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.
2. Loosely, a mailbomb.