Zimbra alternatives and similar tools

Based on the "Collaborative Software" category.

SOGo

5.6 9.6 L1 Zimbra VS SOGo

SOGo is a very fast and scalable modern collaboration suite (groupware). It offers calendaring, address book management, and a full-featured Webmail client along with resource sharing and permission handling. It also makes use of documented standards (IMAP, CalDAV, CardDAV, etc.) and thereby provides native connectivity (without plugins) to many clients such as Microsoft Outlook, Apple iCal, the iPhone, Mozilla Lightning, and a plethora of mobile devices.
Horde Groupware

3.8 0.0 L3 Zimbra VS Horde Groupware

This is the old, deprecated, monolith Horde repository, archived here for historical reasons.

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EGroupware

3.5 9.9 L1 Zimbra VS EGroupware

Web based groupware server written in PHP, forum at https://help.egroupware.org/
Citadel/UX

- Zimbra VS Citadel/UX

Collaboration suite (messaging and groupware) that is descended from the Citadel family of programs.
Kolab

- Zimbra VS Kolab

Another groupware suite.