I was in the newsgroups, a question came up about POP3 providers for Windows. Here are some links I posted.
Windows 2003 has a built-in POP3 component. I completely forgot about this one. Personally, I've never set it up, so I can't give an opinion either way.
Configuring and Comparing the POP3 Service in Microsoft Exchange Server 2003 and Microsoft Windows Server 2003
and of course
Exchange 2003 / 2007 has all the standard mail protocols covered. 🙂