Added Zimbra Collaboration

To Email section

README.md

@@ -116,6 +116,7 @@ See also [Static Site Generators](#static-site-generators) and  [Content Managem
   * [Cypht](http://cypht.org/index.html) - A feed reader for your email accounts - `GPL` 
   * [mailcow](https://github.com/andryyy/mailcow) - Mail server suite based on Dovecot, Postfix and other open source software, that provides a modern Web UI for administration. `GPL`
   * [Roundcube](https://roundcube.net) - A web interface to act as an IMAP frontend ([Screenshots](https://roundcube.net/screens/)) ([Source code](https://github.com/roundcube/roundcubemail/)) `GPLv3` `PHP`
+  * [Zimbra Collaboration] (https://www.zimbra.com/downloads/) - Email, calendar, collaboration server with web interface and lots of integrations. Open source version of Exchange ([Source code] https://www.zimbra.com/open-source) `GPLv2/CPALv1` `Java`
 
 ### XMPP
   * [Converse.js](https://conversejs.org/) - A free and open-source XMPP chat client in your browser ([Source](https://github.com/jcbrand/converse.js)) `MPL` `Javascript`