Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
mailcow - An open source mailserver suite.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
Roundcube - Web-based IMAP email client