Kai:
I think this issue might be the same as the one that I'm encountering in my
post yesterday titled, "Postfix not using LMTP Transport in map".
Can you look at your logs and see if you're getting a similar message to
what I have below?
Host postfix/lmtp[22981]: C0EEEA8:
Greetings!
I have a Postfix + PostfixAdmin + Dovecot + Mailman3 install. My longer
logs and configuration files are pasted here: https://pastebin.com/U0Ciy2i9
Virtual aliases and domains are managed by PostfixAdmin, but stored and
confirmed by Dovecot. Postfix uses dovecot-lmtp to connect,
