smtp:
postfix
qmail
sendmail
exim
pop3/imap
wu-imap
dovercot
(and probably another few hundred, but thats all i know off the top of my head)
if your looking setting up an e-mail server i have a how-to guide post on my local lug website that you can follow and see if that helps, it's useing fedora core 3, dovercot and postfix
http://tlug.dnho.net/?q=node/174 - this one has virus scanning and is a little more on the advanced side
http://tlug.dnho.net/?q=node/159 - this one is pretty basic
there area bunch of other how-to guides there as well so just look for the right one for you
