hMailServer is a free Mail Server for windows. It uses a database backend (currently supports MySQL, MSSQL), supports POP3, IMAP, SMTP, and comes with WebMail support (SquirrelMail). This looks like a fantastic project - I've not had a chance to test this product yet, but I'm looking to change shortly.
We have no control over the content or continued existence of any external on-line resources linked here, and we therefore cannot guarantee that they will function as promised. The appearance of a site on this list does not imply any endorsement by us.