mailcow - An open source mailserver suite.
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.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.