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.
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.
Microsoft 365 - Boost your productivity with reliable access anywhere with services like email, calendar, file sharing, meetings, instant messaging, and Office Online
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
YouTeam - YouTeam is a new, smarter way to outsource.
mailcow - An open source mailserver suite.