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 Stretchhttps://thomas-leister.de/mailserver-debian-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 Some of the techniques may be a little outdated (Also it’s in German) but it still is functional.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.