The Collaborative International Dictionary of English v.0.48:

letter bomb \let"ter bomb`\, letter-bomb \let"ter-bomb`\n.
   A bomb disguised as a letter and sent through the mail,
   usually rigged to explode and kill or harm the recipient when
   opened.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

letter-bomb \letter-bomb\ v. t.
   to send a letter-bomb to.
   [WordNet 1.5]

The Jargon File (version 4.4.7, 29 Dec 2003):

letterbomb


    1. n. A piece of email containing live data intended to do nefarious
    things to the recipient's machine or terminal. It used to be possible, for
    example, to send letterbombs that would lock up some specific kinds of
    terminals when they are viewed, so thoroughly that the user must cycle
    power (see cycle, sense 3) to unwedge them. Under Unix, a letterbomb can
    also try to get part of its contents interpreted as a shell command to the
    mailer. The results of this could range from silly to tragic; fortunately
    it has been some years since any of the standard Unix/Internet mail
    software was vulnerable to such an attack (though, as the Melissa virus
    attack demonstrated in early 1999, Microsoft systems can have serious
    problems). See also Trojan horse; compare nastygram.

    2. Loosely, a mailbomb.


The Free On-line Dictionary of Computing (30 December 2018):

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)