Pegasus Mail  4.52
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.
Pegasus Mail can act as a complete internal mail system on its own without needing further servers or components: it can send and receive Internet mail on its own using standard protocols (SMTP, IMAP and POP3).