Excerpted from the BLURB file...

TkRat is a graphical Mail User Agent (MUA) which handles MIME, POP3
and IMAP4.1.  It is mainly written in C, but the user interface is 
done in Tcl/Tk.  The following is a non-exhaustive list of the 
capabilities:

    * Multilingual interface (English, Swedish and Italian included)
    * MIME support: text/plain, image/gif and message/rfc822
      including multipart/mixed and multipart/alternate, 
      Quoted-printable and Base64 encoding.
    * Supports MIME in headers
    * Composing: (tk's text widget plus many extensions) or an
      external editor of your choice.
    * Message database
    * Virtual folders: mbox, mh, IMAP or POP
    * Message hold: suspend the composing, continued later
    * Watcher: When the program is iconified it checks the mailbox
    * Uses sendmail OR direct SMTP or other MA
    * Supports Delivery Status Notifications - DSN ESMTP sendmail-8.7
    * Supports PGP/MIME and "old-style PGP message receipt"

WWW: http://www.tkrat.org/