A mailbomb (also written mail bomb or mail-bomb) is not a bomb made of metal. In computer talk it is a flood of email — hundreds or thousands of messages — sent to one address or one mail server on purpose, so that the inbox or the machine chokes and cannot work. It is a kind of attack. The word is mail plus bomb, using “bomb” the way computer people do when they mean “hit it with too much.” You may also see it as a verb: to mailbomb someone. Though actually, in older news, mail bomb with a space can mean a real explosive device sent through the post (a letter bomb). The sense here is the email flood, not that. The one-word spelling mailbomb is the computer one. People now often say email bomb for the same thing.