MailEnable is a high perfomance POP3 and SMTP free mail server for the Windows platform. It is free and contains some nice features like spam filtering, SMTP authentication, highly customized relaying options, and a built in list server.
I can’t say I’ve done an exhaustive search for the very best free Windows mail server available on the market but I looked through the list of features contained in MailEnable and it seems to have just about everything you would want in 2005. There is an online demo and plenty of video tutorials available on their site to help you get yourself up to speed and make it useful for you.
[link via Matt Woodward]