This has a flaw for some types of spammer which are aware of mailing
lists and subscribe to the list, then make a post so this kind of
filter doesn't work so well on list environments like yahoo groups,
but
it would cut down on the amount of spam for certain on Debian lists.
We could moderate subscription requests (it's a feature of most
mailing list
software, including the software that is used for Debian lists). Also
we
could moderate the first few postings from a new subscriber.
But of course a spammer could use someone else's email address. The
solution
to this is to moderate every message that has an unknown combination
of From:
field and IP address. There is no software support for this yet.