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.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Microsoft 365 - Boost your productivity with reliable access anywhere with services like email, calendar, file sharing, meetings, instant messaging, and Office Online
Roundcube - Web-based IMAP email client
YouTeam - YouTeam is a new, smarter way to outsource.
Axigen - Axigen is an integrated email, calendaring & collaboration platform, built on a Linux mail server technology.