Stefan Bauer writes:
thank you. Funny. All problematic mails are in the shunt-queue from one specific user.
That's good news (given that things aren't working), something in there will show us why.
the mailman-logs did not log anything helpful.
I thought they should ... not your problem, I'll check and let you know if there's someplace else you should look.
What do you recommend to find the needle in the haystack? I mean, what can cause mailman to fail processing?
Most frequently the issue is a non-ASCII character in the header. A frequent cause is spam filters and virus checkers quoting non-ASCII text in their reports in the header. Nonconforming dates can cause problems for some applications (specifically those that work with list archives), but I don't think they should cause non-delivery, and the date in the sample message is conformant.
You could try running "mailman unshunt", which may produce a helpful error message if for some reason it can't do so. Also check for logs that were updated at that time, which might identify a log with some useful information.
Steve