Make postfix accept domain internal mail traffic

[ralflang]

Allow deployment internal mail

Configure postfix for a mysql backend. use the same tables and credentials as dovecot uses. Share variables where possible.

Configure dovecot and postfix to handle a domain from .env as with the horde userbase as logins/mailboxes.

Postfix should read a mysql table for a list of virtualdomains

Setup accepting outgoing mail from the submission port.

Use database users for plaintext authentication. Do not setup TLS

Setup delivering accepted mail to dovecot via LMTP over TCP

Incoming mail for existing users should be accepted and forwarded to dovecot container's lmtp port.

Configure horde to use the postfix container as smtp server

• use http://$hordeurl/horde/test.php for interactive testing
• use $hordecontainer:/srv/wwwhorde/web/imp/config/backends.local.php to configure SMTP. read backends.php for instructions.

Webmail identity handling

• configure two webmail users manually to use the correct domain name in their email address.
• Test drive sending mail from one webmail user to the other.
  Check horde's config if you can set a default domain in case of missing domain. Use this variable to create a prefs.local.php file and auto-set the user's identity's email address to $login@$virtualdomain

[ikselven]

PR #2 which implements this is merged now. The only missing feature is automatically setting the email address of the default identity.