Relay email (nullmailer)
Overview
Nullmailer is lightweight and simple MTA (mail transport agent) which allows to relay local mails to another server. This is useful if you are in example running private server at home and just want to relay notifications from cron etc. to your external mailbox via your ISPs mail server. Nullmailer allow also to rewrite envelope sender which is needed to pass spam checks many ISPs are nowadays doing.
Installation
Nullmailer can be found only from the testing branch. How to Enable the Testing Repository
# apk add nullmailer@testing
Configuration
Nullmailer is configured using individual files under /etc/nullmailer/, one file per setting.
When mail is sent to any local user i.e. "root", "logcheck", "me@localhost" etc. then mail can be sent to some external address or addresses (comma separated list) instead:
Contents of /etc/nullmailer/adminaddr
When nullmailer relays mail to remote server then envelope sender can be overridden as often mail gets rejected if envelope sender has non-existent domain like root@localhost.lan (note that envelope sender is not the same as From: header which nevertheless stays root@localhost.lan but that is not typically rejected):
Contents of /etc/nullmailer/allmailfrom
Whenever mail nullmailer deals with addresses without hostname or domain defaulthost and defaultdomain files can be used to set defaults, but me will do the same using one file. So mail From: me and To: me will become From: me@example.com and To: me@example.com. Note that this applies to headers which are kind of informational only shown on mail client (but may still lead mail to get rejected if invalid) and envelope sender and recipient must be set correctly in any case using previous configuration files.
Please do not set anything random here, ideally you would own a domain you could use so you are not misusing domain not owned by you even though technically it would work.
Contents of /etc/nullmailer/me
To configure relay server to which mails are sent to add one line per remote server. Typically this would be your ISPs mail server. Optionally supports different authentication schemes:
Contents of /etc/nullmailer/remotes