SMTPD.CONF(5)

NAME
smtpd.conf - SMTP daemon configuration file

DESCRIPTION
smtpd.conf is the configuration file for the mail daemon smtpd(8).

When mail arrives, each “RCPT TO:” command generates a mail envelope. If an envelope matches any of a pre-designated set of criteria (using the match directive), the message is accepted for delivery. A copy of the message, as well as its associated envelopes, is saved in the mail queue and later dispatched according to an associated set of actions (using the action directive). If an envelope does not match any options, it is rejected. The match rules are evaluated sequentially, with the first match winning.

The format of the configuration file is fairly flexible. The current line can be extended over multiple lines using a backslash (‘\’). Comments can be put anywhere in the file using a hash mark (‘#’), and extend to the end of the current line. Care should be taken when commenting out multi-line text: the comment is effective until the end of the entire block. Argument names not beginning with a letter, digit, or underscore, as well as reserved words (such as listen, match, and port), must be quoted. Arguments containing whitespace should be surrounded by double quotes (").

A macro is defined with a command of the form name=value. The macro name can contain letters, digits, and underscores and cannot be a reserved word. Within unquoted arguments, the string $name is later expanded to value.

For example:

    lan_addr = "192.168.0.1"
    listen on $lan_addr
    listen on $lan_addr tls auth

The syntax of smtpd.conf is described below.

action name method [options]
    When the queue runner processes an envelope from the mail queue, it carries out the action name, selected by the match ... action directive when the message was received. The action directive provides configuration data for delivery attempts. Required lookups are performed at the time of each delivery attempt. Consequently, changing an action directive or the files it references and restarting the smtpd(8) daemon causes the changes to take effect for subsequent delivery attempts for the respective dispatcher name, even for messages that were already stuck in the queue prior to the configuration changes.

    The delivery method parameter may be one of the following:

    expand-only
        Only accept the message if a delivery method was specified in an aliases or .forward file.

    forward-only
        Only accept the message if the recipient results in a remote address after the processing of aliases or forward file.

    lmtp destination [rcpt-to]
        Deliver the message to an LMTP server at destination. The location may be expressed as host:port or as a UNIX socket.

        Optionally, rcpt-to might be specified to use the recipient email address (after expansion) instead of the local user in the LMTP session as RCPT TO.

    maildir [pathname] [junk]
        Deliver the message to the maildir in pathname if specified, or by default to ~/Maildir.

        The pathname may contain format specifiers that are expanded before use (see FORMAT SPECIFIERS).

        If the junk argument is provided, the message will be moved to the ‘Junk’ folder if it contains a positive ‘X-Spam’ header. This folder will be created under pathname if it does not yet exist.

    mbox
        Deliver the message to the user's mbox with mail.local(8).

    mda command
        Delegate the delivery to a command that receives the message on its standard input (see MDA COMMANDS).

        The command may contain format specifiers that are expanded before use (see FORMAT SPECIFIERS).

    relay
        Relay the message to another SMTP server.

    The local delivery methods support additional options:

    alias <table>
        Use the mapping table for aliases(5) expansion.

        ttl n {s|m|h|d} Specify how long a message may remain in the queue.

    user username
        Specify the username for performing the delivery, to be looked up with getpwnam(3).

        This is used for virtual hosting where a single username is in charge of handling delivery for all virtual users.

        This option is not usable with the mbox delivery method.

        Only the delivery user's .forward file will be processed.

    userbase <table>
        Use the mapping table for user lookups instead of the getpwnam(3) function.

        The userbase does not apply to the user option.

    virtual <table>
        Use the mapping table for virtual expansion. The aliasing table format is described in table(5).

    wrapper name
        Use the wrapper specified in mda wrapper.

    The relay delivery methods also support additional options:

    backup
        Operate as a backup mail exchanger delivering messages to any mail exchanger with higher priority.

    backup mx name
        Operate as a backup mail exchanger delivering messages to any mail exchanger with higher priority than mail exchanger identified as name.

    ca caname
        For secure connections, use the certificate authority associated with caname (declared in a ca directive) to validate the server's identity.

    helo heloname
        Advertise heloname as the hostname to other mail exchangers during the HELO phase.

    helo-src <table>
        Use the mapping table to look up a hostname matching the source address, to advertise during the HELO phase.

    domain <domains>
        Do not perform MX lookups but look up destination domain in domains and use matching relay url as relay host.

    host relay-url
        Do not perform MX lookups but relay messages to the relay host described by relay-url. The format for relay-url is [proto://[label@]] host[:port]. The following protocols are available:

        smtp
            Normal SMTP session with opportunistic STARTTLS (the default).
        smtp+tls
            Normal SMTP session with mandatory STARTTLS.
        smtp+notls
            Plain text SMTP session without TLS.
        lmtp
            LMTP session. port is required.
        smtps
            SMTP session with forced TLS on connection. The default port is 465.

        Unless noted, port defaults to 25.

        The label corresponds to an entry in a credentials table, as documented in table(5). It is used with the “smtp+tls” and “smtps” protocols for authentication. Server certificates for those protocols are verified by default.

    pki pkiname
        For secure connections, use the certificate associated with pkiname (declared in a pki directive) to prove the client's identity to the remote mail server.

    srs When relaying a mail resulting from a forward, use the Sender Rewriting Scheme to rewrite sender address.

    tls [no-verify]
        Require TLS to be used when relaying, using mandatory STARTTLS by default. When used with a smarthost, the protocol must not be “smtp+notls://”. If no-verify is specified, do not require a valid certificate.

    protocols protostr
        Define the protocol versions to be used for TLS sessions. Refer to the tls_config_parse_protocols(3) manpage for the format of protostr.

    ciphers cipherstr
        Define the list of ciphers that may be used for TLS sessions. Refer to the tls_config_set_ciphers(3) manpage for the format of cipherstr.

    auth <table>
        Use the mapping table for connecting to relay-url using credentials. This option is usable only with host option. The credential table format is described in table(5).

    mail-from mailaddr
        Use mailaddr as the MAIL FROM address within the SMTP transaction.

    src sourceaddr | <sourceaddr>
        Use the string or list table sourceaddr for the source IP address, which is useful on machines with multiple interfaces. If the list contains more than one address, all of them are used in such a way that traffic is routed as efficiently as possible.

admd authservid
    The Administrative Management Domain this mail server belongs to. The authservid will be forwarded to filters using it to identify or mark authentication-results headers. If omitted, it defaults to the server name.

bounce warn-interval delay [, delay ...]
    Send warning messages to the envelope sender when temporary delivery failures cause a message to remain in the queue for longer than delay. Each delay parameter consists of a positive decimal integer and a unit s, m, h, or d. At most four delay parameters can be specified. The default is "bounce warn-interval 4h", sending a single warning after four hours.

ca caname cert cafile
    Associate the Certificate Authority (CA) certificate file cafile with ca entry caname. The ca entry can be referenced in listen on and action ... relay rules.

filter chain-name chain {filter-name [, ...]}
    Register a chain of filters chain-name, consisting of the filters listed in filter-name. Filters in a filter chain are executed in order of declaration for each phase that they are registered for. A filter chain may be used in place of a filter for any directive except filter chains themselves.

filter filter-name phase phase-name match conditions decision
    Register a filter filter-name. A decision about what to do with the mail is taken at phase phase-name when matching conditions. Phases, matching conditions, and decisions are described in MAIL FILTERING, below.

filter filter-name proc proc-name
    Register "proc" filter filter-name backed by the proc-name process.

filter filter-name proc-exec command
    Register and execute "proc" filter filter-name from command, conformant with the smtpd-filters(7) API. If command starts with a slash it is executed with an absolute path, otherwise it will be run from “/usr/local/libexec/smtpd/”.

include "pathname"
    Replace this directive with the content of the additional configuration file at the absolute pathname.

listen on interface [family] [options]
    Listen on the interface for incoming connections, using the same syntax as ifconfig(8). The interface parameter may also be an interface group, an IP address, or a domain name. Listening can optionally be restricted to a specific address family, which can be either inet4 or inet6.

    The options are as follows:

    auth [<authtable>]
        Support SMTPAUTH: clients may only start SMTP transactions after successful authentication. Users are authenticated against either their own normal login credentials or a credentials table authtable, the format of which is described in table(5).

    auth-optional [<authtable>]
        Support SMTPAUTH optionally: clients need not authenticate, but may do so. This allows a listen on directive to both accept incoming mail from untrusted senders and permit outgoing mail from authenticated users (using match auth). It can be used in situations where it is not possible to listen on a separate port (usually the submission port, 587) for users to authenticate.

    ca caname
        For secure connections, use the CA certificate associated with caname (declared in a ca directive) as the CA certificate when verifying client certificates.

    filter name
        Apply filter name on connections handled by this listener.

    hostname hostname
        Use hostname in the greeting banner instead of the default server name.

    hostnames <names>
        Override the server name for specific addresses. The names table contains a mapping of IP addresses to hostnames. If the address on which the connection arrives appears in the mapping, the associated hostname is used.

    mask-src
        Omit the from part when prepending “Received” headers.

    no-dsn
        Disable the DSN (Delivery Status Notification) extension.

    pki pkiname
        For secure connections, use the certificate associated with pkiname (declared in a pki directive) to prove a mail server's identity. This option can be used multiple times to provide alternate certificates for SNI.

    port [port]
        Listen on the given port instead of the default port 25.

    proxy-v2
        Support the PROXYv2 protocol, appropriately rewriting the source address received from proxy.

    received-auth
        In “Received” headers, report whether the session was authenticated and by which local user.

    senders <users> [masquerade]
        Look up the authenticated user in the users mapping table to find the email addresses that user is allowed to submit mail as. In addition, if the masquerade option is provided, the From header is rewritten to match the sender provided in the SMTP session.

    smtps [verify]
        Support SMTPS, by default on port 465. Mutually exclusive with tls. With the verify option, clients must also provide a valid certificate to establish an SMTP session.

    tag tag
        Clients connecting to the listener are tagged with the given tag.

    tls Support STARTTLS, by default on port 25. Mutually exclusive with smtps.

    tls-require [verify]
        Like tls, but force clients to establish a secure connection before being allowed to start an SMTP transaction. With the verify option, clients must also provide a valid certificate to establish an SMTP session.

    protocols protostr
        Define the protocol versions to be used for TLS sessions. Refer to the tls_config_parse_protocols(3) manpage for the format of protostr.

    ciphers cipherstr
        Define the list of ciphers that may be used for TLS sessions. Refer to the tls_config_set_ciphers(3) manpage for the format of cipherstr.

listen on socket [options]
    Listen for incoming SMTP connections on the Unix domain socket /var/run/smtpd.sock. This is done by default, even if the directive is absent.

    The options are as follows:

    filter name
        Apply filter name on connections handled by this listener.

    mask-src
        Omit the from part when prepending “Received” headers.

    no-dsn
        Disable the DSN (Delivery Status Notification) extension.

    tag tag
        Clients connecting to the listener are tagged with the given tag.

match options action name
    If at least one mail envelope matches the options of one match action directive, receive the incoming message, put a copy into each matching envelope, and atomically save the envelopes to the mail spool for later processing by the respective dispatcher name which has to be declared beforehand with the action directive.

    The following matching options are supported and can all be negated:

        []! for any Specify that session may address any destination.

        []! for local Specify that session may address any local domain. This is the default, and may be omitted.

        []! for domain [regex] domain | <domain> Specify that session may address the string or list table domain. The regex keyword is required if domain is a regular expression or table of regular expressions.

        []! for rcpt-to [regex] recipient | <recipient> Specify that session may address the string or list table recipient. The regex keyword is required if recipient is a regular expression or table of regular expressions.

        []! from any Specify that session may originate from any source.

        []! from auth Specify that session may originate from any authenticated user, no matter the source IP address.

        []! from auth [regex] user | <user> Specify that session may originate from authenticated user or user list user, no matter the source IP address. The regex keyword is required if user is a regular expression or table of regular expressions.

        []! from local Specify that session may only originate from a local IP address, or from the local enqueuer. This is the default, and may be omitted.

        []! from mail-from [regex] sender | <sender> Specify that session may originate from sender or sender list sender, no matter the source IP address. The regex keyword is required if sender is a regular expression or table of regular expressions.

        []! from rdns Specify that session may only originate from an IP address that resolves to a reverse DNS.

        []! from rdns [regex] hostname | <hostname> Specify that session may only originate from an IP address that resolves to a reverse DNS matching string or list string hostname. The regex keyword is required if hostname is a regular expression or table of regular expressions.

        []! from socket Specify that session may only originate from the local enqueuer.

        []! from src [regex] address | <address> Specify that session may only originate from string or list table address which can be a specific address or a subnet expressed in CIDR-notation. The regex keyword is required if address is a regular expression or table of regular expressions.

    In addition, the following transaction options may be matched:

        []! auth Matches transactions which have been authenticated.

        []! auth [regex] username | <username> Matches transactions which have been authenticated for user or user list username. The regex keyword is required if username is a regular expression or table of regular expressions.

        []! helo [regex] helo-name | <helo-name> Specify that session's HELO / EHLO should match the string or list table helo-name. The regex keyword is required if helo-name is a regular expression or table of regular expressions.

        []! mail-from [regex] sender | <sender> Specify that transaction's MAIL FROM should match the string or list table sender. The regex keyword is required if sender is a regular expression or table of regular expressions.

        []! rcpt-to [regex] recipient | <recipient> Specify that transaction's RCPT TO should match the string or list table recipient. The regex keyword is required if recipient is a regular expression or table of regular expressions.

        []! tag [regex] tag Matches transactions tagged with the given tag. The regex keyword is required if tag is a regular expression.

        []! tls Specify that transaction should take place in a TLS channel.

match options reject
    Reject the incoming message during the SMTP dialogue. The same options are supported as for the match action directive.

mda wrapper name command
    Associate command with the mail delivery agent wrapper named name. When a local delivery specifies a wrapper, the command associated with the wrapper will be executed instead. The command may contain format specifiers (see FORMAT SPECIFIERS).

mta max-deferred number
    When delivery to a given host is suspended due to temporary failures, cache at most number envelopes for that host such that they can be delivered as soon as another delivery succeeds to that host. The default is 100.

pki pkiname cert certfile
    Associate certificate file certfile with pki entry pkiname. The pki entry defines a keypair configuration that can be referenced in listener rules and relay actions.

    A certificate chain may be created by appending one or many certificates, including a Certificate Authority certificate, to certfile. The creation of certificates is documented in starttls(8).

pki pkiname key keyfile
    Associate the key located in keyfile with pki entry pkiname.

pki pkiname dhe params
    Specify the DHE parameters to use for DHE cipher suites with pki entry pkiname. Valid parameter values are none, legacy, and auto. For legacy, a fixed key length of 1024 bits is used, whereas for auto, the key length is determined automatically. The default is none, which disables DHE cipher suites.

proc proc-name command
    Register an external process named proc-name from command, conformant with the smtpd-filters(7) API. Such processes may be used to share the same instance between multiple filters. If command starts with a slash it is executed with an absolute path, otherwise it will be run from “/usr/local/libexec/smtpd/”.

queue compression
    Store queue files in a compressed format. This may be useful to save disk space.

queue encryption [key]
    Encrypt queue files with EVP_aes_256_gcm(3). If no key is specified, it is read with getpass(3). If the string stdin or a single dash (‘-’) is given instead of a key, the key is read from the standard input.

queue ttl delay
    Set the default expiration time for temporarily undeliverable messages, given as a positive decimal integer followed by a unit s, m, h, or d. The default is four days (4d).

smtp ciphers control
    Set the control string for SSL_CTX_set_cipher_list(3). The default is "HIGH:!aNULL:!MD5".

smtp limit max-mails count
    Limit the number of messages to count for each session. The default is 100.

smtp limit max-rcpt count
    Limit the number of recipients to count for each transaction. The default is 1000.

smtp max-message-size size
    Reject messages larger than size, given as a positive number of bytes or as a string to be parsed with scan_scaled(3). The default is "35M".

smtp sub-addr-delim character
    When resolving the local part of a local email address, ignore the ASCII character and all characters following it. The default is ‘+’.

srs key [backup] secret
    Set the secret key to use for SRS, the Sender Rewriting Scheme. The backup option can be specified to set a secret key to use as a fallback for SRS; this can be used to implement SRS key rotation.

srs ttl delay
    Set the time-to-live delay for SRS envelopes. After this delay, a bounce reply to the SRS address will be discarded to limit risks of forged addresses. The default is four days (4d).

table name [type:]pathname
    Tables provide additional configuration information for smtpd(8) in the form of lists or key-value mappings. The format of the entries depends on what the table is used for. Refer to table(5) for the exhaustive documentation.

    Each table is identified by an arbitrary, unique name.

    If the type is db, information is stored in a file created with makemap(8); if it is file or omitted, information is stored in a plain text file using the format described in table(5). The pathname to the file must be absolute.

table name {value [, ...]}
    Instead of using a separate file, declare a list table containing the given static values. The table must contain at least one value and may declare multiple values as a comma-separated (whitespace optional) list.

table name {key=value [, ...]}
    Instead of using a separate file, declare a mapping table containing the given static key-value pairs. The table must contain at least one key-value pair and may declare multiple pairs as a comma-separated (whitespace optional) list.

  MAIL FILTERING
In a regular workflow, smtpd(8) may accept or reject a message based only on the content of envelopes. Its decisions are about the handling of the message, not about the handling of an active session.

Filtering extends the decision making process by allowing smtpd(8) to stop at each phase of an SMTP session, check that conditions are met, then decide if a session is allowed to move forward.

With filtering, a session may be interrupted at any phase before an envelope is complete. A message may also be rejected after being submitted, regardless of whether the envelope was accepted or not.

The following phases are currently supported:
connect | upon connection, before a banner is displayed
helo | after HELO command is submitted
ehlo | after EHLO command is submitted
mail-from | after MAIL FROM command is submitted
rcpt-to | after RCPT TO command is submitted
data | after DATA command is submitted
commit | after message is fully submitted

At each phase, various conditions may be matched. The fcrdns, rdns, and src data are available in all phases, but other data must have been already submitted before they are available.
fcrdns | forward-confirmed reverse DNS is valid
rdns | session has a reverse DNS
rdns <table> | session has a reverse DNS in table
src <table> | source address is in table
helo <table> | helo name is in table
auth | session is authenticated
auth <table> | session username is in table
mail-from <table> | sender address is in table
rcpt-to <table> | recipient address is in table

These conditions may all be negated by prefixing them with an exclamation mark:
!fcrdns | forward-confirmed reverse DNS is invalid

Any conditions using a table may indicate that the table contains regular expressions by prefixing the table name with the keyword regex.
helo regex <table> | helo name matches a regex in table

Finally, a number of decisions may be taken:
bypass | the session or transaction bypasses filters
disconnect message | the session is disconnected with message
junk | the session or transaction is junked, i.e., an
‘X-Spam: yes’ header is added to any messages
reject message | the command is rejected with message
rewrite value | the command parameter is rewritten with value

Decisions that involve a message require that the message be RFC valid, meaning that they should either start with a 4xx or 5xx status code. Decisions can be taken at any phase, though junking can only happen before a message is committed.

  FORMAT SPECIFIERS
Some configuration directives support expansion of their parameters at runtime. Such directives (for example action maildir, action mda) may use format specifiers which are expanded before delivery or relaying. The following formats are currently supported:
%{sender} | sender email address, may be empty string
%{sender.user} | user part of the sender email address, may be empty
%{sender.domain} | domain part of the sender email address, may be empty
%{rcpt} | recipient email address
%{rcpt.user} | user part of the recipient email address
%{rcpt.domain} | domain part of the recipient email address
%{dest} | recipient email address after expansion
%{dest.user} | user part after expansion
%{dest.domain} | domain part after expansion
%{user.username} | local user
%{user.directory} | home directory of the local user
%{mbox.from} | name used in mbox From separator lines
%{mda} | mda command, only available for mda wrappers

Expansion formats also support partial expansion using the optional bracket notations with substring offset. For example, with recipient domain “example.org”:
%{rcpt.domain[0]} | expands to “e”
%{rcpt.domain[1]} | expands to “x”
%{rcpt.domain[8:]} | expands to “org”
%{rcpt.domain[-3:]} | expands to “org”
%{rcpt.domain[0:6]} | expands to “example”
%{rcpt.domain[0:-4]} | expands to “example”

In addition, modifiers may be applied to the token. For example, with recipient “User+Tag@Example.org”:
%{rcpt:lowercase} | expands to “user+tag@example.org”
%{rcpt:uppercase} | expands to “USER+TAG@EXAMPLE.ORG”
%{rcpt:strip} | expands to “User@Example.org”
%{rcpt:lowercase|strip} | expands to “user@example.org”

For security concerns, expanded values are sanitized and potentially dangerous characters are replaced with ‘’:. In situations where they are desirable, the “raw” modifier may be applied. For example, with recipient “user+t?g@example.org”:
%{rcpt} | expands to “user+t:g@example.org”
%{rcpt:raw} | expands to “user+t?g@example.org”

  MDA COMMANDS
When an action delivery method is mda, smtpd(8) runs the associated command for the delivery with the mail content provided via standard input. The command is expected to read all the mail content.

The exit code of the command reports the outcome of the delivery: status 0 (EX_OK) is a successful delivery; status 71 (EX_OSERR) and 75 (EX_TEMPFAIL) are temporary failures; and all other exit status are considered permanent failures.

The following environment variables are set:

DOMAIN
    The recipient domain.
EXTENSION
    The sub address of the recipient (may be unset).
HOME
    The delivery user's login directory.
LOCAL
    The local part of the recipient user address.
LOGNAME
    The login name of the user.
ORIGINAL_RECIPIENT
    The address of the original recipient.
PATH
    Set to _PATH_DEFPATH. Traditionally /usr/bin:/bin, but expanded to include /usr/sbin, /sbin, /usr/X11R6/bin, /usr/local/bin, and /usr/local/sbin in OpenBSD.
RECIPIENT
    The address of the final recipient.
SENDER
    The address of the sender (might be empty).
SHELL
    Set to /bin/sh.
USER
    Synonym of LOGNAME for backwards compatibility.

FILES
/etc/mail/smtpd.conf
    Default smtpd(8) configuration file.
/etc/mail/mailname
    If this file exists, the first line is used as the server name. Otherwise, the server name is derived from the local hostname returned by gethostname(3), either directly if it is a fully qualified domain name, or by retrieving the associated canonical name through getaddrinfo(3).
/var/run/smtpd.sock
    Unix domain socket for incoming SMTP connections.
/var/spool/smtpd/
    Spool directories for mail during processing.

EXAMPLES
The default smtpd.conf file which ships with OpenBSD listens on the loopback network interface (lo0) and allows for mail from users and daemons on the local machine, as well as permitting email to remote servers. Some more complex configurations are given below.

This first example is similar to the default configuration, but all outgoing mail is forwarded to a remote SMTP server. A secrets file is needed to specify a username and password:

    # touch /etc/mail/secrets
    # chmod 640 /etc/mail/secrets
    # chown root:_smtpd /etc/mail/secrets
    # echo "bob username:password" > /etc/mail/secrets

smtpd.conf would look like this:

    table aliases file:/etc/mail/aliases
    table secrets file:/etc/mail/secrets
    listen on lo0
    action "local_mail" mbox alias <aliases>
    action "outbound" relay host smtp+tls://bob@smtp.example.com \
    	auth <secrets>
    match from local for local action "local_mail"
    match from local for any action "outbound"

In this second example, the aim is to permit mail delivery and relaying only for users that can authenticate (using their normal login credentials). An RSA certificate must be provided to prove the server's identity. The mail server listens on all interfaces the default routes point to. Mail with a local destination is sent to an external MDA. First, the RSA certificate is created:

    # openssl genrsa -out /etc/ssl/private/mail.example.com.key 4096
    # openssl req -new -x509 -key /etc/ssl/private/mail.example.com.key \
    	-out /etc/ssl/mail.example.com.crt -days 365
    # chmod 600 /etc/ssl/mail.example.com.crt
    # chmod 600 /etc/ssl/private/mail.example.com.key

In the example above, a certificate valid for one year was created. The configuration file would look like this:

    pki mail.example.com cert "/etc/ssl/mail.example.com.crt"
    pki mail.example.com key "/etc/ssl/private/mail.example.com.key"
    table aliases file:/etc/mail/aliases
    listen on lo0
    listen on egress tls pki mail.example.com auth
    action mda_with_aliases mda "/path/to/mda -f -" alias <aliases>
    action mda_without_aliases mda "/path/to/mda -f -"
    action "outbound" relay
    match for local action mda_with_aliases
    match from any for domain example.com action mda_without_aliases
    match for any action "outbound"
    match auth from any for any action "outbound"

For sites that wish to sign messages using DKIM, the following example uses opensmtpd-filter-dkimsign for DKIM signing:

    table aliases file:/etc/mail/aliases
    filter "dkimsign" proc-exec "filter-dkimsign -d <domain> -s <selector> \
    	-k /etc/mail/dkim/private.key" user _dkimsign group _dkimsign
    listen on socket filter "dkimsign"
    listen on lo0 filter "dkimsign"
    action "local_mail" mbox alias <aliases>
    action "outbound" relay
    match for local action "local_mail"
    match for any action "outbound"

Alternatively, the opensmtpd-filter-rspamd package may be used to provide integration with rspamd, a third-party daemon which provides multiple antispam features as well as DKIM signing. As well as configuring rspamd itself, it requires use of the proc-exec keyword:

    filter "rspamd" proc-exec "filter-rspamd"

Sites that accept non-local messages may be able to cut down on the volume of spam received by rejecting forged messages that claim to be from the local domain. The following example uses a list table other-relays to specify the IP addresses of relays that may legitimately originate mail with the owner's domain as the sender.

    table aliases file:/etc/mail/aliases
    table other-relays file:/etc/mail/other-relays
    listen on lo0
    listen on egress
    action "local_mail" mbox alias <aliases>
    action "outbound" relay
    match for local action "local_mail"
    match for any action "outbound"
    match !from src <other-relays> mail-from "@example.com" for any \
          reject
    match from any for domain example.com action "local_mail"

SEE ALSO
mailer.conf(5), table(5), smtpd-filters(7), makemap(8), smtpd(8)

HISTORY
smtpd(8) first appeared in OpenBSD 4.6.