I don't know how often you encounter this type of problem, but I receive
only a handful of questions each month from list owners who are unable to
match delivery errors with subscriber addresses. Consequently, I find it
easier to deal with these as exceptions rather than attempt to modify
Standard Operating Procedures for each of over 1700 lists on our server.
When a question of this nature arises, I advise the list owner to choose
the subscriber address which seems most likely to be the source of the
error and send a test message to that address. If the owner has chosen
correctly, the test message should bounce in the same manner that the list
messages are bouncing. If, after this test, the list owner is still unable
to identify the subscriber whose address is causing the problem, I use the
following job template to send a test message to every address on the list.
This job will send a seperate message to each subscriber address on the
list, regardless of option settings, with the subscriber's address on the
"To:" line. Delivery error messages should be delivered directly to the
list owner.

----- begin job template -----
DISTRIBUTE MAIL [log in to unmask] PW=zzzzzzzz
//To DD *
(replace this line with subscriber addresses, one per line)
/*
//Data DD *,EOF
Date:
From:    [log in to unmask]
Subject: xxxxxxxx address probe
To:

This message is an address probe sent to subscribers on the xxxxxxxx
list in order to identify the subscriber whose address is generating
delivery error messages. Please do not reply to this message.
----- end job template -----

--
Paul Russell
Senior Systems Administrator
University of Notre Dame