I was in the newsgroups, a question came up about POP3 providers for Windows. Here are some links I posted.
http://mailenable.com
http://smartertools.com
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.
http://www.windowsnetworking.com/articles_tutorials/Windows_POP3_Service.html
Configuring and Comparing the POP3 Service in Microsoft Exchange Server 2003 and Microsoft Windows Server 2003
http://www.microsoft.com/downloads/details.aspx?familyid=46E9CDD0-95F0-4DB6-A4D2-874F4ABB09E5&displaylang=en
and of course
Exchange 2003 / 2007 has all the standard mail protocols covered. 🙂
Pondering Thought 