Sendmail

Easy to use but advanced IP PBX system, VoIP gateway and conference server.

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

IMAP and POP3 server written primarily with security in mind.

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

Modular SIP server (registrar/proxy/router/etc).

Portable and lightweight Internet Relay Chat server for small or private networks.