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.