[Exim] mail "loop"

             We are running exim 4.05 on a Redhat 7.2 box. Our mail
servers work flawlessly except for one unsual problem: occasionally
(I'd say about once per every 4000 messages) the sending host does
not seem to realize that the email it has sent us was successfully
delivered. So it sends it again and again and again. The mail is
successfully delivered to the inbox of the user on our end - sometimes
hundreds of times. There is nothing queued up in exim however. It is the
sending host (eg hotmail or yahoo or whatever) that is re-queing the
message and delivering it over and over with each of their queue runs.
So I'm not sure how to troubleshoot this because I haven't got much to
look at. I am able to break the loop by refusing mail from the particular
sender via a filter. I then remove the user from the filter once the loop is
broken. So the sending host obviously gets the blocking message and
removes the message from their queue, but without that filter it does not
receive a notification of successful delivery. Any idea where I can look
to find out what is happening with one of these rogue messages? Thanks!