Source : Free On-Line Dictionary of Computing

letterbomb
     
         1. An {e-mail} message containing {live data}
        intended to do nefarious things to the recipient's computer or
        terminal.  It is possible, for example, to send letterbombs
        that will lock up some specific kinds of terminals when they
        are viewed, so thoroughly that the user must turn the terminal
        off to unwedge it.  Under {Unix}, a letterbomb can also try to
        get part of its contents interpreted as a shell command.  The
        results of this could range from silly to tragic.
     
        See also {Trojan horse}; compare {nastygram}, {talk bomb}.
     
        2. Loosely, a {mailbomb}.
     
        [{Jargon File}]
     
        (1998-01-16)