hMailServer is a free e-mail server for Microsoft Windows. It's used by Internet service providers, companies, governments, schools and enthusiasts in all parts of the world.
It supports the common e-mail protocols (IMAP, SMTP and POP3) and can easily be integrated with many existing web mail systems. It has flexible score-based spam protection and can attach to your virus scanner to scan all incoming and outgoing email.
hMailServer comes with an easy-to-use and broad API which can be used for integration with other software in your environment. As an example, many Internet service providers connect hMailServer to their existing administration system to reduce manual work involved with managing user accounts. hMailServer also has a built-in Active Directory integration.