Hello again,
thanks for your help so far with the strange unable-to-determine-caller's-group-id-thing. In fact I found out that the problem was that the either qmail or mailman expected the GID of user "popuser" to be 110. (it was 30). So I changed that by changing the GID in /etc/group and /etc/passwd to 110 and by performing this
find / -gid 30 -print -exec chgrp 110 {} \; >gid30to110.log
so far so good. NOW I get at least no "deferral" but "success" delivery message in my qmail log but with this error behind it:
delivery 890: success: Failure_to_exec_script._WANTED_gid_12,_GOT_gid_110./Failed_to_start_/usr /lib/mailman/mail/mailman./did_0+0+1/
:'-( (gid 12 == mail)
Since using a combination of Suse9.3, PLESK and a (obviously?) special distribution of mailmnan that comes with this combination I'm not able to compile this with those funky parameters like -with-mail-gid=12 or something.
What can I do? Thanks in advance - Regards Maxi