Thanks for all your assistance. I got the behaviour confirmed from 
different sources confirmed as a listserv problem.  We are currently
evaluating possibilies to upgrade. Meanwhile we switched the archive
off and replaced it by a simple archive mailbox, that is subscribed
to the the list. Anyway we need the archive only for documentary
puposes.

And, to pick up that thread of the discussion also, we will keep
the list Send=public. Spam and viruses are an annoyance, but only 
that way we are reachable for everybody, what is an important 
objective.

Oliver