Pete's info is excellent, but in case it makes things more clear, I'll try
to say the same thing in different words :-)

While there are perverse cases, most delivery problems can be solved.  In
this case, no one is complaining about missing list messages, but we apparently
have one of our distributions bouncing back to Listserv.

Oddly, Listserv thinks the bounce came from someone not subscribed!

There can be several reasons for this, including

   - Listserv or a list owner has recently removed the subscription (of the
   address Listserv reports in the Daily Summary).  I have "Changelog=" set on
   for all my lists so it is a simple matter of looking at the "Changelog"
   audit trail to see if the mail address had recent activity such as
unsubscribe.
   For this case, the subscription is now gone ... we're not sending new posts
   to the address, but we may still be getting bounces from older post
   distributions for several days and can simply ignore them.
   - One of your subscribers forwards their mail to a second e-mail
   address.  Then, if all is working, the distribution to the subscribed
   address goes to the mail system of the subscriber and is then automatically
   forwarded or redirected to the "second e-mail address".  As list owner, you
   have no idea this is happening, as it happens to their mail after Listserv
   distributes your list post.

So, continuing with the 2nd scenario above (the subscriber forwards their
e-mail elsewhere) ...

If problems happen at the subscriber-address mail system, Listserv
generally handles it perfectly and you see the subscriber in the Daily
Summary.  If problems occur at the "second e-mail address" site, we have
the problem you have just seen ... the bounce comes from a
non-subscriber.   How do we find the subscriber?

Assuming it is important enough to spend your time on this puzzle, there
are ways to solve it.  Pete described one:  instead of instructing Listserv
to handle the bounces, instruct Listserv (with the "Auto-Delete=" list
configuration statement) to forward future bounces to the "Errors-To
="people (your
e-mail address, for instance).  Now, on the next bounce message, instead of
getting a summary from Listserv, you have the exact and full bounce
messageto study.

Look carefully at the bounce message.  There may be clues in it as to which
list distribution message it is.  If it's an old message, just ignore it.  If
you are lucky, the bounce message contains the distributed message and your
list has placed the subscribers name/address in the message and your puzzle
is solved. (Note: this requires "Mail-Merge= Y" and something like a "bottom
banner" that lists the subscriber ... common in commercial e-mail lists,
but not so common otherwise because such a setup is not available for your
list or Listserv site (it is less efficient than Listserv distributing exactly
the same message to all subscribers)).

If the visible text of the bounce message doesn't reveal the subscriber, your
next action is to look at the bounce message "mail header lines".  Your
e-mail reading facility will have a way to display these mail header lines,
but each mail reading facility seems to have it's own method.  For example,
Gmail has an entry in the pull down menu that's part of the "Reply"
button.  Pull down the menu and select "Show original".  Another example, the
Thunderbird e-mail program has an "other actions" pull down menu;
select the"view source" entry to see
these mail header lines.

In any case, once you are viewing the mail header lines, you will see much
more information about the e-mail, including where it has been.  To see the
"where it has been" information, look at the "Received:" lines.  The ones
near the top are the most recent, that is, your mail system, followed by
the mail system that produced the bounce, etc.

If you are lucky, the bounce message will include "Received:" lines from
not only the bounce message, but the Listserv distribution and even
original poster (Alas, some bounce messages give only vague reference to
the mail it says caused the bounce).   So look at each "Received:" line.
Scan down them until you see the lines that refer to your Listserv site or
the original poster.    Study it/them and move up, looking carefully at
each "Received:" line.  If you are lucky, you may see your subscriber's
address.  Case closed.   If not, look up through the "Received:" lines to
see where the mail first went after Listserv.  This new site may be where
all your list distributions go before going off site, or may be the
subscribers' site.  If the site is the one that handles all of your list
mail, skip it and go up to the next "Received:" line.  Repeat until you get
out of your mail server addresses.  You now have the mail domain address of
your subscriber ... not necessarily their e-mail address, but the part that
is to the right of the "@" in their address.  If you are lucky, you have
one or a few subscribers at that mail domain and can focus your attention
on that/those subscribers.

Also, if you are lucky, the subscriber site may put your subscriber e-mail
address in one of its lines, such as the one that shows the forward to the
next site in the path the e-mail took.   If not, your luck is running low
and another method must be used.  :-(

If this reading of "Received:" lines doesn't produce an answer, you still
have options.  If you are unsure about the "Received:" lines, have someone
experienced look at your bounce message, such as your Listserv site maintainer
or your postmaster.

An alternative to looking at the bounce messages if to use "address probing",
which may be available at your Listserv site.  See the LSoft doc or your
Listserv site maintainer for help in setting up your list for address
probing.  Normally only a small portion of your subscribers are probed for
each list distribution, but this can be cranked up for a case such as we're
discussing.  The advantage of this method is that it usually works without
dissecting a bounce message and sometimes works even when the bounce
message dissection fails us.  A Listserv address probe constructs the
distributed message in such a way that the subscriber address is part of
the e-mail address used to return the bounce to Listserv.  The disadvantage
of an address probe on every subscriber is that it is much less efficient
for Listserv and the outgoing mail system than normal operations. So the
address probe information is a bit like the "Mail-Merge=" method, but works
even when the bounce message fails to contain the original distribution
content.  With a bounce from a piece of mail using the address probe,
Listserv will report the subscriber address in the Daily Summary.

Finally, I should mention a case where your subscription address is only
slightly different from the bounce address, but the mail wasn't
forwardedby your subscriber.  In such a case, a forwarding or address
renaming/remapping was done internally at the subscribers mail site.  Maybe
the domain x.edu was remapped to x.schools.com or northcampus.x.edu .  In
such cases, a search of all your subscribers looking for a similar part to
the left of the "@" will lead you to your subscriber (or a good bet ...
sending a message to your suspected problem subscriber might bounce,
verifying your suspicion).  Likewise, a search using just a portion of the
bounce address domain might lead you to the subscriber or a small set of
possibilities.

Sorry for the length of this post.  Parts have been cut and pasted from a
discussion found useful by some of my list owners that like to solve such
problems themselves.


Cheers, Wayne

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