Sounds like an (auto) forward problem.  If you can't figure out
who the subscriber is, then I suggest turning off monitoring and
(then) receive the original "bounce" directly to the * ERRORS-TO=
address (at least until the next bounce).
 
This could reveal the RECEIVED line headers which should indicate
the point of forwarding to the misdirected (or obsolete) userid.
 
/Pete Weiss in Summer-like downtown University Park, PA USA