Notes/Domino is a bear to set up, but once it's done, it's great. It's much more than just email, though. Mercury is good, and of course, Groupwise is excellent - and yes, it does run on Windows as well as Netware. I know of several shops here who transferred their Groupwise on Netware to Groupwise on Windows and none of their users ever noticed. But again, Groupwise, like Notes, is much more than just email. There are lots of good open source email packages out there, but Sendmail is the one most often used. Unix (and Linux) do require a little learning, but it's worth it.
Good luck.
Iolair MacWalter
Director of IT