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.
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).
Apache OpenOffice - Apache OpenOffice is an open source, office document productivity suite.
Roundcube - Web-based IMAP email client
SSuite Office - SSuite Office is called to be one of the most accomplished software developers for both beginners and professionals alike.