What is the X in X-mas?
A notification message returned to the sender by a site unable to relay e-mail to the intended recipient or the next link in a bang path. Reasons might include a nonexistent or misspelled user name or a down relay site. Bounce messages can themselves fail, with occasionally ugly results; see sorcerer's apprentice mode and software laser. The terms "bounce mail" and "barfmail" are also common.