Careful;
That is how this whole thing started, I could go into it, but here
is as good a write up as I have read:
http://www.jwz.org/doc/groupware.html
It's not an exchange/outlook killer, but it will hopefully be simple but useful web mail/calendar.
Thunderbird is also a pretty key client for us & the various other free software mail projects; I wish the Mozilla Messaging co. all the luck in the world.