GNU General Public License v3.0

roundcubemail

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.

GNU General Public License v2.0

sogo

Cypht: Lightweight Open Source webmail aggregator [PHP, JS].  Supports IMAP/SMTP, JMAP and soon EWS

roundcubemail

sogo

cypht

dermail-webmail