If you want a Windows mail server the open source solution is http://www.hmailserver.com/ which requires setting up with a mysql database.
Mysql is also open source

i personally prefer using linux for a mail server simply because its more stable than windows, has lower resource demands and is less prone to virus infection... considering that many viruses are transmitted through email using an OS that is easy to infect is unwise imho

There are a very very wide range of mailservers for Linux which can suit any need

http://www.hypexr.org/linux_mail_server.php

I generally use ubuntu linux, clamav for virus scanning, spamassain for spam and fetchmail as the mail server.

Although Linux can be a pain to configure, once it is done it does not require constant attention unlike a windows box