Jens Günther writes:
2023-08-26T08:21:59.522238+02:00 postfix/smtpd[2843414]: NOQUEUE: reject: RCPT from unknown[192.168.176.6]: 450 4.7.25 Client host rejected: cannot find your hostname, [192.168.176.6]; from=<$admin@mydomain.de> to=<$recipient> proto=ESMTP helo=<mailman-web>
There are a bunch of $myxxxx variables in Postfix, I think $myhosts might be the right one. If you add the IP 192.168.176.6 to that variable, it will be whitelisted and this problem should go away. There may be better ways to handle this, but don't mess with hostnames. Find the "right" way to permit that IP should do the job.
I mean the hostname for the core-container must be there, since mail is accepted :-D
That's not the way this works. Postfix accepts the mail because it's arrives at the physical host, then looks up routing for the mailbox and discovers it should be delivered via LMTP to the core container's IP. No hostname needed.
Steve