Wiktionary
mail bomb
n. 1 An explosive device sent through the post; a letter bomb or parcel bomb 2 (context computing English) A large number of e-mails sent to one destination, or an e-mail with a very large attachment vb. To send a mail bomb (in either sense)
Usage examples of "mail bomb".
March parked and took his provisions and Buhler's mail bomb, or whatever it was, and walked carefully up a steep path into the forest.
The matter of the mail bomb that had destroyed half of another apartment the following year did cause me to begin wondering whether the statistical nature of reality might not be under a strain in my vicinity at that season.