Free alternative to Microsoft Exchange

"hMailServer is a free e-mail server for Microsoft Windows. It supports all the common e-mail protocols (IMAP, SMTP and POP3) and comes with an easy-to-use COM library that can be used for integration with other software. It also has support for virtual domains, distribution lists, antivirus, antispam, aliases, distributed domains and much more. E-mail data is stored in a database server, MySQL or MS SQL, depending on your choice."

Now, I understand most people these days would more likely want to just use a free e-mail service (Google Apps ) than to host their own mail server.  However, in case you do, be sure to check this one out.  It's nothing fancy, but it gets the job done...for free. 

http://www.hmailserver.com/