bounce message

bounce message n.  [UNIX] 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.