The Xbiff program provides graphical notification of new e-mail. It only
handles mail stored in a filesystem accessible file, not via IMAP, POP
or other remote access protocols.

It displays a little image of a mailbox in the X Window System. When
there is no mail, the flag on the mailbox is down. When mail arrives,
the flag goes up and the mailbox beeps. By default, pressing any mouse
button in the image forces xbiff to remember the current size of the
mail file as being the "empty" size and to lower the flag.