Andrew suggested giving them Google apps accounts. I
think it's a great
solution--it allows people to use gmail or pretty much any mail client
they want.
However, it forces them to use the google mail servers, which may be
less-than-desirable for some of us (me included) for various reasons.
Better to handle blacklisting, and let people use their own mail servers
or whatever else they'd like.
--
Mark Holmquist
Contractor, Wikimedia Foundation
mtraceur(a)member.fsf.org
http://marktraceur.info