postfix SMTP server cipher
list at mandatory TLS security levels. This list works in addition to the exclusions listed
with smtpd_tls_exclude_ciphers (see there for syntax details).

This feature is available in Postfix 2.3 and later.

smtpd_tls_mandatory_protocols (default: see postconf -d output)
TLS protocols accepted by the Postfix SMTP server with mandatory TLS encryption. If the list
is empty, the server supports all available TLS protocol versions. A non-empty value is a
list of protocol names to include or exclude, separated by whitespace, commas or colons.

As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set the
lowest acceptable TLS protocol version and/or the highest acceptable TLS protocol version.
To set the lower bound include an element of the form: ">=version" where version is a either
one of the TLS protocol names listed above, or a hexadecimal number corresponding to the de‐
sired TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound,
use "<=version". There must be no whitespace between the ">=" or "<=" symbols and the proto‐
col name or number.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtpd_tls_mandatory_protocols = >=TLSv1.2, <=0305
# Allow only TLS 1.2 and up:
smtpd_tls_mandatory_protocols = >=0x0303

With Postfix < 3.6 there is no support for a minimum or maximum version, and the protocol
range is configured via protocol exclusions. To require at least TLS 1.0, set
"smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3". Listing the protocols to include, rather
than protocols to exclude, is supported, but not recommended. The exclusion form more accu‐
rately matches the underlying OpenSSL interface.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this protocol via
"!TLSv1.3" is supported since Postfix 3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and
3.3.2).

Example:

# Preferred syntax with Postfix >= 3.6:
smtpd_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
# Legacy syntax:
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

This feature is available in Postfix 2.3 and later.

smtpd_tls_protocols (default: see postconf -d output)
TLS protocols accepted by the Postfix SMTP server with opportunistic TLS encryption. If the
list is empty, the server supports all available TLS protocol versions. A non-empty value is
a list of protocol names to include or exclude, separated by whitespace, commas or colons.

As of Postfix 3.6, the preferred way to limit the range of acceptable protocols is to set the
lowest acceptable TLS protocol version and/or the highest acceptable TLS protocol version.
To set the lower bound include an element of the form: ">=version" where version is a either
one of the TLS protocol names listed above, or a hexadecimal number corresponding to the de‐
sired TLS protocol version (0301 for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound,
use "<=version". There must be no whitespace between the ">=" or "<=" symbols and the proto‐
col name or number.

Hexadecimal example (Postfix >= 3.6):

# Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtpd_tls_protocols = >=TLSv1, <=0305
# Allow only TLS 1.0 and up:
smtpd_tls_protocols = >=0x0301

With Postfix < 3.6 there is no support for a minimum or maximum version, and the protocol
range is configured via protocol exclusions. To require at least TLS 1.0, set
"smtpd_tls_protocols = !SSLv2, !SSLv3". Listing the protocols to include, rather than proto‐
cols to exclude, is supported, but not recommended. The exclusion form more accurately
matches the underlying OpenSSL interface.

Support for "TLSv1.3" was introduced in OpenSSL 1.1.1. Disabling this protocol via
"!TLSv1.3" is supported since Postfix 3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and
3.3.2).

Example:
# Preferred syntax with Postfix >= 3.6:
smtpd_tls_protocols = >=TLSv1, <=TLSv1.3
# Legacy syntax:
smtpd_tls_protocols = !SSLv2, !SSLv3

This feature is available in Postfix 2.6 and later.

smtpd_tls_received_header (default: no)
Request that the Postfix SMTP server produces Received: message headers that include infor‐
mation about the protocol and cipher used, as well as the remote SMTP client CommonName and
client certificate issuer CommonName. This is disabled by default, as the information may be
modified in transit through other mail servers. Only information that was recorded by the
final destination can be trusted.

This feature is available in Postfix 2.2 and later.

smtpd_tls_req_ccert (default: no)
With mandatory TLS encryption, require a trusted remote SMTP client certificate in order to
allow TLS connections to proceed. This option implies "smtpd_tls_ask_ccert = yes".

When TLS encryption is optional, this setting is ignored with a warning written to the mail
log.

This feature is available in Postfix 2.2 and later.

smtpd_tls_security_level (default: empty)
The SMTP TLS security level for the Postfix SMTP server; when a non-empty value is specified,
this overrides the obsolete parameters smtpd_use_tls and smtpd_enforce_tls. This parameter is
ignored with "smtpd_tls_wrappermode = yes".

Specify one of the following security levels:

none TLS will not be used.

may Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not re‐
quire that clients use TLS encryption.

encrypt
Mandatory TLS encryption: announce STARTTLS support to remote SMTP clients, and re‐
quire that clients use TLS encryption. According to RFC 2487 this MUST NOT be applied
in case of a publicly-referenced SMTP server. Instead, this option should be used only
on dedicated servers.

Note 1: the "fingerprint", "verify" and "secure" levels are not supported here. The Postfix
SMTP server logs a warning and uses "encrypt" instead. To verify remote SMTP client certifi‐
cates, see TLS_README for a discussion of the smtpd_tls_ask_ccert, smtpd_tls_req_ccert, and
permit_tls_clientcerts features.

Note 2: The parameter setting "smtpd_tls_security_level = encrypt" implies
"smtpd_tls_auth_only = yes".

Note 3: when invoked via "sendmail -bs", Postfix will never offer STARTTLS due to insuffi‐
cient privileges to access the server private key. This is intended behavior.

This feature is available in Postfix 2.3 and later.

smtpd_tls_session_cache_database (default: empty)
Name of the file containing the optional Postfix SMTP server TLS session cache. Specify a
database type that supports enumeration, such as btree or sdbm; there is no need to support
concurrent access. The file is created if it does not exist. The smtpd(8) daemon does not
use this parameter directly, rather the cache is implemented indirectly in the tlsmgr(8) dae‐
mon. This means that per-smtpd-instance master.cf overrides of this parameter are not effec‐
tive. Note that each of the cache databases supported by tlsmgr(8) daemon: $smtpd_tls_ses‐
sion_cache_database, $smtp_tls_session_cache_database (and with Postfix 2.3 and later
$lmtp_tls_session_cache_database), needs to be stored separately. It is not at this time pos‐
sible to store multiple caches in a single database.

Note: dbm databases are not suitable. TLS session objects are too large.

As of Postfix 2.11 the preferred mechanism for session resumption is RFC 5077 TLS session
tickets, which don't require server-side storage. Consequently, for Postfix >= 2.11 this pa‐
rameter should generally be left empty. TLS session tickets require an OpenSSL library (at
least version 0.9.8h) that provides full support for this TLS extension. See also
smtpd_tls_session_cache_timeout.

Example:

smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_scache

This feature is available in Postfix 2.2 and later.

smtpd_tls_session_cache_timeout (default: 3600s)
The expiration time of Postfix SMTP server TLS session cache information. A cache cleanup is
performed periodically every $smtpd_tls_session_cache_timeout seconds. As with
$smtpd_tls_session_cache_database, this parameter is implemented in the tlsmgr(8) daemon and
therefore per-smtpd-instance master.cf overrides are not possible.

As of Postfix 2.11 this setting cannot exceed 100 days. If set <= 0, session caching is dis‐
abled, not just via the database, but also via RFC 5077 TLS session tickets, which don't re‐
quire server-side storage. If set to a positive value less than 2 minutes, the minimum value
of 2 minutes is used instead. TLS session tickets require an OpenSSL library (at least ver‐
sion 0.9.8h) that provides full support for this TLS extension.

This feature is available in Postfix 2.2 and later, and updated for TLS session ticket sup‐
port in Postfix 2.11.

smtpd_tls_wrappermode (default: no)
Run the Postfix SMTP server in TLS "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 (submissions/smtps) is
reserved for this purpose.

This feature is available in Postfix 2.2 and later.

smtpd_upstream_proxy_protocol (default: empty)
The name of the proxy protocol used by an optional before-smtpd proxy agent. When a proxy
agent is used, this protocol conveys local and remote address and port information. Specify
"smtpd_upstream_proxy_protocol = haproxy" to enable the haproxy protocol; version 2 is sup‐
ported with Postfix 3.5 and later.

NOTE: To use the nginx proxy with smtpd(8), 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 para‐
meter.

This feature is available in Postfix 2.10 and later.

smtpd_use_tls (default: no)
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not require that
clients use TLS encryption.

Note: when invoked via "sendmail -bs", Postfix will never offer STARTTLS due to insufficient
privileges to access the server private key. This is intended behavior.

This feature is available in Postfix 2.2 and later. With Postfix 2.3 and later use
smtpd_tls_security_level instead.

smtputf8_autodetect_classes (default: sendmail, verify)
Detect that a message requires SMTPUTF8 support for the specified mail origin classes. This
is a workaround to avoid chicken-and-egg problems during the initial SMTPUTF8 roll-out in en‐
vironments with pre-existing mail flows that contain UTF8. Those mail flows should not break
because Postfix suddenly refuses to deliver such mail to down-stream MTAs that don't announce
SMTPUTF8 support.

The problem is that Postfix cannot rely solely on the sender's declaration that a message re‐
quires SMTPUTF8 support, because UTF8 may be introduced during local processing (for example,
the client hostname in Postfix's Received: header, adding @$myorigin or .$mydomain to an in‐
complete address, address rewriting, alias expansion, automatic BCC recipients, local for‐
warding, and changes made by header checks or Milter applications).

For now, the default is to enable "SMTPUTF8 required" autodetection only for Postfix sendmail
command-line submissions and address verification probes. This may change once SMTPUTF8 sup‐
port achieves world domination. However, sites that add UTF8 content via local processing
(see above) should autodetect the need for SMTPUTF8 support for all email.

Specify one or more of the following:

sendmail
Submission with the Postfix sendmail(1) 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, RFC 6532, and
RFC 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 parame‐
ter disables locally-generated bounces, changes the handling of negative responses from re‐
mote servers, content filters or plugins, and prevents the Postfix SMTP server from rejecting
mail permanently by changing 5xx reply codes into 4xx. However, soft_bounce is no cure for
address rewriting mistakes or mail routing mistakes.

Note: "soft_bounce = yes" is in some cases implemented by modifying server responses. There‐
fore, the response that Postfix logs may differ from the response that Postfix actually sends
or receives.

Example:

soft_bounce = yes

stale_lock_time (default: 500s)
The time after which a stale exclusive mailbox lockfile is removed. This is used for deliv‐
ery to file or mailbox.

stress (default: empty)
This feature is documented in the STRESS_README document.

This feature is available in Postfix 2.5 and later.

strict_7bit_headers (default: no)
Reject mail with 8-bit text in message headers. This blocks mail from poorly written applica‐
tions.

This feature should not be enabled on a general purpose mail server, because it is likely to
reject legitimate email.

This feature is available in Postfix 2.0 and later.

strict_8bitmime (default: no)
Enable both strict_7bit_headers and strict_8bitmime_body.

This feature should not be enabled on a general purpose mail server, because it is likely to
reject legitimate email.

This feature is available in Postfix 2.0 and later.

strict_8bitmime_body (default: no)
Reject 8-bit message body text without 8-bit MIME content encoding information. This blocks
mail from poorly written applications.

Unfortunately, this also rejects majordomo approval requests when the included request con‐
tains valid 8-bit MIME mail, and it rejects bounces from mailers that do not MIME encapsulate
8-bit content (for example, bounces from qmail or from old versions of Postfix).

This feature should not be enabled on a general purpose mail server, because it is likely to
reject legitimate email.

This feature is available in Postfix 2.0 and later.

strict_mailbox_ownership (default: yes)
Defer delivery when a mailbox file is not owned by its recipient. The default setting is not
backwards compatible.

This feature is available in Postfix 2.5.3 and later.

strict_mime_encoding_domain (default: no)
Reject mail with invalid Content-Transfer-Encoding: information for the message/* or multi‐
part/* MIME content types. This blocks mail from poorly written software.

This feature should not be enabled on a general purpose mail server, because it will reject
mail after a single violation.

This feature is available in Postfix 2.0 and later.

strict_rfc821_envelopes (default: no)
Require that addresses received in SMTP MAIL FROM and RCPT TO commands are enclosed with <>,
and that those addresses do not contain RFC 822 style comments or phrases. This stops mail
from poorly written software.

By default, the Postfix SMTP server accepts RFC 822 syntax in MAIL FROM and RCPT TO ad‐
dresses.

strict_smtputf8 (default: no)
Enable stricter enforcement of the SMTPUTF8 protocol. The Postfix SMTP server accepts UTF8
sender or recipient addresses only when the client requests an SMTPUTF8 mail transaction.

This feature is available in Postfix 3.0 and later.

sun_mailtool_compatibility (default: no)
Obsolete SUN mailtool compatibility feature. Instead, use "mailbox_delivery_lock = dotlock".

swap_bangpath (default: yes)
Enable the rewriting of "site!user" into "user@site". This is necessary if your machine is
connected to UUCP networks. It is enabled by default.

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

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

• The message is received from a network client that matches $lo‐
cal_header_rewrite_clients,

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

To get the behavior before Postfix version 2.2, specify "local_header_rewrite_clients = sta‐
tic: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 de‐
fault facility. Examples are errors while parsing the command line arguments, and errors
while accessing the Postfix main.cf configuration file.

syslog_name (default: see postconf -d output)
A prefix that is prepended to the process name in syslog records, so that, for example,
"smtpd" becomes "prefix/smtpd".

Warning: a non-default syslog_name setting takes effect only after a Postfix process has com‐
pleted initialization. Errors during process initialization will be logged with the default
name. Examples are errors while parsing the command line arguments, and errors while access‐
ing the Postfix main.cf configuration file.

tcp_windowsize (default: 0)
An optional workaround for routers that break TCP window scaling. Specify a value > 0 and <
65536 to enable this feature. With Postfix TCP servers (smtpd(8), qmqpd(8)), this feature 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 enable
Postfix TCP servers with the updated tcp_windowsize setting:

# postconf -e tcp_windowsize=65535 master_service_disable=
# postfix reload

If you skip these steps with a running Postfix system, then the tcp_windowsize change will
work only for Postfix TCP clients (smtp(8), 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 specified
with *_tls_CApath or *_tls_CAfile. The default is "no"; this prevents Postfix from trusting
third-party certificates and giving them relay permission with permit_tls_all_clientcerts.

This feature is available in Postfix 2.4.15, 2.5.11, 2.6.8, 2.7.2 and later versions. Specify
"tls_append_default_CA = yes" for backwards compatibility, to avoid breaking certificate ver‐
ification with sites that don't use permit_tls_all_clientcerts.

tls_config_file (default: default)
Optional configuration file with baseline OpenSSL settings. OpenSSL loads any SSL settings
found in the configuration file for the selected application name (see tls_config_name) or
else the built-in application name "openssl_conf" when no application name is specified, or
no corresponding configuration section is present.

With OpenSSL releases 1.1.1 and 1.1.1a, applications (including Postfix) can neither specify
an alternative configuration file, nor avoid loading the default configuration file.

With OpenSSL 1.1.1b or later, this parameter may be set to one of:

default (default)
Load the system-wide "openssl.cnf" configuration file.

none (recommended, OpenSSL 1.1.1b or later only)
This setting disables loading of the system-wide "openssl.cnf" file.

/absolute-path (OpenSSL 1.1.1b or later only)
Load the configuration file specified by /absolute-path. With this setting it is an
error for the file to not contain any settings for the selected tls_config_name.
There is no fallback to the default "openssl_conf" name.

Failures in processing of the built-in default configuration file, are silently ignored. Any
errors in loading a non-default configuration file are detected by Postfix, and cause TLS
support to be disabled.

The OpenSSL configuration file format is not documented here, beyond giving two examples.

Example: Default settings for all applications.

# The name 'openssl_conf' is the default application name
# The section name to the right of the '=' sign is arbitrary,
# any name will do, so long as it refers to the desired section.
#
# The name 'system_default' selects the settings applied internally
# by the SSL library as part of SSL object creation. Applications
# can then apply any additional settings of their choice.
#
# In this example, TLS versions prior to 1.2 are disabled by default.
#
openssl_conf = system_wide_settings
[system_wide_settings]
ssl_conf = ssl_library_settings
[ssl_library_settings]
system_default = initial_ssl_settings
[initial_ssl_settings]
MinProtocol = TLSv1.2

Example: Custom settings for an application named "postfix".

# The mapping from an application name to the corresponding configuration
# section must appear near the top of the file, (in what is sometimes called
# the "default section") prior to the start of any explicitly named
# "[sections]". The named sections can appear in any order and don't nest.
#
postfix = postfix_settings
[postfix_settings]
ssl_conf = postfix_ssl_settings
[postfix_ssl_settings]
system_default = baseline_postfix_settings
[baseline_postfix_settings]
MinProtocol = TLSv1

This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6, 3.6.10, and 3.5.20.

tls_config_name (default: empty)
The application name passed by Postfix to OpenSSL library initialization functions. This
name is used to select the desired configuration "section" in the OpenSSL configuration file
specified via the tls_config_file parameter. When empty, or when the selected name is not
present in the configuration file, the default application name ("openssl_conf") is used as a
fallback.

This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6, 3.6.10, and 3.5.20.

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 underly‐
ing OpenSSL library, otherwise the Postfix SMTP client will not support DANE TLSA security.

Specify a list of digest names separated by commas and/or whitespace. Each digest name may
be followed by an optional "=<number>" suffix. For example, "sha512" may instead be speci‐
fied as "sha512=2" and "sha256" may instead be specified as "sha256=1". The optional number
must match the <a href="https://www.iana.org/assignments/dane-parameters/dane-parame‐
ters.xhtml#matching-types" >IANA assigned TLSA matching type number the algorithm in ques‐
tion. Postfix will check this constraint for the algorithms it knows about. Additional
matching type algorithms registered with IANA can be added with explicit numbers provided
they are supported by OpenSSL.

Invalid list elements are logged with a warning and disable DANE support. TLSA RRs that
specify digests not included in the list are ignored with a warning.

Note: It is unwise to omit sha256 from the digest list. This digest algorithm is the only
mandatory to implement digest algorithm in RFC 6698, and many servers are expected to publish
TLSA records with just sha256 digests. Unless one of the standard digests is seriously com‐
promised and servers have had ample time to update their TLSA records you should not omit any
standard digests, just arrange them in order from strongest to weakest.

This feature is available in Postfix 2.11 and later.

tls_dane_trust_anchor_digest_enable (default: yes)
Enable support for RFC 6698 (DANE TLSA) DNS records that contain digests of trust-anchors
with certificate usage "2". Do not change this setting from its default value.

This feature is available in Postfix 2.11 through 3.1. It has been withdrawn in Postfix 3.2,
as trust-anchor TLSA records are now widely used and have proved sufficiently reliable.
Postfix 3.2 and later ignore this configuration parameter and behaves as though it were set
to "yes".

tls_disable_workarounds (default: see postconf -d output)
List or bit-mask of OpenSSL bug work-arounds to disable.

The OpenSSL toolkit includes a set of work-arounds for buggy SSL/TLS implementations. Appli‐
cations, such as Postfix, that want to maximize interoperability ask the OpenSSL library to
enable the full set of recommended work-arounds.

From time to time, it is discovered that a work-around creates a security issue, and should
no longer be used. If upgrading OpenSSL to a fixed version is not an option or an upgrade is
not available in a timely manner, or in closed environments where no buggy clients or servers
exist, it may be appropriate to disable some or all of the OpenSSL interoperability
work-arounds. This parameter specifies which bug work-arounds to disable.

If the value of the parameter is a hexadecimal long integer starting with "0x", the bug
work-arounds corresponding to the bits specified in its value are removed from the SSL_OP_ALL
work-around bit-mask (see openssl/ssl.h and SSL_CTX_set_options(3)). You can specify more
bits than are present in SSL_OP_ALL, excess bits are ignored. Specifying 0xFFFFFFFF disables
all bug-workarounds on a 32-bit system. This should also be sufficient on 64-bit systems, un‐
til OpenSSL abandons support for 32-bit systems and starts using the high 32 bits of a 64-bit
bug-workaround mask.

Otherwise, the parameter is a white-space or comma separated list of specific named bug
work-arounds chosen from the list below. It is possible that your OpenSSL version includes
new bug work-arounds added after your Postfix source code was last updated, in that case you
can only disable one of these via the hexadecimal syntax above.

CRYPTOPRO_TLSEXT_BUG
New with GOST support in OpenSSL 1.0.0.

DONT_INSERT_EMPTY_FRAGMENTS
See SSL_CTX_set_options(3)

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 8422).
It is unwise to list only "bleeding-edge" curves supported by a small subset of clients. The
default list is suitable for most users.

Postfix skips curve names that are unknown to OpenSSL, or that are known but not yet imple‐
mented. This makes it possible to "anticipate" support for curves that should be used once
they become available. In particular, in some OpenSSL versions, the new RFC 8031 curves
"X25519" and "X448" may be known by name, but ECDH support for either or both may be missing.
These curves may appear in the default value of this parameter, even though they'll only be
usable with later versions of OpenSSL.

See also the "tls_ffdhe_auto_groups" parameter, which supports customizing the list of FFDHE
groups enabled with TLS 1.3. That setting is introduced with Postfix 3.8, when built against
OpenSSL 3.0 or later.

This feature is available in Postfix 3.2 and later, when it is compiled and linked with
OpenSSL 1.0.2 or later on platforms where EC algorithms have not been disabled by the vendor.

tls_eecdh_strong_curve (default: prime256v1)
The elliptic curve used by the Postfix SMTP server for sensibly strong ephemeral ECDH key ex‐
change. This curve is used by the Postfix SMTP server when "smtpd_tls_eecdh_grade = strong".
The phrase "sensibly strong" means approximately 128-bit security based on best known at‐
tacks. The selected curve must be implemented by OpenSSL (as reported by ecparam(1) with the
"-list_curves" option) and be one of the curves listed in Section 5.1.1 of RFC 8422. You
should not generally change this setting. Remote SMTP client implementations must support
this curve for EECDH key exchange to take place. It is unwise to choose only "bleeding-edge"
curves supported by only a small subset of clients.

The default "strong" curve is rated in NSA Suite B for information classified up to SECRET.

Note: elliptic curve names are poorly standardized; different standards groups are assigning
different names to the same underlying curves. The curve with the X9.62 name "prime256v1" is
also known under the SECG name "secp256r1", but OpenSSL does not recognize the latter name.

This feature is available in Postfix 2.6 and later, when it is compiled and linked with
OpenSSL 1.0.0 or later on platforms where EC algorithms have not been disabled by the vendor.

tls_eecdh_ultra_curve (default: secp384r1)
The elliptic curve used by the Postfix SMTP server for maximally strong ephemeral ECDH key
exchange. This curve is used by the Postfix SMTP server when "smtpd_tls_eecdh_grade = ultra".
The phrase "maximally strong" means approximately 192-bit security based on best known at‐
tacks. This additional strength comes at a significant computational cost, most users should
instead set "smtpd_tls_eecdh_grade = strong". The selected curve must be implemented by
OpenSSL (as reported by ecparam(1) with the "-list_curves" option) and be one of the curves
listed in Section 5.1.1 of RFC 8422. You should not generally change this setting. Remote
SMTP client implementations must support this curve for EECDH key exchange to take place. It
is unwise to choose only "bleeding-edge" curves supported by only a small subset of clients.

This default "ultra" curve is rated in NSA Suite B for information classified up to TOP SE‐
CRET.

This feature is available in Postfix 2.6 and later, when it is compiled and linked with
OpenSSL 1.0.0 or later on platforms where EC algorithms have not been disabled by the vendor.

tls_export_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "export" or higher grade ciphers. Ignored as of Postfix 3.8. In
earlier Postfix releases this defined the meaning of the "export" setting in smtpd_tls_ci‐
phers, smtpd_tls_mandatory_ciphers, smtp_tls_ciphers, smtp_tls_mandatory_ciphers,
lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change
this setting.

This feature is available in Postfix 2.3 and later.

tls_fast_shutdown_enable (default: yes)
A workaround for implementations that hang Postfix while shutting down a TLS session, until
Postfix times out. With this enabled, Postfix will not wait for the remote TLS peer to re‐
spond to a TLS 'close' notification. This behavior is recommended for TLSv1.0 and later.

tls_ffdhe_auto_groups (default: see postconf -d output)
The prioritized list of finite-field Diffie-Hellman ephemeral (FFDHE) key exchange groups
supported by the Postfix SMTP client and server. OpenSSL 3.0 adds support for FFDHE key
agreement in TLS 1.3. In OpenSSL 1.1.1, TLS 1.3 was only supported with elliptic-curve based
key agreement. The "tls_ffdhe_auto_groups" parameter makes it possible to configure the list
of FFDHE groups that the Postfix client or server will enable in OpenSSL 3.0 and up. This
parameter has no effect when Postfix is built against earlier OpenSSL versions.

The default list of FFDHE groups that Postfix enables in OpenSSL 3.0 and up includes just the
2048 and 3072-bit groups. Stronger FFDHE groups perform poorly and EC groups are a much bet‐
ter choice for the same security level. Postfix ignores group names that are unknown to
OpenSSL, or that are known but not yet implemented. The FFDHE groups are largely a backup,
in case some peer does not support EC key exchange, or EC key exchange needs to be disabled
for some pressing reason.

Setting this parameter empty disables FFDHE support in TLS 1.3. Whether FFDHE key agreement
is enabled in TLS 1.2 and earlier depends on whether any of the "kDHE" ciphers are included
in the cipherlist.

Conversely, setting "tls_eecdh_auto_curves" empty disables TLS 1.3 EC key agreement in
OpenSSL 3.0 and later. Note that at least one of "tls_eecdh_auto_curves" and
"tls_ffdhe_auto_groups" must be non-empty, this is required by OpenSSL 3.0. If both are in‐
advertently set empty, Postfix will fall back to the compiled-in defaults.

All the default groups and EC curves should sufficiently strong to make "pruning" the de‐
faults unwise. At a minimum, "X25519" and "P-256" (a.k.a. "prime256v1") should be among the
enabled EC curves, while "dhe2048" and "dhe3072" should be among the FFDHE groups.

This feature is available in Postfix 3.8 and later, when it is compiled and linked with
OpenSSL 3.0 or later.

tls_high_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "high" grade ciphers. This defines the meaning of the "high" set‐
ting in smtpd_tls_ciphers, smtpd_tls_mandatory_ciphers, smtp_tls_ciphers, smtp_tls_manda‐
tory_ciphers, lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. You are strongly encouraged
not to change this setting.

This feature is available in Postfix 2.3 and later.

tls_legacy_public_key_fingerprints (default: no)
A temporary migration aid for sites that use certificate public-key fingerprints with Postfix
2.9.0..2.9.5, which use an incorrect algorithm. This parameter has no effect on the certifi‐
cate fingerprint support that is available since Postfix 2.2.

Specify "tls_legacy_public_key_fingerprints = yes" temporarily, pending a migration from con‐
figuration files with incorrect Postfix 2.9.0..2.9.5 certificate public-key finger prints, to
the correct fingerprints used by Postfix 2.9.6 and later. To compute the correct certificate
public-key fingerprints, see TLS_README.

This feature is available in Postfix 2.9.6 and later.

tls_low_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "low" or higher grade ciphers. Ignored as of Postfix 3.8. In
earlier Postfix releases this defined 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 not to change this setting.

This feature is available in Postfix 2.3 and later.

tls_medium_cipherlist (default: see postconf -d output)
The OpenSSL cipherlist for "medium" or higher grade ciphers. This defines the meaning of the
"medium" setting in smtpd_tls_ciphers, smtpd_tls_mandatory_ciphers, smtp_tls_ciphers,
smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. This is the
default cipherlist for mandatory TLS encryption in the TLS client (with anonymous ciphers
disabled when verifying server certificates). This is the default cipherlist for opportunis‐
tic TLS with Postfix releases after the middle of 2015. You are strongly encouraged not to
change this setting.

This feature is available in Postfix 2.3 and later.

tls_null_cipherlist (default: eNULL:!aNULL)
The OpenSSL cipherlist for "NULL" grade ciphers that provide authentication without encryp‐
tion. This defines the meaning of the "null" setting in smtpd_tls_mandatory_ciphers,
smtp_tls_mandatory_ciphers and lmtp_tls_mandatory_ciphers. You are strongly encouraged not
to change this setting.

This feature is available in Postfix 2.3 and later.

tls_preempt_cipherlist (default: no)
With SSLv3 and later, use the Postfix SMTP server's cipher preference order instead of the
remote client's cipher preference order.

By default, the OpenSSL server selects the client's most preferred cipher that the server
supports. With SSLv3 and later, the server may choose its own most preferred cipher that is
supported (offered) by the client. Setting "tls_preempt_cipherlist = yes" enables server ci‐
pher preferences.

While server cipher selection may in some cases lead to a more secure or performant cipher
choice, there is some risk of interoperability issues. In the past, some SSL clients have
listed lower priority ciphers that they did not implement correctly. If the server chooses a
cipher that the client prefers less, it may select a cipher whose client implementation is
flawed. Most notably Windows 2003 Microsoft Exchange servers have flawed implementations of
DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA. Enabling server cipher-suite
selection may create interoperability issues with Windows 2003 Microsoft Exchange clients.

This feature is available in Postfix 2.8 and later, in combination with OpenSSL 0.9.7 and
later.

tls_random_bytes (default: 32)
The number of bytes that tlsmgr(8) reads from $tls_random_source when (re)seeding the in-mem‐
ory pseudo random number generator (PRNG) pool. The default of 32 bytes (256 bits) is good
enough for 128bit symmetric keys. If using EGD or a device file, a maximum of 255 bytes is
read.

This feature is available in Postfix 2.2 and later.

tls_random_exchange_name (default: see postconf -d output)
Name of the pseudo random number generator (PRNG) state file that is maintained by tlsmgr(8).
The file is created when it does not exist, and its length is fixed at 1024 bytes.

As of version 2.5, Postfix no longer uses root privileges when opening this file, and the de‐
fault file location was changed from ${config_directory}/prng_exch to ${data_direc‐
tory}/prng_exch. As a migration aid, an attempt to open the file under a non-Postfix direc‐
tory is redirected to the Postfix-owned data_directory, and a warning is logged.

This feature is available in Postfix 2.2 and later.

tls_random_prng_update_period (default: 3600s)
The time between attempts by tlsmgr(8) to save the state of the pseudo random number genera‐
tor (PRNG) to the file specified with $tls_random_exchange_name.

This feature is available in Postfix 2.2 and later.

tls_random_reseed_period (default: 3600s)
The maximal time between attempts by tlsmgr(8) to re-seed the in-memory pseudo random number
generator (PRNG) pool from external sources. The actual time between re-seeding attempts is
calculated using the PRNG, and is between 0 and the time specified.

This feature is available in Postfix 2.2 and later.

tls_random_source (default: see postconf -d output)
The external entropy source for the in-memory tlsmgr(8) pseudo random number generator (PRNG)
pool. Be sure to specify a non-blocking source. If this source is not a regular file, the
entropy source type must be prepended: egd:/path/to/egd_socket for a source with EGD compat‐
ible socket interface, or dev:/path/to/device for a device file.

Note: on OpenBSD systems specify dev:/dev/arandom when dev:/dev/urandom gives timeout errors.

This feature is available in Postfix 2.2 and later.

tls_server_sni_maps (default: empty)
Optional lookup tables that map names received from remote SMTP clients via the TLS Server
Name Indication (SNI) extension to the appropriate keys and certificate chains. This parame‐
ter is implemented in the Postfix TLS library, and applies to both smtpd(8) and the SMTP
server mode of tlsproxy(8).

When this parameter is non-empty, the Postfix SMTP server enables SNI extension processing,
and logs SNI values that are invalid or don't match an entry in the specified tables. When
an entry does match, the SNI name is logged as part of the connection summary at log levels 1
and higher.

The lookup key is either the verbatim SNI domain name or an ancestor domain prefixed with a
leading dot. For internationalized domains, the lookup key must be in IDNA 2008 A-label form
(as required in the TLS SNI extension).

The syntax of the lookup value is the same as with the smtp_tls_chain_files parameter (see
there for additional details), but here scoped to just TLS connections in which the client
sends a matching SNI domain name.

Example:

/etc/postfix/main.cf:
#
# The indexed SNI table must be created with "postmap -F"
#
indexed = ${default_database_type}:${config_directory}/
tls_server_sni_maps = ${indexed}sni

/etc/postfix/sni:
#
# The example.com domain has both an RSA and ECDSA certificate
# chain. The chain files MUST start with the private key,
# with the certificate chain next, starting with the leaf
# (server) certificate, and then the issuer certificates.
#
example.com /etc/postfix/sni-chains/rsa2048.example.com.pem,
/etc/postfix/sni-chains/ecdsa-p256.example.com.pem
#
# The example.net domain has a wildcard certificate, and two
# additional DNS names. So its certificate chain is also used
# with any subdomain, plus the additional names.
#
example.net /etc/postfix/sni-chains/example.net.pem
.example.net /etc/postfix/sni-chains/example.net.pem
example.info /etc/postfix/sni-chains/example.net.pem
example.org /etc/postfix/sni-chains/example.net.pem

Note that the SNI lookup tables should also have entries for the domains that correspond to
the Postfix SMTP server's default certificate(s). This ensures that the remote SMTP client's
TLS SNI extension gets a positive response when it specifies one of the Postfix SMTP server's
default domains, and ensures that the Postfix SMTP server will not log an SNI name mismatch
for such a domain. The Postfix SMTP server's default certificates are then only used when
the client sends no SNI or when it sends SNI with a domain that the server knows no certifi‐
cate(s) for.

The mapping from an SNI domain name to a certificate chain is indirect. In the input source
files for "cdb", "hash", "btree" or other tables that are converted to on-disk indexed files
via postmap(1), 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 interpreted
at run-time, and don't have a separate source format, the table value is again a list files,
that are loaded into memory when the table is opened.

With tables whose content is managed outside of Postfix, such as LDAP, MySQL, PostgreSQL,
socketmap and tcp, the value must be a concatenation of the desired PEM keys and certificate
chains, that is then further encoded to yield a single-line base64 string. Creation of such
tables and secure storage (the value includes private key material) are outside the responsi‐
bility of Postfix.

With "socketmap" and "tcp" the data will be transmitted in the clear, and there is no query
access control, so these are generally unsuitable for storing SNI chains. With LDAP and SQL,
you should restrict read access and use TLS to protect the sensitive data in transit.

Typically there is only one private key and its chain of certificates starting with the
"leaf" certificate corresponding to that key, and continuing with the appropriate intermedi‐
ate issuer CA certificates, with each certificate ideally followed by its issuer. Servers
that have keys and certificates for more than one algorithm (e.g. both an RSA key and an
ECDSA key, or even RSA, ECDSA and Ed25519) can use multiple chains concatenated together,
with the key always listed before the corresponding certificates.

This feature is available in Postfix 3.4 and later.

tls_session_ticket_cipher (default: Postfix >= 3.0: aes-256-cbc, Postfix < 3.0: aes-128-cbc)
Algorithm used to encrypt RFC5077 TLS session tickets. This algorithm must use CBC mode,
have a 128-bit block size, and must have a key length between 128 and 256 bits. The default
is aes-256-cbc. Overriding the default to choose a different algorithm is discouraged.

Setting this parameter empty disables session ticket support in the Postfix SMTP server. An‐
other way to disable session ticket support is via the tls_ssl_options parameter.

This feature is available in Postfix 3.0 and later.

tls_ssl_options (default: empty)
List or bit-mask of OpenSSL options to enable.

The OpenSSL toolkit provides a set of options that applications can enable to tune the
OpenSSL behavior. Some of these work around bugs in other implementations and are on by de‐
fault. You can use the tls_disable_workarounds parameter to selectively disable some or all
of the bug work-arounds, making OpenSSL more strict at the cost of non-interoperability with
SSL clients or servers that exhibit the bugs.

Other options are off by default, and typically enable or disable features rather than bug
work-arounds. These may be turned on (with care) via the tls_ssl_options parameter. The
value is a white-space or comma separated list of named options chosen from the list below.
The names are not case-sensitive, you can use lower-case if you prefer. The upper case val‐
ues below match the corresponding macro name in the ssl.h header file with the SSL_OP_ prefix
removed. It is possible that your OpenSSL version includes new options added after your
Postfix source code was last updated, in that case you can only enable one of these via the
hexadecimal syntax below.

You should only enable features via the hexadecimal mask when the need to control the feature
is critical (to deal with a new vulnerability or a serious interoperability problem). Post‐
fix DOES NOT promise backwards compatible behavior with respect to the mask bits. A feature
enabled via the mask in one release may be enabled by other means in a later release, and the
mask bit will then be ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better solution.

If the value of the parameter is a hexadecimal long integer starting with "0x", the options
corresponding to the bits specified in its value are enabled (see openssl/ssl.h and
SSL_CTX_set_options(3)). You can only enable options not already controlled by other Postfix
settings. For example, you cannot disable protocols or enable server cipher preference. Do
not attempt to enable 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 resump‐
tion. 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 attack.
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 exam‐
ple.net may be:

example.com. IN MX 0 example.com.mx1.example.net.
example.com. IN MX 0 example.com.mx2.example.net.

and the TLS certificate may be for "*.example.net". The "*" then corresponds with multiple
labels in the mail server domain name. While multi-label wildcards are not widely supported,
and are not blessed by any standard, there is little to be gained by disallowing their use in
this context.

Notes:

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

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

• Earlier Postfix implementations behave as if "tls_wildcard_matches_multiple_labels =
no".

This feature is available in Postfix 2.11 and later.

tlsmgr_service_name (default: tlsmgr)
The name of the tlsmgr(8) service entry in master.cf. This service maintains TLS session
caches and other information in support of TLS.

This feature is available in Postfix 2.11 and later.

tlsproxy_client_CAfile (default: $smtp_tls_CAfile)
A file containing CA certificates of root CAs trusted to sign either remote TLS server cer‐
tificates or intermediate CA certificates. See smtp_tls_CAfile for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_CApath (default: $smtp_tls_CApath)
Directory with PEM format Certification Authority certificates that the Postfix tlsproxy(8)
client uses to verify a remote TLS server certificate. See smtp_tls_CApath for further de‐
tails.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_cert_file (default: $smtp_tls_cert_file)
File with the Postfix tlsproxy(8) 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 cer‐
tificates is via the "tlsproxy_client_chain_files" parameter.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_eckey_file (default: $smtp_tls_eckey_file)
File with the Postfix tlsproxy(8) 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. Use tlsproxy_client_security_level instead.

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_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 - 3.6. It was renamed to tlsproxy_client_secu‐
rity_level in Postfix 3.7.

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 des‐
tination and by remote TLS server hostname. See smtp_tls_per_site for further details.

This feature is available in Postfix 3.4 and later.

tlsproxy_client_policy (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 - 3.6. It was renamed to tlsproxy_client_policy_maps
in Postfix 3.7.

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 3.7 and later. It was previously called tl‐
sproxy_client_policy.

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 3.7 and later. It was previously called tl‐
sproxy_client_level.

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. Use tlsproxy_client_security_level instead.

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. Use tlsproxy_tls_security_level
instead.

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 further de‐
tails.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_CApath (default: $smtpd_tls_CApath)
A directory containing (PEM format) CA certificates of root CAs trusted to sign either remote
SMTP client certificates or intermediate CA certificates. See smtpd_tls_CApath for further
details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_always_issue_session_ids (default: $smtpd_tls_always_issue_session_ids)
Force the Postfix tlsproxy(8) 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 cer‐
tificates is via the "tlsproxy_tls_chain_files" parameter.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_chain_files (default: $smtpd_tls_chain_files)
Files with the Postfix tlsproxy(8) 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 opportunistic
TLS encryption. See smtpd_tls_ciphers for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dcert_file (default: $smtpd_tls_dcert_file)
File with the Postfix tlsproxy(8) 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 ex‐
port-grade cipher suites are by default not used.

With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no longer supported, and this
parameter is silently ignored.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dkey_file (default: $smtpd_tls_dkey_file)
File with the Postfix tlsproxy(8) 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 fur‐
ther details. With Postfix >= 3.4 the preferred way to configure tlsproxy server keys and
certificates is via the "tlsproxy_tls_chain_files" parameter.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eckey_file (default: $smtpd_tls_eckey_file)
File with the Postfix tlsproxy(8) 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 further de‐
tails.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_protocols (default: $smtpd_tls_mandatory_protocols)
The SSL/TLS protocols accepted by the Postfix tlsproxy(8) server with mandatory TLS encryp‐
tion. If the list is empty, the server supports all available SSL/TLS protocol versions. See
smtpd_tls_mandatory_protocols for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_protocols (default: $smtpd_tls_protocols)
List of TLS protocols that the Postfix tlsproxy(8) server will exclude or include with oppor‐
tunistic TLS encryption. See smtpd_tls_protocols for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_req_ccert (default: $smtpd_tls_req_ccert)
With mandatory TLS encryption, require a trusted remote SMTP client certificate in order to
allow TLS connections to proceed. See smtpd_tls_req_ccert for further details.

This feature is available in Postfix 2.8 and later.

tlsproxy_tls_security_level (default: $smtpd_tls_security_level)
The SMTP TLS security level for the Postfix tlsproxy(8) 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 expiration time
for the SMTP server cache shared by all three services, namely smtpd_tls_session_cache_time‐
out.

This feature is available in Postfix 2.8 and later.

tlsproxy_use_tls (default: $smtpd_use_tls)
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not require that
clients use TLS encryption. See smtpd_use_tls for further details. Use tlsproxy_tls_secu‐
rity_level instead.

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 ter‐
minated by a built-in watchdog timer. This is a safety mechanism that prevents tlsproxy(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.

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 main‐
tains a record of mail deliveries and produces a mail delivery report when verbose delivery
is requested with "sendmail -v".

This feature is available in Postfix 2.1 and later.

transport_delivery_slot_cost (default: $default_delivery_slot_cost)
A transport-specific override for the default_delivery_slot_cost parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_delivery_slot_cost parameters will not show up in "postconf" command output
before Postfix version 2.9. This limitation applies to many parameters whose name is a com‐
bination of a master.cf service name and a built-in suffix (in this case: "_deliv‐
ery_slot_cost").

transport_delivery_slot_discount (default: $default_delivery_slot_discount)
A transport-specific override for the default_delivery_slot_discount parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_delivery_slot_discount parameters will not show up in "postconf" command out‐
put before Postfix version 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this case: "_deliv‐
ery_slot_discount").

transport_delivery_slot_loan (default: $default_delivery_slot_loan)
A transport-specific override for the default_delivery_slot_loan parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_delivery_slot_loan parameters will not show up in "postconf" command output
before Postfix version 2.9. This limitation applies to many parameters whose name is a com‐
bination of a master.cf service name and a built-in suffix (in this case: "_deliv‐
ery_slot_loan").

transport_destination_concurrency_failed_cohort_limit (default: $default_destination_concur‐
rency_failed_cohort_limit)
A transport-specific override for the default_destination_concurrency_failed_cohort_limit pa‐
rameter value, where transport is the master.cf name of the message delivery transport.

Note: some transport_destination_concurrency_failed_cohort_limit parameters will not show up
in "postconf" command output before Postfix version 2.9. This limitation applies to many pa‐
rameters whose name is a combination of a master.cf service name and a built-in suffix (in
this case: "_destination_concurrency_failed_cohort_limit").

This feature is available in Postfix 2.5 and later.

transport_destination_concurrency_limit (default: $default_destination_concurrency_limit)
A transport-specific override for the default_destination_concurrency_limit parameter value,
where transport is the master.cf name of the message delivery transport.

Note: some transport_destination_concurrency_limit parameters will not show up in "postconf"
command output before Postfix version 2.9. This limitation applies to many parameters whose
name is a combination of a master.cf service name and a built-in suffix (in this case: "_des‐
tination_concurrency_limit").

transport_destination_concurrency_negative_feedback (default: $default_destination_concurrency_nega‐
tive_feedback)
A transport-specific override for the default_destination_concurrency_negative_feedback para‐
meter 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 para‐
meters whose name is a combination of a master.cf service name and a built-in suffix (in this
case: "_destination_concurrency_negative_feedback").

This feature is available in Postfix 2.5 and later.

transport_destination_concurrency_positive_feedback (default: $default_destination_concurrency_posi‐
tive_feedback)
A transport-specific override for the default_destination_concurrency_positive_feedback para‐
meter 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 para‐
meters whose name is a combination of a master.cf service name and a built-in suffix (in this
case: "_destination_concurrency_positive_feedback").

This feature is available in Postfix 2.5 and later.

transport_destination_rate_delay (default: $default_destination_rate_delay)
A transport-specific override for the default_destination_rate_delay parameter value, where
transport is the master.cf name of the message delivery transport.

Note: some transport_destination_rate_delay parameters will not show up in "postconf" command
output before Postfix version 2.9. This limitation applies to many parameters whose name is
a combination of a master.cf service name and a built-in suffix (in this case: "_destina‐
tion_rate_delay").

This feature is available in Postfix 2.5 and later.

transport_destination_recipient_limit (default: $default_destination_recipient_limit)
A transport-specific override for the default_destination_recipient_limit parameter value,
where transport is the master.cf name of the message delivery transport.

Note: some transport_destination_recipient_limit parameters will not show up in "postconf"
command output before Postfix version 2.9. This limitation applies to many parameters whose
name is a combination of a master.cf service name and a built-in suffix (in this case: "_des‐
tination_recipient_limit").

transport_extra_recipient_limit (default: $default_extra_recipient_limit)
A transport-specific override for the default_extra_recipient_limit parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_extra_recipient_limit parameters will not show up in "postconf" command out‐
put before Postfix version 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this case: "_extra_recipi‐
ent_limit").

transport_initial_destination_concurrency (default: $initial_destination_concurrency)
A transport-specific override for the initial_destination_concurrency parameter value, where
transport is the master.cf name of the message delivery transport.

Note: some transport_initial_destination_concurrency parameters will not show up in "post‐
conf" command output before Postfix version 2.9. This limitation applies to many parameters
whose name is a combination of a master.cf service name and a built-in suffix (in this case:
"_initial_destination_concurrency").

This feature is available in Postfix 2.5 and later.

transport_maps (default: empty)
Optional lookup tables with mappings from recipient address to (message delivery transport,
next-hop destination). See transport(5) for details.

Specify zero or more "type:table" lookup tables, separated by whitespace or comma. Tables
will be searched in the specified order until a match is found. If you use this feature with
local files, run "postmap /etc/postfix/transport" after making a change.

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

For safety reasons, as of Postfix 2.3 this feature does not allow $number substitutions in
regular expression maps.

Examples:

transport_maps = dbm:/etc/postfix/transport
transport_maps = hash:/etc/postfix/transport

transport_minimum_delivery_slots (default: $default_minimum_delivery_slots)
A transport-specific override for the default_minimum_delivery_slots parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_minimum_delivery_slots parameters will not show up in "postconf" command out‐
put before Postfix version 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this case: "_minimum_deliv‐
ery_slots").

transport_recipient_limit (default: $default_recipient_limit)
A transport-specific override for the default_recipient_limit parameter value, where trans‐
port is the master.cf name of the message delivery transport.

Note: some transport_recipient_limit parameters will not show up in "postconf" command output
before Postfix version 2.9. This limitation applies to many parameters whose name is a com‐
bination of a master.cf service name and a built-in suffix (in this case: "_recipi‐
ent_limit").

transport_recipient_refill_delay (default: $default_recipient_refill_delay)
A transport-specific override for the default_recipient_refill_delay parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_recipient_refill_delay parameters will not show up in "postconf" command out‐
put before Postfix version 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this case: "_recipient_re‐
fill_delay").

This feature is available in Postfix 2.4 and later.

transport_recipient_refill_limit (default: $default_recipient_refill_limit)
A transport-specific override for the default_recipient_refill_limit parameter value, where
transport is the master.cf name of the message delivery transport.

Note: transport_recipient_refill_limit parameters will not show up in "postconf" command out‐
put before Postfix version 2.9. This limitation applies to many parameters whose name is a
combination of a master.cf service name and a built-in suffix (in this case: "_recipient_re‐
fill_limit").

This feature is available in Postfix 2.4 and later.

transport_retry_time (default: 60s)
The time between attempts by the Postfix queue manager to contact a malfunctioning message
delivery transport.

transport_time_limit (default: $command_time_limit)
A transport-specific override for the command_time_limit parameter value, where transport is
the master.cf name of the message delivery transport.

Note: transport_time_limit parameters will not show up in "postconf" command output before
Postfix version 2.9. This limitation applies to many parameters whose name is a combination
of a master.cf service name and a built-in suffix (in this case: "_time_limit").

transport_transport_rate_delay (default: $default_transport_rate_delay)
A transport-specific override for the default_transport_rate_delay parameter value, where the
initial transport in the parameter name is the master.cf name of the message delivery trans‐
port.

Note: transport_transport_rate_delay parameters will not show up in "postconf" command output
before Postfix version 2.9. This limitation applies to many parameters whose name is a com‐
bination of a master.cf service name and a built-in suffix (in this case: "_trans‐
port_rate_delay").

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.

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 Postfix
2.4-2.7, specify an empty value to disable this feature.

Example:

# Default value before Postfix 2.8.
# Note: the ":" and ";" are both required.
undisclosed_recipients_header = To: undisclosed-recipients:;

unknown_address_reject_code (default: 450)
The numerical response code when the Postfix SMTP server rejects a sender or recipient ad‐
dress because its domain is unknown. This is one of the possible replies from the restric‐
tions reject_unknown_sender_domain and reject_unknown_recipient_domain.

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

unknown_address_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unknown_sender_domain or reject_unknown_recipi‐
ent_domain fail due to a temporary error condition. Specify "defer" to defer the remote SMTP
client request immediately. With the default "defer_if_permit" action, the Postfix SMTP
server continues to look for opportunities to reject mail, and defers the client request only
if it would otherwise be accepted.

This feature is available in Postfix 2.6 and later.

unknown_client_reject_code (default: 450)
The numerical Postfix SMTP server response code when a client without valid address <=> name
mapping is rejected by the reject_unknown_client_hostname restriction. The SMTP server always
replies with 450 when the mapping failed due to a temporary error condition.

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

unknown_helo_hostname_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unknown_helo_hostname fails due to a temporary
error condition. Specify "defer" to defer the remote SMTP client request immediately. With
the default "defer_if_permit" action, the Postfix SMTP server continues to look for opportu‐
nities to reject mail, and defers the client request only if it would otherwise be accepted.

This feature is available in Postfix 2.6 and later.

unknown_hostname_reject_code (default: 450)
The numerical Postfix SMTP server response code when the hostname specified with the HELO or
EHLO command is rejected by the reject_unknown_helo_hostname restriction.

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

unknown_local_recipient_reject_code (default: 550)
The numerical Postfix SMTP server response code when a recipient address is local, and $lo‐
cal_recipient_maps specifies a list of lookup tables that does not match the recipient. A
recipient address is local when its domain matches $mydestination, $proxy_interfaces or
$inet_interfaces.

The default setting is 550 (reject mail) but it is safer to initially use 450 (try again
later) so you have time to find out if your local_recipient_maps settings are OK.

Example:

unknown_local_recipient_reject_code = 450

This feature is available in Postfix 2.0 and later.

unknown_relay_recipient_reject_code (default: 550)
The numerical Postfix SMTP server reply code when a recipient address matches $relay_domains,
and relay_recipient_maps specifies a list of lookup tables that does not match the recipient
address.

This feature is available in Postfix 2.0 and later.

unknown_virtual_alias_reject_code (default: 550)
The Postfix SMTP server reply code when a recipient address matches $virtual_alias_domains,
and $virtual_alias_maps specifies a list of lookup tables that does not match the recipient
address.

This feature is available in Postfix 2.0 and later.

unknown_virtual_mailbox_reject_code (default: 550)
The Postfix SMTP server reply code when a recipient address matches $virtual_mailbox_domains,
and $virtual_mailbox_maps specifies a list of lookup tables that does not match the recipient
address.

This feature is available in Postfix 2.0 and later.

unverified_recipient_defer_code (default: 450)
The numerical Postfix SMTP server response when a recipient address probe fails due to a tem‐
porary error condition.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

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

This feature is available in Postfix 2.6 and later.

unverified_recipient_reject_code (default: 450)
The numerical Postfix SMTP server response when a recipient address is rejected by the re‐
ject_unverified_recipient restriction.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

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

This feature is available in Postfix 2.1 and later.

unverified_recipient_reject_reason (default: empty)
The Postfix SMTP server's reply when rejecting mail with reject_unverified_recipient. Do not
include the numeric SMTP reply code or the enhanced status code. By default, the response in‐
cludes actual address verification details.

Example:

unverified_recipient_reject_reason = Recipient address lookup failed

This feature is available in Postfix 2.6 and later.

unverified_recipient_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unverified_recipient fails due to a temporary
error condition. Specify "defer" to defer the remote SMTP client request immediately. With
the default "defer_if_permit" action, the Postfix SMTP server continues to look for opportu‐
nities to reject mail, and defers the client request only if it would otherwise be accepted.

This feature is available in Postfix 2.6 and later.

unverified_sender_defer_code (default: 450)
The numerical Postfix SMTP server response code when a sender address probe fails due to a
temporary error condition.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

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

This feature is available in Postfix 2.6 and later.

unverified_sender_reject_code (default: 450)
The numerical Postfix SMTP server response code when a recipient address is rejected by the
reject_unverified_sender restriction.

Unlike elsewhere in Postfix, you can specify 250 in order to accept the address anyway.

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

This feature is available in Postfix 2.1 and later.

unverified_sender_reject_reason (default: empty)
The Postfix SMTP server's reply when rejecting mail with reject_unverified_sender. Do not in‐
clude the numeric SMTP reply code or the enhanced status code. By default, the response in‐
cludes actual address verification details.

Example:

unverified_sender_reject_reason = Sender address lookup failed

This feature is available in Postfix 2.6 and later.

unverified_sender_tempfail_action (default: $reject_tempfail_action)
The Postfix SMTP server's action when reject_unverified_sender fails due to a temporary error
condition. Specify "defer" to defer the remote SMTP client request immediately. With the de‐
fault "defer_if_permit" action, the Postfix SMTP server continues to look for opportunities
to reject mail, and defers the client request only if it would otherwise be accepted.

This feature is available in Postfix 2.6 and later.

use_srv_lookup (default: empty)
Enables discovery for the specified service(s) using DNS SRV records. For example, with
"use_srv_lookup = submission" and "relayhost = example.com:submission", the Postfix SMTP
client will look up DNS SRV records for _submission._tcp.example.com, and will relay email
through the hosts and ports that are specified with those records. See RFC 2782 for details
of the host selection process.

Specify zero or more service names separated by comma and/or whitespace. Any name in the ser‐
vices(5) database may be specified, though in practice only submission, submissions, and smtp
make sense.

When SRV record lookup is enabled with use_srv_lookup, you can enclose a domain name in "[]"
to force IP address lookup instead of SRV record lookup.

Example 1: MUA-to-MTA submission using SRV record lookup for the "submission" service for do‐
main "example.com". This uses the default SMTP delivery agent with STARTTLS, and looks up SRV
records for "_submission._tcp.example.com".

/etc/postfix/main.cf:
use_srv_lookup = submission
relayhost = example.com:submission
smtp_tls_security_level = may
...see SASL_README for sasl configuration...

Example 2: MUA-to-MTA submission using SRV record lookup for the "submissions" service for
domain "example.org". This uses a dedicated SMTP delivery agent (smtp-wraptls) with tls_wrap‐
permode turned on, and looks up SRV records for "_submissions._tcp.example.org".

Note: specify the older name "smtps" instead of "submissions" when a provider has DNS SRV
records like "_smtps._tcp.example.org" instead of "_submissions._tcp.example.org".

/etc/postfix/main.cf:
use_srv_lookup = submissions
default_transport = smtp-wraptls:example.org:submissions
...see SASL_README for sasl configuration...

/etc/postfix/master.cf:
smtp-wraptls unix ... ... ... ... ... smtp
-o { smtp_tls_wrappermode = yes }
-o { smtp_tls_security_level = encrypt }

Example 3: Sender-dependent selection for a combination of MUA-to-MTA submission services.
This combines examples 1 and 2 with examples of how to disable SRV and look up IP address
records for "smtp-relay.example.net" and "smtp-relay.other.example". Again, specify the
older name "smtps" instead of "submissions" when a provider has DNS SRV records like
"_smtps._tcp.example.org" instead of "_submissions._tcp.example.org".

/etc/postfix/main.cf:
use_srv_lookup = submission, submissions
sender_dependent_default_transport_maps = inline:{
# Destinations that support SRV record lookup.
{ user1 AT example.com = smtp:example.com:submission }
{ user2 AT example.org = smtp-wraptls:example.org:submissions }
# Use [destination] to force IP address lookups.
{ user3 AT example.net = smtp:[smtp-relay.example.net]:submission }
{ user4 AT other.example =
smtp-wraptls:[smtp-relay.other.example]:submissions } }
...see SASL_README for sasl configuration...

Example 4: MTA-to-MTA traffic, using SRV record lookup for the SMTP service. This is useful
for Postfix tests, and may be useful in environments where ports are dynamically assigned to
servers.

/etc/postfix/main.cf:
use_srv_lookup = smtp
# Fall back to MX record lookup when SRV records are unavailable.
#allow_srv_lookup_fallback = yes
#ignore_srv_lookup_error = yes

This feature is available in Postfix 3.8 and later.

verp_delimiter_filter (default: -=+)
The characters Postfix accepts as VERP delimiter characters on the Postfix sendmail(1) com‐
mand line and in SMTP commands.

This feature is available in Postfix 1.1 and later.

virtual_alias_address_length_limit (default: 1000)
The maximal length of an email address after virtual alias expansion. This stops virtual
aliasing loops that increase the address length exponentially.

This feature is available in Postfix 3.0 and later.

virtual_alias_domains (default: $virtual_alias_maps)
Postfix is the final destination for the specified list of virtual alias domains, that is,
domains 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-exis‐
tent recipients. See also the virtual alias domain class in the ADDRESS_CLASS_README file

This feature is available in Postfix 2.0 and later. The default value is backwards compatible
with Postfix version 1.1.

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

Specify a list of host or domain names, "/file/name" or "type:table" patterns, separated by
commas and/or whitespace. A "/file/name" pattern is replaced by its contents; a "type:table"
lookup table is matched when a table entry matches a host or domain name (the lookup result
is ignored). Continue long lines by starting the next line with whitespace. Specify "!pat‐
tern" to exclude a host or domain name from the list. The form "!/file/name" is supported
only in Postfix version 2.4 and later.

See also the VIRTUAL_README and ADDRESS_CLASS_README documents for further information.

Example:

virtual_alias_domains = virtual1.tld virtual2.tld

virtual_alias_expansion_limit (default: 1000)
The maximal number of addresses that virtual alias expansion produces from each original re‐
cipient.

This feature is available in Postfix 2.1 and later.

virtual_alias_maps (default: $virtual_maps)
Optional lookup tables that alias specific mail addresses or domains to other local or remote
addresses. 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 compatible
with Postfix version 1.1.

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

If you use this feature with indexed files, run "postmap /etc/postfix/virtual" after changing
the file.

Examples:

virtual_alias_maps = dbm:/etc/postfix/virtual
virtual_alias_maps = hash:/etc/postfix/virtual

virtual_alias_recursion_limit (default: 1000)
The maximal nesting depth of virtual alias expansion. Currently the recursion limit is ap‐
plied only to the left branch of the expansion graph, so the depth of the tree can in the
worst case reach the sum of the expansion and recursion limits. This may change in the fu‐
ture.

This feature is available in Postfix 2.1 and later.

virtual_delivery_status_filter (default: $default_delivery_status_filter)
Optional filter for the virtual(8) delivery agent to change the delivery status code or ex‐
planatory text of successful or unsuccessful deliveries. See default_delivery_status_filter
for details.

This feature is available in Postfix 3.0 and later.

virtual_destination_concurrency_limit (default: $default_destination_concurrency_limit)
The maximal number of parallel deliveries to the same destination via the virtual message de‐
livery transport. This limit is enforced by the queue manager. The message delivery transport
name is the first field in the entry in the master.cf file.

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

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

virtual_gid_maps (default: empty)
Lookup tables with the per-recipient group ID for virtual(8) 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 speci‐
fied 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 sub‐
stitution of $1 etc. in regular expression lookup tables, because that would open a security
hole.

Note 2: for security reasons, the virtual(8) 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 control
map doesn't litter the file system with mailboxes. While virtual_mailbox_base could be set
to "/", this setting isn't recommended.

This parameter is specific to the virtual(8) 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 the 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) deliv‐
ery agent. The SMTP server validates recipient addresses with $virtual_mailbox_maps and re‐
jects mail for non-existent recipients. See also the virtual mailbox domain class in the AD‐
DRESS_CLASS_README file.

This parameter expects the same syntax as the mydestination configuration parameter.

This feature is available in Postfix 2.0 and later. The default value is backwards compatible
with Postfix version 1.1.

virtual_mailbox_limit (default: 51200000)
The maximal size in bytes of an individual virtual(8) 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 avail‐
able 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 speci‐
fied domain that does not have a specific "user AT domain.tld" entry.

With the default "virtual_mailbox_domains = $virtual_mailbox_maps", lookup tables also need
entries with a left-hand side of "domain.tld" to satisfy virtual_mailbox_domain lookups (the
right-hand side is required but will not be used).

The remainder of this text is specific to the virtual(8) delivery agent. It does not apply
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_mail‐
box_base is unconditionally prepended to this path.

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

virtual_maps (default: empty)
Optional lookup tables with a) names of domains for which all addresses are aliased to ad‐
dresses in other local or remote domains, and b) addresses that are aliased to addresses in
other local or remote domains. Available before Postfix version 2.0. With Postfix version
2.0 and later, this is replaced by separate controls: virtual_alias_domains and vir‐
tual_alias_maps.

virtual_minimum_uid (default: 100)
The minimum user ID value that the virtual(8) delivery agent accepts as a result from $vir‐
tual_uid_maps table lookup. Returned values less than this will be rejected, and the message
will be deferred.

This parameter is specific to the virtual(8) 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.

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 speci‐
fied domain that does not have a specific "user AT domain.tld" entry.

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

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)

Generated by phpman v4.10.0-7-g98e9fd5 · Markdown · JSON · MCP Author: Che Dong Under GNU General Public License
2026-09-05 00:09 @216.73.217.1
CrawledBy Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; ClaudeBot/1.0; +claudebot@anthropic.com)