Adora is intended to be a full-featured email client. This means it will send and recieve messages over the internet, store them as files on your computer and allow you to view them and organize them at your whim. While Adora is not yet a complete, stable software package, it has a lot of functionality already.
Exmh is an X user interface for MH mail. MH provides a set of UNIX commands that manage folders and mail messages.
Mama Mua is an IMAP4 message user agent written in Python. It uses Tkinter and Pmw for the user interface. The standard Python run-time libraries do the rest. It is designed to be a mail client for a local network mail server.
SMAN recieve mails from a POP3 server, sends mail by an SMTP server, MIME attachments, address book, TO, CC, BCC lists, draft messages, and more.
TkMail is an X windows interface to reading mail built using Tcl/Tk and Perl. Reading, sending, and managing mail messages can almost all be done using only the mouse (except for the body of the message, of course). TkMail supports the standard mailx format for folders.
TkRat a powerful, but yet easy to use, graphical Mail User Agent.