hMailServer is a free, open source, 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.
Licensed under AGPLv3, hMailServer can be used for free in most commercial scenarios. It's source code is available for anyone to read or extend at GitHub.
hMailServer comes with a set of built-in anti spam features such as SPF, SURBL. It can also be integrated with 3rd party anti spam systems such as SpamAssassin and ASSP.
Any webmail system which supports IMAP and SMTP can be used with hMailServer. RoundCube and SquirrelMail are popular webmail systems which are often used with hMailServer.