I would like to shutout a very comprehensible Guide on how to set up an Email Server System.

One I actually used for my System

Mailserver mit Dovecot, Postfix, MySQL und Rspamd unter Debian 9 Stretch

In my experience this is a working guide. Some of the techniques may be a little outdated (Also it’s in German) but it still is functional. Go check it out if you want to.