Some things you may do that might help with diagnostics on future
instances of the problem:

  1) in your list header, include these keywords:

 Mail-Merge= Yes
 Mail-Via= Direct
 Auto-delete= Yes,Semi-Auto,Delay(4),Max(30),Probe(1)

in particular the Probe(1) will cause very frequent passive probing,
and the headers on a probe message are much more likely to reveal the
subscriber involved, if it comes back to you.  This passive probing is
non-invasive, the subscribers generally don't even know it is happening.
 You may choose other delay and max values based on your list activity
level.

The mail-merge is required, and mail-via is useful, in connection with
my next suggestion:

   2) include in the bottom_banner and bottom_banner_html templates a
string like this: your subscriber address: &*TO;
so that every message will have the subscriber address in the body
text, and if that comes back it will be quite clear who the item flowed
through. This may increase the workload on your list host server and
outgoing mail server, but unless you are paying for CPU time by the
second or bandwidth by the kilobyte, I believe this effect is of no
significance - and the troubleshooting advantages are considerable.

You might also resort to the mail server log file around the time of
the message coming in, to see if there is any correlation to something
going out - but if it is in the middle of a list message distribution,
perceiving this relationship is very very difficult.

One other tactic is to put all the subscriber addresses into the BCC:
field of a message directly from your personal email client and see what
kind of responses arrive; depending on the size of the list, this could
be a cumbersome task indeed, but is very likely to give you the identity
of the "reflecting" subscriber.

The problem might also mysteriously go away - it may be caused by a
vacation auto-reply somewhere that will be turned off when the
vacationer gets back to their office.



>>> [log in to unmask] 7/11/2007 12:50:59 AM >>>
Colleagues,

I have a pesky problem that maybe a set of fresher eyes can help me
with. When posting to one of our lists, the posting makes it to the list
just fine, but the following is sent back to the sender: