SOGo is fully supported and trusted groupware server with a focus
on scalability and open standards. SOGo is released under the GNU
GPL/LGPL v2 and above.

SOGo provides a rich AJAX-based Web interface and supports multiple
native clients through the use of standard protocols such as CalDAV,
CardDAV and GroupDAV.

SOGo is the missing component of your infrastructure; it sits in
the middle of your servers to offer your users an uniform and
complete interface to access their information. It has been deployed
in production environments where thousands of users are involved.

This is SOGo 2.  (What are the differences between version 2 and 5?
Version 5 has a modern, fully responsive Web frontend. Version 2 is
not responsive and has an aging design. Both versions 2 and 5 share a
common implementation of the communication protocols supported in SOGo
and SOPE: LDAP, IMAP, SQL, CardDAV, CalDAV, and Microsoft Enterprise
ActiveSync.)

Homepage:
https://www.sogo.nu/
