GNUnet

Fast, scalable, enterprise mail/groupware server providing ESMTP, IMAP, POP3, webmail, mailing list, basic web-based calendaring and scheduling services.

Message transfer agent (MTA) developed at the University of Cambridge.

General-purpose, lightweight, minimalist WebRTC Server.

Feature-rich email archiving solution.

Distributed, secure messenger with audio and video chat capabilities.

Fast, light and extensible chat client.