tkbiff allows arbitrary commands to be executed upon mail reception. If you like programs such as xbiff and xbiff++ but wish they were more flexible, then you'll like tkbiff. Unlike other biffs, tkbiff is fully customizable. tkbiff also doesn't waste your valuable screen space with icons; instead, it shows you the mail itself. It supports UNIX, Mac, and Windows, IMAP, POP, and UNIX-style mail files, and SSL and APOP.
TkRat is a graphical Mail User Agent (MUA) which handles MIME. It is mainly written in C, but the user interface is done in Tcl/Tk. The program currently understands UNIX mailboxes, POP, IMAP, and mh folders. Messages are sent via SMTP or any user configured program (sendmail, for example). It has support for disconnected IMAP operations.