Spruce is an X11 mail client written with the Gtk+ widget set. It is geared towards being extremely small and fast, supporting all the common features that users want or need without sacrificing usability or functionality
Microsoft Exchange compatible server and client libraries
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. OpenChange client-side library is used in existing messaging clients and is the solution in new projects to communicate natively with Microsoft Exchange and Exchange-compatible servers. OpenChange server is a transparent Microsoft Exchange server replacement using native Exchange protocols and does not require any plugin installation in Outlook.
elmutt - mutt with elm features
Multimedia Rich Unified Communication based on Asterisk, Sendmail, MySQL, PHP, Flash, AIR and Flex
Hey Everyone! Nirvana has moved to http://sourceforge.net/projects/nirvana It isn't here anymore.
XCmail is a MIME and multi POP3 server capable mailtool for X11 using the Xclasses layout library. XCmail was designed completely object orientated and by this may be improved easily. <p> XCmail has its own server in the internet. Take a look on it.