Jacob:

You did not indicate whether you scanned your subscription list for all
userids related to the reinjected email.  If you have and cannot find a
'candidate' then I suggest a  LISTSERV(R) Distribute MAIL job to all
subscribers.  This would very likely produce a similar problem, but this
time you might have more info available in the RFC headers.

Finally, you could "force" a PROBE.

/Pete