.\" $OpenBSD: smtpd.conf.5,v 1.125 2015/03/06 17:36:47 millert Exp $ .\" .\" Copyright (c) 2008 Janne Johansson .\" Copyright (c) 2009 Jacek Masiulaniec .\" Copyright (c) 2012 Gilles Chehade .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES .\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. .\" .\" .Dd $Mdocdate: March 6 2015 $ .Dt SMTPD.CONF 5 .Os .Sh NAME .Nm smtpd.conf .Nd Simple Mail Transfer Protocol daemon configuration file .Sh DESCRIPTION .Nm is the configuration file for the mail daemon .Xr smtpd 8 . .Pp The current line can be extended over multiple lines using a backslash .Pq Sq \e . Comments can be put anywhere in the file using a hash mark .Pq Sq # , 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. .Pp Argument names not beginning with a letter, digit, or underscore must be quoted. Arguments containing whitespace should be surrounded by double quotes .Pq \&" . .Pp Macros can be defined that will later be expanded in context. Macro names must start with a letter, digit, or underscore, and may contain any of those characters. Macro names may not be reserved words (for example .Ar listen , .Ar accept , .Ar port ) . Macros are not expanded inside quotes. .Pp For example: .Bd -literal -offset indent lan_addr = "192.168.0.1" listen on $lan_addr listen on $lan_addr tls auth .Ed .Pp Additional configuration files can be included with the .Ic include keyword, for example: .Bd -literal -offset indent include "/etc/mail/smtpd.conf.local" .Ed .Pp The syntax of .Nm is described below. .Bl -tag -width Ds .It Ic accept | reject .Xr smtpd 8 accepts and rejects messages based on information gathered during the SMTP session. .Pp For each message processed by the daemon, the filter rules are evaluated in sequential order, from first to last. The first matching rule decides what action is taken. If no rule matches the message, the default action is to reject the message. An exclamation mark may be specified to perform a reverse match. .Pp Following the accept/reject decision comes the optional tag matching: .Bl -tag -width Ds .It Xo .Ic tagged .Op Ic \&! .Ar tag .Xc If specified, the rule will only be matched if the client session was tagged with .Ar tag . .El .Pp After that the client's IP address filter is specified: .Bl -tag -width Ds .It Ic from any Make the rule match regardless of the IP of connecting client. .It Xo .Ic from .Op Ic \&! .Ic local .Xc The rule matches only locally originating connections. This is the default, and may be omitted. .It Xo .Ic from .Op Ic \&! .Ic source .Pf < Ar table Ns > .Xc The rule matches if the connection is made from a client whose address is declared in the table .Ar table . .El .Pp In addition, finer filtering may be achieved on the sender if desired: .Bl -tag -width Ds .It Xo .Ic sender .Op Ic \&! .Pf < Ar senders Ns > .Xc If specified, the rule will only be matched if the sender email address is found in the table .Ar senders . The table may contain complete email addresses or apply to an entire domain if prefixed with @. .El .Pp Next comes the selection based on the domain the message is sent to: .Bl -tag -width Ds .It Ic for any Op Ic alias No < Ns Ar aliases Ns > Make the rule match regardless of the domain it is sent to. If specified, the table .Ar aliases is used for looking up alternative destinations for all addresses. .It Ic for any virtual No < Ns Ar vmap Ns > Make the rule match regardless of the domain it is sent to. The .Ar vmap table will be used as the virtual domain mapping. .It Xo .Ic for .Op Ic \&! .Ic domain .Ar domain .Op Ic alias No < Ns Ar aliases Ns > .Xc This rule applies to mail destined for the specified .Ar domain . This parameter supports the .Sq * wildcard, so that a single rule for all sub-domains can be used, for example: .Bd -literal -offset indent accept for domain "*.example.com" deliver to mbox .Ed .Pp If specified, the table .Ar aliases is used for looking up alternative destinations for addresses in this .Ar domain . .It Xo .Ic for .Op Ic \&! .Ic domain .Pf < Ar domains Ns > .Op Ic alias No < Ns Ar aliases Ns > .Xc This rule applies to mail destined to domains which are part of the table .Ar domains . .Pp If specified, the table .Ar aliases is used for looking up alternative destinations for addresses in these .Ar domains . .It Xo .Ic for .Op Ic \&! .Ic domain .Ar domain .Ic virtual No < Ns Ar users Ns > .Xc This rule applies to mail destined for the specified virtual .Ar domain . This parameter supports the .Sq * wildcard, so that a single rule for all sub-domains can be used, for example: .Bd -literal -offset indent accept for domain "*.example.com" \e virtual deliver to mbox .Ed .Pp The table .Ar users holds a key-value mapping of virtual to system users. For an example of how to configure the .Ar users table, see .Xr makemap 8 . .It Xo .Ic for .Op Ic \&! .Ic domain .Pf < Ar domains Ns > Ic virtual No < Ns Ar users Ns > .Xc This rule applies to mail destined for the virtual domains specified in the table .Ar domains . .Pp The table .Ar users holds a key-value mapping of virtual to system users. For an example of how to configure the .Ar users table, see .Xr makemap 8 . .It Xo .Ic for .Op Ic \&! .Ic local .Op Ic alias No < Ns Ar aliases Ns > .Xc This rule applies to mail destined to .Dq localhost and to the default server name. See the .Sx FILES entry for .Pa /etc/mail/mailname below for details of how the server name is determined. .It Xo .Ic for .Op Ic \&! .Ic local .Ic virtual No < Ns Ar vmap Ns > .Xc This rule applies to mail destined to .Dq localhost and to the default server name. The .Ar vmap table will be used as the virtual domain mapping. .El .Pp Further filtering may be achieved on specific recipients if desired: .Bl -tag -width Ds .It Xo .Ic recipient .Op Ic \&! .Pf < Ar recipients Ns > .Xc If specified, the rule will only be matched if the recipient email address is found in the table .Ar recipients . The table may contain complete email addresses or apply to an entire domain if prefixed with .Sq @ . .El .Pp If the method of delivery is local, a user database may be specified to override the system database: .Bl -tag -width Ds .It Op Ic userbase No < Ns Ar table Ns > Look up users in the table .Ar table instead of performing system lookups using the .Xr getpwnam 3 function. .El .Pp Finally, the method of delivery is specified: .Bl -tag -width Ds .It Ic deliver to lmtp Op Ar host : Ns Ar port | socket Mail is delivered to .Ar host : Ns Ar port , or to the .Ux .Ar socket over LMTP. .It Ic deliver to maildir Op Ar path Mail is added to a maildir. Its location, .Ar path , may contain format specifiers that are expanded before use .Pq see Sx FORMAT SPECIFIERS . If .Ar path is not provided, then .Pa ~/Maildir is assumed. .It Ic deliver to mbox Mail is delivered to the local user's system mailbox in .Pa /var/mail . .It Ic deliver to mda Ar program Mail is piped to the specified .Ar program , which is run with the privileges of the user the message is destined to. This parameter may use conversion specifiers that are expanded before use .Pq see Sx FORMAT SPECIFIERS . .It Xo .Bk -words .Ic relay .Op Ic backup Op Ar mx .Op Ic as Ar address .Op Ic source No < Ns Ar source Ns > .Op Ic hostname Ar name .Op Ic hostnames No < Ns Ar names Ns > .Op Ic pki Ar pkiname .Op Ic tls | verify .Ek .Xc .Pp Mail is relayed. The routing decision is based on the DNS system. .Pp If the .Ic backup parameter is specified, the current server will act as a backup server for the target domain. Accepted mails are only relayed through servers with a lower preference value in the MX record for the domain than the one specified in .Ar mx . If .Ar mx is not specified, the default server name will be assumed. .Pp If the .Ic as parameter is specified, .Xr smtpd 8 will rewrite the sender advertised in the SMTP session. .Ar address may be a user, a domain prefixed with .Sq @ , or an email address, causing smtpd to rewrite the user-part, the domain-part, or the entire address, respectively. .Pp If the .Ic source parameter is specified, .Xr smtpd 8 will explicitly bind to an address found in the table referenced by .Ar source when connecting to the relay. If the table contains more than one address, they are picked in turn each time a new connection is opened. .Pp By default, when connecting to a remote server, .Xr smtpd 8 advertises its default server name. A .Ic hostname parameter may be specified to advertise the alternate hostname .Ar name . If the .Ic source parameter is used, the .Ic hostnames parameter may be specified to advertise a hostname based on the source address. Table .Ar names contains a mapping of IP addresses to hostnames and .Xr smtpd 8 will automatically select the name that matches its source address when connected to the remote server. The .Ic hostname and .Ic hostnames parameters are mutually exclusive. .Pp When relaying, STARTTLS is always attempted if available on remote host and OpenSMTPD will try to present a certificate matching the outgoing hostname if one is registered in the pki. If .Ic pki is specified, the certificate registered for .Ar pkiname is used instead. .Pp If .Ic tls is specified, OpenSMTPD will refuse to relay unless the remote host provides STARTTLS. .Pp If .Ic verify is specified, OpenSMTPD will refuse to relay unless the remote host provides STARTTLS and the certificate it presented has been verified. .Pp Note that the .Ic tls and .Ic verify options are mutually exclusive and should only be used in private networks as they will prevent proper relaying on the Internet. .It Xo .Ic relay via .Ar host .Op Ic auth No < Ns Ar auth Ns > .Op Ic as Ar address .Op Ic source No < Ns Ar source Ns > .Op Ic hostname Ar name .Op Ic hostnames No < Ns Ar names Ns > .Op Ic pki Ar pkiname .Op Ic verify .Xc .Pp Mail is relayed through the specified .Ar host expressed as a URL. For example: .Bd -literal -offset indent smtp://mx1.example.org # use SMTP smtp://mx1.example.org:4321 # use SMTP \e # with port 4321 lmtp://localhost:2026 # use LMTP \e # with port 2026 .Ed .Pp The communication channel may be secured using one of the secure schemas. For example: .Bd -literal -offset indent tls://mx1.example.org # use TLS smtps://mx1.example.org # use SMTPS secure://mx1.example.org # try SMTPS and \e # fallback to TLS .Ed .Pp In addition, credentials for authenticated relaying may be provided when using a secure schema. For example: .Bd -literal -offset indent tls+auth://label@mx.example.org # over TLS smtps+auth://label@mx.example.org # over SMTPS secure+auth://label@mx.example.org # over either \e # SMTPS or TLS .Ed .Pp If a pki entry exists for the outgoing hostname, or one is provided with .Ar pkiname , the associated certificate will be sent to the remote server. .Pp If an SMTPAUTH session with .Ar host is desired, the .Ic auth parameter is used to specify the .Ar auth table that holds the credentials. Credentials will be looked up using the label provided in the URL. .Pp If the .Ic as parameter is specified, .Xr smtpd 8 will rewrite the sender advertised in the SMTP session. .Ar address may be a user, a domain prefixed with .Sq @ , or an email address, causing smtpd to rewrite the user-part, the domain-part, or the entire address, respectively. .Pp If the .Ic source parameter is specified, .Xr smtpd 8 will explicitly bind to an address found in the table referenced by .Pf < Ar source Ns > when connecting to the relay. If the table contains more than one address, they are picked in turn each time a new connection is opened. .Pp By default, when connecting to a remote server, .Xr smtpd 8 advertises its default server name. A .Ic hostname parameter may be specified to advertise the alternate hostname .Ar name . If the .Ic source parameter is used, the .Ic hostnames parameter may be specified to advertise a hostname based on the source address. Table .Ar names contains a mapping of IP addresses to hostnames and .Xr smtpd 8 will automatically select the name that matches its source address when connected to the remote server. The .Ic hostname and .Ic hostnames parameters are mutually exclusive. .El .Pp If .Ic verify is specified, OpenSMTPD will refuse to relay unless the remote host provides STARTTLS and the certificate it presented has been verified. The relay URL must specify TLS for this option to be valid. .Pp Additional per-rule adjustments available: .Bl -tag -width Ds .It Xo .Ic expire .Sm off .Ar n .Brq Cm s | m | h | d .Sm on .Xc Specify how long a message that matched this rule can stay in the queue. .El .It Xo .Ic bounce-warn .Sm off .Ar n .Brq Cm s | m | h | d .Oo , .Sm on .Ar ... .Oc .Xc Specify the delays for which temporary failure reports must be generated when messages are stuck in the queue. For example: .Bd -literal -offset indent bounce-warn 1h, 6h, 2d .Ed .Pp will generate a failure report when an envelope is in the queue for more than one hour, six hours and two days. The default is 4h. .It Xo .Ic expire .Sm off .Ar n .Brq Cm s | m | h | d .Sm on .Xc Specify how long a message can stay in the queue. The default value is 4 days. For example: .Bd -literal -offset indent expire 4d # expire after 4 days expire 10h # expire after 10 hours .Ed .It Xo .Ic limit mta .Op Ic for Ic domain Ar domain .Ar family .Xc Instruct .Xr smtpd 8 to only use the specified address .Ar family for outgoing connections. Accepted values are .Ic inet4 and .Ic inet6 . If a .Ar domain is specified, the restriction only applies when connecting to MXs for this domain. .It Xo .Ic limit scheduler max-inflight .Ar num .Xc Suspend the scheduling of envelopes for deliver/relay until the number of inflight envelopes falls below .Ar num . Changing the default value might degrade performances. .It Xo .Bk -words .Ic listen on Ar interface .Op Ar family .Op Ic port Ar port .Op Ic tls | tls-require | tls-require verify | smtps | secure .Op Ic pki Ar pkiname .Op Ic auth | auth-optional Op < Ns Ar authtable Ns > .Op Ic tag Ar tag .Op Ic hostname Ar hostname .Op Ic hostnames No < Ns Ar names Ns > .Op Ic mask-source .Op Ic no-dsn .Ek .Xc .Pp Specify an .Ar interface and .Ar port to listen on. An interface group, an IP address or a domain name may be used in place of .Ar interface . The .Ar family parameter can be used to listen only on specific address family. Accepted values are .Ic inet4 and .Ic inet6 . .Pp Secured connections are provided either using STARTTLS .Pq Ic tls , by default on port 25, or SMTPS .Pq Ic smtps , by default on port 465. .Ic tls-require may be used to force clients to establish a secure connection before being allowed to start an SMTP transaction. .Pp If .Ic tls-require verify is specified, the client must provide a valid certificate to be able to establish an SMTP session. .Pp .Ic secure may be specified to provide both STARTTLS and SMTPS services. Host certificates may be used for these connections, and must be priorly declared using the pki directive. If .Ic pki is specified, a certificate matching .Ic name is searched for. .Pp If the .Ic auth parameter is used, then a client may only start an SMTP transaction after a successful authentication. Any remote sender that passed SMTPAUTH is treated as if it was the server's local user that was sending the mail. This means that filter rules using .Ic from local will be matched. If .Ic auth-optional is specified, then SMTPAUTH is not required to establish an SMTP transaction. This is only useful to let a listener accept incoming mail from untrusted senders and outgoing mail from authenticated users in situations where it is not possible to listen on the submission port. .Pp Both .Ic auth and .Ic auth-optional accept an optional table as a parameter. When provided, credentials are looked up in this table. Credentials format is described in .Xr table 5 . .Pp If the .Ic tag parameter is used, then clients connecting to the listener will be tagged .Ar tag . .Pp If the .Ic hostname parameter is used, then it will be used in the greeting banner instead of the default server name. .Pp The .Ic hostnames parameter overrides the server name for specific addresses. Table .Ar names contains a mapping of IP addresses to hostnames and .Xr smtpd 8 will use the hostname that matches the address on which the connection arrives if it is found in the mapping. .Pp If the .Ic mask-source parameter is used, then the listener will skip the .Ic from part when prepending the .Dq Received header. .Pp If the .Ic no-dsn parameter is used, DSN (Delivery Status Notification) extension will not be enabled. .It Ic max-message-size Ar n Specify a maximum message size of .Ar n bytes. The argument may contain a multiplier, as documented in .Xr scan_scaled 3 . The default maximum message size is 35MB if none is specified. .It Ic pki Ar hostname Ic certificate Ar certfile Associate the certificate located in .Ar certfile with .Ar hostname . .Pp A certificate chain may be created by appending one or many certificates, including a Certificate Authority certificate, to .Ar certfile . .Pp Creation of certificates is documented in .Xr starttls 8 . .It Ic pki Ar hostname Ic key Ar keyfile Associate the key located in .Ar keyfile with .Ar hostname . .It Ic pki Ar hostname Ic ca Ar cafile Associate a custom CA certificate .Ar cafile with .Ar hostname . .It Ic pki Ar hostname Ic dhparams Ar dhfile Associate the Diffie-Hellman parameters located in .Ar dhfile with .Ar hostname . .Pp The parameters are used for ephemeral key exchange. If not specified, OpenSMTPD will use safely generated builtin parameters. .Pp Creation of Diffie-Hellman parameters is documented in .Xr openssl 1 . .It Ic queue compression Enable transparent compression of envelopes and messages. The only supported algorithm at the moment is gzip. Envelopes and messages may be inspected using the .Xr smtpctl 8 or .Xr gzcat 1 utilities. .It Ic queue encryption Op key Ar key Enable transparent encryption of envelopes and messages. .Ar key must be a 16-byte random key in hexadecimal representation. It can be obtained using the .Xr openssl 1 utility as follow: .Bd -literal -offset indent $ openssl rand \-hex 16 .Ed .Pp If the .Ar key parameter is not specified, it is read with .Xr getpass 3 at startup. If .Ar key is .Ic stdin , then it is read from the standard input at startup. .Pp The only supported algorithm is AES-256 in GCM mode. Envelopes and messages may be inspected using the .Xr smtpctl 8 utility. .Pp Queue encryption can be used with queue compression and will always perform compression before encryption. .It Ic table Ar name Oo Ar type : Oc Ns Ar config Tables are used to provide additional configuration information for .Xr 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 .Xr table 5 for the exhaustive documentation. .Pp The table is identified using table name .Ar name ; the name itself is arbitrarily chosen. .Pp .Ar type specifies the table backend, and should be one of the following: .Pp .Bl -tag -width "fileXXX" -compact .It db Information is stored in a file created using .Xr makemap 8 . .It file Information is stored in a plain text file using the same format as used to generate .Xr makemap 8 mappings. This is the default. .El .Pp .Ar config specifies a configuration file for the table data. It must be an absolute path to a file for the .Dq file and .Dq db table types. .It Ic table Ar name Brq Ar value Op , Ar ... Tables containing list of static values may be declared using an inlined notation. .Pp The table is identified using table name .Ar name ; the name itself is arbitrarily chosen. .Pp The table must contain at least one value and may declare many values as a list of comma separated strings. .It Ic table Ar name Brq Ar key Ns = Ns Ar value Op , Ar ... Tables containing static key-value mappings may be declared using an inlined notation. .Pp The table is identified using table name .Ar name ; the name itself is arbitrarily chosen. .Pp The table must contain at least one key-value mapping and may declare many mappings as a list of comma separated .Ar key Ns = Ns Ar value descriptions. .El .Ss FORMAT SPECIFIERS Some configuration directives support expansion of their parameters at runtime. Such directives (for example .Ic deliver to maildir , .Ic deliver to mda ) may use format specifiers which will be expanded before delivery or relaying. The following formats are currently supported: .Bl -column %{user.directory} -offset indent .It %{sender} Ta sender email address .It %{sender.user} Ta user part of the sender email address .It %{sender.domain} Ta domain part of the sender email address .It %{rcpt} Ta recipient email address .It %{rcpt.user} Ta user part of the recipient email address .It %{rcpt.domain} Ta domain part of the recipient email address .It %{dest} Ta recipient email address after expansion .It %{dest.user} Ta user part after expansion .It %{dest.domain} Ta domain part after expansion .It %{user.username} Ta local user .It %{user.directory} Ta home directory of the local user .El .Pp Expansion formats also support partial expansion using the optional bracket notations with substring offset. For example, with recipient domain .Dq example.org : .Bl -column %{rcpt.domain[0:-4]} -offset indent .It %{rcpt.domain[0]} Ta expands to Dq e .It %{rcpt.domain[1]} Ta expands to Dq x .It %{rcpt.domain[8:]} Ta expands to Dq org .It %{rcpt.domain[-3:]} Ta expands to Dq org .It %{rcpt.domain[0:6]} Ta expands to Dq example .It %{rcpt.domain[0:-4]} Ta expands to Dq example .El .Pp In addition, modifiers may be applied to the token. For example, with recipient .Dq User+Tag@Example.org : .Bl -column %{rcpt:lowercase|strip} -offset indent .It %{rcpt:lowercase} Ta expands to Dq user+tag@example.org .It %{rcpt:uppercase} Ta expands to Dq USER+TAG@EXAMPLE.ORG .It %{rcpt:strip} Ta expands to Dq User@Example.org .It %{rcpt:lowercase|strip} Ta expands to Dq user@example.org .El .Pp For security concerns, expanded values are sanitized and potentially dangerous characters are replaced with .Sq \&: . In situations where they are desirable, the .Dq raw modifier may be applied. For example, with recipient .Dq user+t?g@example.org : .Bl -column %{rcpt:raw} -offset indent .It %{rcpt} Ta expands to Dq user+t:g@example.org .It %{rcpt:raw} Ta expands to Dq user+t?g@example.org .El .Sh FILES .Bl -tag -width "/etc/mail/smtpd.confXXX" .It Pa /etc/mail/smtpd.conf Default .Xr smtpd 8 configuration file. .It Pa /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 .Xr gethostname 3 , either directly if it is a fully qualified domain name, or by retrieving the associated canonical name through .Xr getaddrinfo 3 . .It Pa /var/spool/smtpd/ Spool directories for mail during processing. .El .Sh EXAMPLES The default .Nm file which ships with .Ox 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. .Pp This first example is the same as 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: .Bd -literal -offset indent # touch /etc/mail/secrets # chmod 640 /etc/mail/secrets # chown root:_smtpd /etc/mail/secrets # echo "label username:password" > /etc/mail/secrets # makemap /etc/mail/secrets .Ed .Pp .Nm would look like this: .Bd -literal -offset indent listen on lo0 table aliases db:/etc/mail/aliases.db table secrets db:/etc/mail/secrets.db accept for local alias deliver to mbox accept for any relay via tls+auth://label@smtp.example.com \e auth .Ed .Pp In this second example, the aim is to permit mail relaying for any user 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 route(s) point to. Mail with a local destination should be sent to an external mda. First, the RSA certificate is created: .Bd -literal -offset indent # openssl genrsa \-out /etc/ssl/private/mail.example.com.key 4096 # openssl req \-new \-x509 \-key /etc/ssl/private/mail.example.com.key \e \-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 .Ed .Pp In the example above, a certificate valid for one year was created. The configuration file would look like this: .Bd -literal -offset indent pki mail.example.com certificate "/etc/ssl/mail.example.com.crt" pki mail.example.com key "/etc/ssl/private/mail.example.com.key" listen on lo0 listen on egress tls pki mail.example.com auth table aliases db:/etc/mail/aliases.db accept for local alias deliver to mda "/path/to/mda \-f \-" accept from any for domain example.com \e deliver to mda "/path/to/mda \-f \-" accept for any relay .Ed .Pp For sites that wish to sign messages using DKIM, the .Em dkimproxy package may be used as a filter. The following example is the same as the default configuration, but all outgoing mail is passed to dkimproxy_out on port 10027 for signing. The signed messages are received on port 10028 and tagged for relaying. .Bd -literal -offset indent listen on lo0 listen on lo0 port 10028 tag DKIM table aliases db:/etc/mail/aliases.db accept for local alias deliver to mbox accept tagged DKIM for any relay accept from local for any relay via smtp://127.0.0.1:10027 .Ed .Pp 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 table .Em other-relays can be used to specify the IP addresses of relays that may legitimately originate mail with your domain as the sender. .Bd -literal -offset indent listen on lo0 listen on egress table aliases db:/etc/mail/aliases.db table other-relays "/etc/mail/other-relays" accept for local alias deliver to mbox accept from local for any relay reject from ! source sender "@example.com" for any accept from any for domain example.com \e alias deliver to mbox .Ed .Sh SEE ALSO .Xr mailer.conf 5 , .Xr table 5 , .Xr makemap 8 , .Xr smtpd 8 .Sh HISTORY .Xr smtpd 8 first appeared in .Ox 4.6 .