Run postfix with SMTP authentication (sasldb) in a Docker container.
TLS and OpenDKIM support is optional. Fail2ban can be enabled.

1. Pull image

   docker pull danilsmirnov/postfix

1. Create postfix container with smtp authentication

   docker run -p 25:25 \
   	-e MAIL_DOMAIN=example.com -e SMTP_USER=user:pwd \
   	--name postfix -d danilsmirnov/postfix
   # Set multiple user credentials: -e SMTP_USER=user1:pwd1,user2:pwd2,...,userN:pwdN

2. Set mail host defferent from mail domain

   docker run -p 25:25 \
   	-e MAIL_DOMAIN=example.com -e MAIL_HOST=mail.example.com -e SMTP_USER=user:pwd \
   	--name postfix -d danilsmirnov/postfix

3. Enable OpenDKIM: save your domain key mail.private in /path/to/domainkeys

   docker run -p 25:25 \
   	-e MAIL_DOMAIN=example.com -e MAIL_HOST=mail.example.com -e SMTP_USER=user:pwd \
   	-v /path/to/domainkeys:/etc/opendkim/domainkeys \
   	--name postfix -d danilsmirnov/postfix
   # Set DKIM_SELECTOR variable if not okay with default "mail" selector

   To sign more domains add their key file(s) named $DKIM_SELECTOR._domainkey.$DOMAIN.private, e.g.:

   mail._domainkey.subdomain.example.com.private
   mail._domainkey.*.example.com.private

   (Wildcard subdomain refers to its parent domain's DNS record.)

4. Enable TLS(587): save your SSL certificates mail.example.com.key and mail.example.com.crt to /path/to/certs

   docker run -p 587:587 \
   	-e MAIL_DOMAIN=example.com -e MAIL_HOST=mail.example.com -e SMTP_USER=user:pwd \
   	-v /path/to/certs:/etc/postfix/certs \
   	--name postfix -d danilsmirnov/postfix

5. Enable Fail2ban with postfix-sasl jail to ban brute-force attackers

   docker run -p 25:25 \
   	-e MAIL_DOMAIN=example.com -e MAIL_HOST=mail.example.com -e SMTP_USER=user:pwd \
   	-e FAIL2BAN=enabled --cap-add NET_ADMIN \
   	--name postfix -d danilsmirnov/postfix
   # Note: NET_ADMIN capability must be granted to the container
   # FAIL2BAN_BANTIME, FAIL2BAN_FINDTIME and FAIL2BAN_MAXRETRY could be set as well

6. Add your custom configuration script /configure.sh

   docker run -p 25:25 \
   	-e MAIL_DOMAIN=example.com -e MAIL_HOST=mail.example.com -e SMTP_USER=user:pwd \
   	-v /path/to/script:/configure.sh \
   	--name postfix -d danilsmirnov/postfix

   E.g., add an alias to forward mail to:

   postconf -e "virtual_alias_maps = hash:/etc/postfix/virtual"
   echo "mailbox@${MAIL_DOMAIN} [email protected]" > /etc/postfix/virtual
   postmap /etc/postfix/virtual

• Login credential should be set to ([email protected], password) in SMTP client
• You can assign the port of MTA on the host machine to one other than 25 (postfix how-to)
• Read the reference below to find out how to generate domain keys and add public key to the domain's DNS records

• Overview of changes and improvements
• Postfix SASL Howto
• How To Install and Configure DKIM with Postfix on Debian Wheezy

• catatnight