JXchange is an mail server multiplataform, written in Java language, like MS-Exchange. This project includes SMTP server, POP3 server, IMAP server, web management interface and webmail designed to run in almost all OS.
PyWebMail is a Python library and POP3 gateway that enable working with WebMail servers like with POP3 or IMAP mail server. Currently Mail.Com, Yahoo! Mail, Gmail, Netscape.Net, HotMail and some minor servers support is finished.
SIMON is a very simple MTA and POP3 implementation. It is completely designed from the ground up and is useful for departmental or small to medium sized companies.
MSWeb is a client for Microsoft's Outlook Web Access (OWA). It is intended to deliver mail from a MS exchange server to a UNIX computer. For those who are opposed to 50 MB mailreaders.