bounce message

[UNIX] n. Notification message returned to sender by a site unable to relay email to the intended Internet address recipient or the next link in a bang path (see bounce, sense 1). Reasons might include a nonexistent or misspelled username 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.