hej @stephan.wild
the main issue with Mailserver Logfiles is, that you will not have one Logfile per E-Mail. You get multiple Log Messages for one E-Mail. Each contain a different working-state.
Integrate such a GROK Pattern ( https://github.com/whyscream/postfix-grok-patterns ) into Graylog is easy, but then you would need to group the messages together that represent one Mail. That is not easy.
Currently I do not know any FOSS Log Management Solution that can work with vanilla/unmodified Mailserver Logfiles and give some meaningful feedback/analyse on them.
You can do this, but not without heavy customization and building of scripts.