christopher.claus@tgcamberg1848.de writes:
I identified the restriction
smtpd_sender_restrictions = reject_unknown_reverse_client_hostname which blocks the connection from mailman in postfix. I have no idea why mailman sends its mails by IP instead by containernam/hostname as before. Probably this relates with IPv6 activation ....
I am sure that Mailman has nothing to do with this problem, and almost sure that Postfix is not responsible for it. This is not a murder case, where the reporter of the crime is the obvious suspect. It is an Internet failure case, where the #1 suspect is a constant: DNS.
What is very likely happening is that IPv6 is preferred over IPv4, and your DNS doesn't have a PTR record for the IPv6 address of the mailman container. That means the reverse lookup fails, and the message is blocked.
Steve