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.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Roundcube - Web-based IMAP email client
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.