Pegasus Mail: Pegasus Mail is a free, standards-based electronic mail client suitable for use by single or multiple users on single computers or on local area networks. A proven product, it has served millions of users since it was released in 1990. It is extremely feature-rich and powerful, yet remains small and fast.
Mercury Mail Transport System: Mercury is a free, standards-based mail server solution, providing comprehensive, fast server support for all major Internet e-mail protocols. It is supplied in two versions, one hosted on Windows systems, the other running as a set of NLMs on Novell NetWare file servers.
Some examples of environments where Pegasus Mail and Mercury offer the right solution.
And why are these commercial-quality packages free? Click here for the history of Pegasus Mail.