Perhaps bounce them to the sender with a notice to fix their mail
program...
> Even better: automatically add the
> sender address of such messages to the not allowed list?
Very very bad. You would accidentally blacklist even people who run a
single-user local gateway. (Everyone who is on a large number of
mailing lists knows that such gateways can be a very good idea.)
Not to mention organization or city local gateways.
olaf