docker-mailserver/README.md

docker-mailserver

A fullstack but simple mail server (smtp, imap, antispam, antivirus...). Only configuration files, no SQL database. Keep it simple and versioned. Easy to deploy and upgrade.

ANNOUNCEMENT

Some time VERY SOON we will merge the next branch based on Debian Buster to master. That means the docker image latest will change. The change may break things!

The following possibly breaking changes are known:

• Filebeat is removed and should be handled by another container, see Wiki.
• Dovecot will be downgraded a little bit (same major version) so that we can use the official Debian version.

If you want to stick to the old version a while longer, either switch to stable or to a specific version. If you run into problems, please raise issues and ask for help. Don't forget to provide details.

Includes:

• Postfix with smtp or ldap auth
• Dovecot for sasl, imap (and optional pop3) with ssl support, with ldap auth
  • Dovecot is installed from the Dovecot Community Repo
• saslauthd with ldap auth
• Amavis
• Spamassasin supporting custom rules
• ClamAV with automatic updates
• OpenDKIM
• OpenDMARC
• Fail2ban
• Fetchmail
• Postscreen
• Postgrey
• basic Sieve support using dovecot
• LetsEncrypt and self-signed certificates
• Setup script to easily configure and maintain your mailserver
• persistent data and state (but think about backups!)
• Integration tests
• Automated builds on docker hub
• Plus addressing (a.k.a. extension delimiters) works out of the box: email for you+extension@example.com go to you@example.com

Why I created this image: Simple mail server with Docker

Before you open an issue, please have a look this README, the Wiki and Postfix/Dovecot documentation.

Requirements

Recommended:

• 1 CPU
• 1-2GB RAM
• Swap enabled for the container

Minimum:

• 1 CPU
• 512MB RAM

Note: You'll need to deactivate some services like ClamAV to be able to run on a host with 512MB of RAM. Even with 1G RAM you may run into problems without swap, see FAQ.

Usage

Get latest image

docker pull tvial/docker-mailserver:latest

Get the tools

Download the docker-compose.yml, the .env and the setup.sh files:

curl -o setup.sh https://raw.githubusercontent.com/tomav/docker-mailserver/master/setup.sh; chmod a+x ./setup.sh

curl -o docker-compose.yml https://raw.githubusercontent.com/tomav/docker-mailserver/master/docker-compose.yml.dist

curl -o .env https://raw.githubusercontent.com/tomav/docker-mailserver/master/.env.dist

curl -o env-mailserver https://raw.githubusercontent.com/tomav/docker-mailserver/master/env-mailserver.dist

Create a docker-compose environment

• Edit the files .env and env-mailserver to your liking:
  • .env contains the configuration for docker-compose
  • env-mailserver contains the configuration for the mailserver container
  • These files supports only simple VAR=VAL lines (see Documentation).
  • Don't quote your values.
  • Variable substitution is not supported (e.g. OVERRIDE_HOSTNAME=$HOSTNAME.$DOMAINNAME).
• Install docker-compose in the version 1.7 or higher.

Note: If you want to use a bare domain (host name equals domain name) see FAQ.

Start Container

docker-compose up -d mail

Create your mail accounts

./setup.sh email add <user@domain> [<password>]

Generate DKIM keys

./setup.sh config dkim

Now the keys are generated, you can configure your DNS server by just pasting the content of config/opendkim/keys/domain.tld/mail.txt in your domain.tld.hosts zone.

Restart and update the container

docker-compose down
docker pull tvial/docker-mailserver:latest
docker-compose up -d mail

You're done!

And don't forget to have a look at the remaining functions of the setup.sh script

SPF/Forwarding Problems

If you got any problems with SPF and/or forwarding mails, give SRS a try. You enable SRS by setting ENABLE_SRS=1. See the variable description for further information.

For informational purposes:

Your config folder will be mounted in /tmp/docker-mailserver/. To understand how things work on boot, please have a look at start-mailserver.sh

restart: always ensures that the mail server container (and ELK container when using the mail server together with ELK stack) is automatically restarted by Docker in cases like a Docker service or host restart or container exit.

Exposed ports

• 25 receiving email from other mailservers
• 465 SSL Client email submission
• 587 TLS Client email submission
• 143 StartTLS IMAP client
• 993 TLS/SSL IMAP client
• 110 POP3 client
• 995 TLS/SSL POP3 client

Note: Port 25 is only for receiving email from other mailservers and not for submitting email. You need to use port 465 or 587 for this.

Examples with just the relevant environmental variables:

version: '2'

services:
  mail:
    image: tvial/docker-mailserver:latest
    hostname: mail
    domainname: domain.com
    container_name: mail
    ports:
      - "25:25"
      - "143:143"
      - "587:587"
      - "993:993"
    volumes:
      - maildata:/var/mail
      - mailstate:/var/mail-state
      - maillogs:/var/log/mail
      - ./config/:/tmp/docker-mailserver/
    environment:
      - ENABLE_SPAMASSASSIN=1
      - ENABLE_CLAMAV=1
      - ENABLE_FAIL2BAN=1
      - ENABLE_POSTGREY=1
      - ONE_DIR=1
      - DMS_DEBUG=0
    cap_add:
      - NET_ADMIN
      - SYS_PTRACE

volumes:
  maildata:
    driver: local
  mailstate:
    driver: local
  maillogs:
    driver: local

for ldap setup:

version: '2'

services:
  mail:
    image: tvial/docker-mailserver:latest
    hostname: mail
    domainname: domain.com
    container_name: mail
    ports:
      - "25:25"
      - "143:143"
      - "587:587"
      - "993:993"
    volumes:
      - maildata:/var/mail
      - mailstate:/var/mail-state
      - maillogs:/var/log/mail
      - ./config/:/tmp/docker-mailserver/
    environment:
      - ENABLE_SPAMASSASSIN=1
      - ENABLE_CLAMAV=1
      - ENABLE_FAIL2BAN=1
      - ENABLE_POSTGREY=1
      - ONE_DIR=1
      - DMS_DEBUG=0
      - ENABLE_LDAP=1
      - LDAP_SERVER_HOST=ldap # your ldap container/IP/ServerName
      - LDAP_SEARCH_BASE=ou=people,dc=localhost,dc=localdomain
      - LDAP_BIND_DN=cn=admin,dc=localhost,dc=localdomain
      - LDAP_BIND_PW=admin
      - LDAP_QUERY_FILTER_USER=(&(mail=%s)(mailEnabled=TRUE))
      - LDAP_QUERY_FILTER_GROUP=(&(mailGroupMember=%s)(mailEnabled=TRUE))
      - LDAP_QUERY_FILTER_ALIAS=(|(&(mailAlias=%s)(objectClass=PostfixBookMailForward))(&(mailAlias=%s)(objectClass=PostfixBookMailAccount)(mailEnabled=TRUE)))
      - LDAP_QUERY_FILTER_DOMAIN=(|(&(mail=*@%s)(objectClass=PostfixBookMailAccount)(mailEnabled=TRUE))(&(mailGroupMember=*@%s)(objectClass=PostfixBookMailAccount)(mailEnabled=TRUE))(&(mailalias=*@%s)(objectClass=PostfixBookMailForward)))
      - DOVECOT_PASS_FILTER=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=%n))
      - DOVECOT_USER_FILTER=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=%n))
      - ENABLE_SASLAUTHD=1
      - SASLAUTHD_MECHANISMS=ldap
      - SASLAUTHD_LDAP_SERVER=ldap
      - SASLAUTHD_LDAP_BIND_DN=cn=admin,dc=localhost,dc=localdomain
      - SASLAUTHD_LDAP_PASSWORD=admin
      - SASLAUTHD_LDAP_SEARCH_BASE=ou=people,dc=localhost,dc=localdomain
      - POSTMASTER_ADDRESS=postmaster@localhost.localdomain
      - POSTFIX_MESSAGE_SIZE_LIMIT=100000000
    cap_add:
      - NET_ADMIN
      - SYS_PTRACE

volumes:
  maildata:
    driver: local
  mailstate:
    driver: local
  maillogs:
    driver: local

Environment variables

Please check how the container starts to understand what's expected. Also if an option doesn't work as documented here, check if you are running the latest image!

Value in bold is the default value.

General

DMS_DEBUG

• 0 => Debug disabled
• 1 => Enables debug on startup

ENABLE_CLAMAV

• 0 => Clamav is disabled
• 1 => Clamav is enabled

ONE_DIR

• 0 => state in default directories
• 1 => consolidate all states into a single directory (/var/mail-state) to allow persistence using docker volumes

ENABLE_POP3

• empty => POP3 service disabled
• 1 => Enables POP3 service

ENABLE_FAIL2BAN

• 0 => fail2ban service disabled
• 1 => Enables fail2ban service

If you enable Fail2Ban, don't forget to add the following lines to your docker-compose.yml:

cap_add:
  - NET_ADMIN

Otherwise, iptables won't be able to ban IPs.

SMTP_ONLY

• empty => all daemons start
• 1 => only launch postfix smtp

SSL_TYPE

• empty => SSL disabled
• letsencrypt => Enables Let's Encrypt certificates
• custom => Enables custom certificates
• manual => Let you manually specify locations of your SSL certificates for non-standard cases
• self-signed => Enables self-signed certificates
• any other value => SSL required, settings by default

Please read the SSL page in the wiki for more information.

TLS_LEVEL

• empty => modern
• modern => Enables TLSv1.2 and modern ciphers only. (default)
• intermediate => Enables TLSv1, TLSv1.1 and TLSv1.2 and broad compatibility ciphers.
• old => NOT implemented. If you really need it, then customize the TLS ciphers overriding postfix and dovecot settings [ wiki](https://github.com/tomav/docker-mailserver/wiki/

SPOOF_PROTECTION

Configures the handling of creating mails with forged sender addresses.

• empty => Mail address spoofing allowed. Any logged in user may create email messages with a forged sender address. See also Wikipedia(not recommended, but default for backwards compatibility reasons)
• 1 => (recommended) Mail spoofing denied. Each user may only send with his own or his alias addresses. Addresses with extension delimiters are not able to send messages.

ENABLE_SRS

Enables the Sender Rewriting Scheme. SRS is needed if your mail server acts as forwarder. See postsrsd for further explanation.

• 0 => Disabled
• 1 => Enabled

PERMIT_DOCKER

Set different options for mynetworks option (can be overwrite in postfix-main.cf) WARNING: Adding the docker network's gateway to the list of trusted hosts, e.g. using the network or connected-networks option, can create an open relay, for instance if IPv6 is enabled on the host machine but not in Docker.

• empty => localhost only
• host => Add docker host (ipv4 only)
• network => Add the docker default bridge network (172.16.0.0/12); WARNING: docker-compose might use others (e.g. 192.168.0.0/16) use PERMIT_DOCKER=connected-networks in this case
• connected-networks => Add all connected docker networks (ipv4 only)

Note: you probably want to set POSTFIX_INET_PROTOCOLS=ipv4 to make it work fine with Docker.

VIRUSMAILS_DELETE_DELAY

Set how many days a virusmail will stay on the server before being deleted

• empty => 7 days

ENABLE_POSTFIX_VIRTUAL_TRANSPORT

This Option is activating the Usage of POSTFIX_DAGENT to specify a ltmp client different from default dovecot socket.

• empty => disabled
• 1 => enabled

POSTFIX_DAGENT

Enabled by ENABLE_POSTFIX_VIRTUAL_TRANSPORT. Specify the final delivery of postfix

• empty: fail
• lmtp:unix:private/dovecot-lmtp (use socket)
• lmtps:inet:<host>:<port> (secure lmtp with starttls, take a look at https://sys4.de/en/blog/2014/11/17/sicheres-lmtp-mit-starttls-in-dovecot/)
• lmtp:<kopano-host>:2003 (use kopano as mailstore)
• etc.

POSTFIX_MAILBOX_SIZE_LIMIT

Set the mailbox size limit for all users. If set to zero, the size will be unlimited (default).

• empty => 0 (no limit)

POSTFIX_MESSAGE_SIZE_LIMIT

Set the message size limit for all users. If set to zero, the size will be unlimited (not recommended!)

• empty => 10240000 (~10 MB)

ENABLE_MANAGESIEVE

• empty => Managesieve service disabled
• 1 => Enables Managesieve on port 4190

OVERRIDE_HOSTNAME

• empty => uses the hostname command to get the mail server's canonical hostname
• => Specify a fully-qualified domainname to serve mail for. This is used for many of the config features so if you can't set your hostname (e.g. you're in a container platform that doesn't let you) specify it in this environment variable.

POSTMASTER_ADDRESS

• empty => postmaster@domain.com
• => Specify the postmaster address

POSTSCREEN_ACTION

• 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.
• ignore => 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 statistics without blocking mail.

DOVECOT_MAILBOX_FORMAT

• maildir => uses very common Maildir format, one file contains one message
• sdbox => (experimental) uses Dovecot high-performance mailbox format, one file contains one message
• mdbox ==> (experimental) uses Dovecot high-performance mailbox format, multiple messages per file and multiple files per box

This option has been added in November 2019. Using other format than Maildir is considered as experimental in docker-mailserver and should only be used for testing purpose. For more details, please refer to Dovecot Documentation.

POSTFIX_INET_PROTOCOLS

• all => All possible protocols.
• ipv4 => Use only IPv4 traffic. Most likely you want this behind Docker.
• ipv6 => Use only IPv6 traffic.

Note: More details in http://www.postfix.org/postconf.5.html#inet_protocols

Reports

PFLOGSUMM_TRIGGER

Enables regular pflogsumm mail reports.

• not set => No report
• daily_cron => Daily report for the previous day
• logrotate => Full report based on the mail log when it is rotated

This is a new option. The old REPORT options are still supported for backwards compatibility. If this is not set and reports are enabled with the old options, logrotate will be used.

PFLOGSUMM_RECIPIENT

Recipient address for pflogsumm reports.

• not set => Use REPORT_RECIPIENT or POSTMASTER_ADDRESS
• => Specify the recipient address(es)

PFLOGSUMM_SENDER

From address for pflogsumm reports.

• not set => Use REPORT_SENDER or POSTMASTER_ADDRESS
• => Specify the sender address

LOGWATCH_INTERVAL

Interval for logwatch report.

• none => No report is generated
• daily => Send a daily report
• weekly => Send a report every week

LOGWATCH_RECIPIENT

Recipient address for logwatch reports if they are enabled.

• not set => Use REPORT_RECIPIENT or POSTMASTER_ADDRESS
• => Specify the recipient address(es)

REPORT_RECIPIENT (deprecated)

Enables a report being sent (created by pflogsumm) on a regular basis.

• 0 => Report emails are disabled unless enabled by other options
• 1 => Using POSTMASTER_ADDRESS as the recipient
• => Specify the recipient address

REPORT_SENDER (deprecated)

Change the sending address for mail report

• empty => mailserver-report@hostname
• => Specify the report sender (From) address

REPORT_INTERVAL (deprecated)

changes the interval in which logs are rotated and a report is being sent (deprecated).

• daily => Send a daily report
• weekly => Send a report every week
• monthly => Send a report every month

Note: This variable used to control logrotate inside the container and sent the pflogsumm report when the logs were rotated. It is still supported for backwards compatibility, but the new option LOGROTATE_INTERVAL has been added that only rotates the logs.

LOGROTATE_INTERVAL

Defines the interval in which the mail log is being rotated.

• daily => Rotate daily.
• weekly => Rotate weekly.
• monthly => Rotate monthly.

Note that only the log inside the container is affected. The full log output is still available via docker logs mail (or your respective container name). If you want to control logrotation for the docker generated logfile see: Docker Logging Drivers.

Also note that by default the logs are lost when the container is recycled. To keep the logs, mount a volume.

Finally the logrotate interval may affect the period for generated reports. That is the case when the reports are triggered by log rotation.

Spamassassin

ENABLE_SPAMASSASSIN

• 0 => Spamassassin is disabled
• 1 => Spamassassin is enabled

SA_TAG

• 2.0 => add spam info headers if at, or above that level

Note: this spamassassin setting needs ENABLE_SPAMASSASSIN=1

SA_TAG2

• 6.31 => add 'spam detected' headers at that level

Note: this spamassassin setting needs ENABLE_SPAMASSASSIN=1

SA_KILL

• 6.31 => triggers spam evasive actions

Note: this spamassassin setting needs ENABLE_SPAMASSASSIN=1. By default, the mailserver is configured to quarantine spam emails. If emails are quarantined, they are compressed and stored in a location dependent on the ONE_DIR setting above. If ONE_DIR=1 the location is /var/mail-state/lib-amavis/virusmails/. If ONE_DIR=0 it is /var/lib/amavis/virusmails/. These paths are inside the docker container. To inhibit this behaviour and deliver spam emails, set this to a very high value e.g. 100.0.

SA_SPAM_SUBJECT

• ***SPAM*** => add tag to subject if spam detected

Note: this spamassassin setting needs ENABLE_SPAMASSASSIN=1

SA_SHORTCIRCUIT_BAYES_SPAM

• 1 => will activate spamassassin short circuiting for bayes spam detection.

This will uncomment the respective line in /etc/spamassasin/local.cf

Note: activate this only if you are confident in your bayes database for identifying spam.

SA_SHORTCIRCUIT_BAYES_HAM

• 1 => will activate spamassassin short circuiting for bayes ham detection

This will uncomment the respective line in /etc/spamassasin/local.cf

Note: activate this only if you are confident in your bayes database for identifying ham.

Fetchmail

ENABLE_FETCHMAIL

• 0 => fetchmail disabled
• 1 => fetchmail enabled

FETCHMAIL_POLL

• 300 => fetchmail The number of seconds for the interval

LDAP

ENABLE_LDAP

• empty => LDAP authentification is disabled
• 1 => LDAP authentification is enabled
• NOTE:
  • A second container for the ldap service is necessary (e.g. docker-openldap)
  • For preparing the ldap server to use in combination with this container this article may be helpful

LDAP_START_TLS

• empty => no
• yes => LDAP over TLS enabled for Postfix

LDAP_SERVER_HOST

• empty => mail.domain.com
• => Specify the dns-name/ip-address where the ldap-server
• NOTE: If you going to use the mailserver in combination with docker-compose you can set the service name here

LDAP_SEARCH_BASE

• empty => ou=people,dc=domain,dc=com
• => e.g. LDAP_SEARCH_BASE=dc=mydomain,dc=local

LDAP_BIND_DN

• empty => cn=admin,dc=domain,dc=com
• => take a look at examples of SASL_LDAP_BIND_DN

LDAP_BIND_PW

• empty => admin
• => Specify the password to bind against ldap

LDAP_QUERY_FILTER_USER

• e.g. (&(mail=%s)(mailEnabled=TRUE))
• => Specify how ldap should be asked for users

LDAP_QUERY_FILTER_GROUP

• e.g. (&(mailGroupMember=%s)(mailEnabled=TRUE))
• => Specify how ldap should be asked for groups

LDAP_QUERY_FILTER_ALIAS

• e.g. (&(mailAlias=%s)(mailEnabled=TRUE))
• => Specify how ldap should be asked for aliases

LDAP_QUERY_FILTER_DOMAIN

• e.g. (&(|(mail=*@%s)(mailalias=*@%s)(mailGroupMember=*@%s))(mailEnabled=TRUE))
• => Specify how ldap should be asked for domains

DOVECOT_TLS

• empty => no
• yes => LDAP over TLS enabled for Dovecot

Dovecot

The following variables overwrite the default values for /etc/dovecot/dovecot-ldap.conf.ext.

DOVECOT_USER_FILTER

• e.g. (&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=%n))

DOVECOT_USER_ATTRS

• e.g. homeDirectory=home,qmailUID=uid,qmailGID=gid,mailMessageStore=mail
• => Specify the directory to dovecot attribute mapping that fits your directory structure.
• Note: This is necessary for directories that do not use the Postfix Book Schema.
• Note: The left-hand value is the directory attribute, the right hand value is the dovecot variable.
• More details on the Dovecot Wiki

DOVECOT_PASS_FILTER

• e.g. (&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=%n))

DOVECOT_PASS_ATTRS

• e.g. uid=user,userPassword=password
• => Specify the directory to dovecot variable mapping that fits your directory structure.
• Note: This is necessary for directories that do not use the Postfix Book Schema.
• Note: The left-hand value is the directory attribute, the right hand value is the dovecot variable.
• More details on the Dovecot Wiki

Postgrey

ENABLE_POSTGREY

• 0 => postgrey is disabled
• 1 => postgrey is enabled

POSTGREY_DELAY

• 300 => greylist for N seconds

Note: This postgrey setting needs ENABLE_POSTGREY=1

POSTGREY_MAX_AGE

• 35 => delete entries older than N days since the last time that they have been seen

Note: This postgrey setting needs ENABLE_POSTGREY=1

POSTGREY_AUTO_WHITELIST_CLIENTS

• 5 => whitelist host after N successful deliveries (N=0 to disable whitelisting)

Note: This postgrey setting needs ENABLE_POSTGREY=1

POSTGREY_TEXT

• Delayed by postgrey => response when a mail is greylisted

Note: This postgrey setting needs ENABLE_POSTGREY=1

SASL Auth

ENABLE_SASLAUTHD

• 0 => saslauthd is disabled
• 1 => saslauthd is enabled

SASLAUTHD_MECHANISMS

• empty => pam
• ldap => authenticate against ldap server
• shadow => authenticate against local user db
• mysql => authenticate against mysql db
• rimap => authenticate against imap server
• NOTE: can be a list of mechanisms like pam ldap shadow

SASLAUTHD_MECH_OPTIONS

• empty => None
• e.g. with SASLAUTHD_MECHANISMS rimap you need to specify the ip-address/servername of the imap server ==> xxx.xxx.xxx.xxx

SASLAUTHD_LDAP_SERVER

• empty => localhost

SASLAUTHD_LDAP_SSL

• empty or 0 => ldap:// will be used
• 1 => ldaps:// will be used

SASLAUTHD_LDAP_BIND_DN

• empty => anonymous bind
• specify an object with privileges to search the directory tree
• e.g. active directory: SASLAUTHD_LDAP_BIND_DN=cn=Administrator,cn=Users,dc=mydomain,dc=net
• e.g. openldap: SASLAUTHD_LDAP_BIND_DN=cn=admin,dc=mydomain,dc=net

SASLAUTHD_LDAP_PASSWORD

• empty => anonymous bind

SASLAUTHD_LDAP_SEARCH_BASE

• empty => Reverting to SASLAUTHD_MECHANISMS pam
• specify the search base

SASLAUTHD_LDAP_FILTER

• empty => default filter (&(uniqueIdentifier=%u)(mailEnabled=TRUE))
• e.g. for active directory: (&(sAMAccountName=%U)(objectClass=person))
• e.g. for openldap: (&(uid=%U)(objectClass=person))

SASL_PASSWD

• empty => No sasl_passwd will be created
• string => /etc/postfix/sasl_passwd will be created with the string as password

SRS (Sender Rewriting Scheme)

SRS_SENDER_CLASSES

An email has an "envelope" sender (indicating the sending server) and a "header" sender (indicating who sent it). More strict SPF policies may require you to replace both instead of just the envelope sender.

More info.

• envelope_sender => Rewrite only envelope sender address
• header_sender => Rewrite only header sender (not recommended)
• envelope_sender,header_sender => Rewrite both senders

SRS_EXCLUDE_DOMAINS

• empty => Envelope sender will be rewritten for all domains
• provide comma separated list of domains to exclude from rewriting

SRS_SECRET

• empty => generated when the container is started for the first time
• provide a secret to use in base64
• you may specify multiple keys, comma separated. the first one is used for signing and the remaining will be used for verification. this is how you rotate and expire keys
• if you have a cluster/swarm make sure the same keys are on all nodes
• example command to generate a key: dd if=/dev/urandom bs=24 count=1 2>/dev/null | base64

SRS_DOMAINNAME

• empty => Derived from OVERRIDE_HOSTNAME, DOMAINNAME, or the container's hostname
• Set this if auto-detection fails, isn't what you want, or you wish to have a separate container handle DSNs

Default Relay Host

DEFAULT_RELAY_HOST

• empty => don't set default relayhost setting in main.cf
• default host and port to relay all mail through. Format: [example.com]:587 (don't forget the brackets if you need this to be compatible with $RELAY_USER and $RELAY_PASSWORD, explained below).

Multi-domain Relay Hosts

RELAY_HOST

• empty => don't configure relay host
• default host to relay mail through

RELAY_PORT

• empty => 25
• default port to relay mail through

RELAY_USER

• empty => no default
• default relay username (if no specific entry exists in postfix-sasl-password.cf)

RELAY_PASSWORD

• empty => no default
• password for default relay user