I had the same problem with anon.penet.fi. Their postmaster
tells me that posting or cc-ing to a list with an anon
subscriber can cause this, and that a work-around is to change
the subscriber's address from anxxxx to naxxxx (which prevents
the replies from being made anonymous). I forgot the address
of firewalls' listmaster, but maybe he could do that, or get
in touch with the subscriber (BTW, what's the local policy on
anon subscribers ?).
The problem is that firewalls uses majordomo for list administration,
and I don't know that it's smart enough.