Table of Contents
Mail Servers
Free
Cyrus IMAP (License: BSD style)
Dovecot IMAP en POP3. (License: LGPL 2.1 and others)
DBMail IMAP and POP3. git repository. (License: GNU GPL v2)
Courier Mail Server (License: GNU GPL)
OpenChange is a portable Open Source implementation of Microsoft Exchange server and Exchange protocols. It provides a complete solution to interoperate with Microsoft Outlook clients or Microsoft Exchange servers. (License: GNU GPL v3)
hMailServer is a free e-mail server for Microsoft Windows.
Perdition is a fully featured POP3 and IMAP4 proxy server. It is able to handle both SSL and non-SSL connections and redirect users to a real-server based on a database lookup. (License: GNU GPL)
Obtuse smtpd/smtpfwdd (development stopped in 1997)
osmtpd based on Obtuse smtpd 2.0
Emm Mail Manager This is emm/sm, an electronic mail manager for Unix. Emm is very similar (by design) to the version of MM from the DEC-20, but fits smoothly into the Unix environment.
Commercial
Axigen
MDaemon Email Server software for Windows
MailEnable For Microsoft Windows. Free Standard edition available.
Scalix Linux Email, Calendaring and Messaging Platform. Free Community Edition available.
Open-Xchange There is a free open-source edition, but it isn't easy to find it on their website, so I'm listing it under Commercial.
Zimbra, also provides an Open Source Edition