Do you get the same message, for the same email address, repeated over time, or is this particular one just the latest example of the problem?  You may need to ask your LISTSERV site administrator to scan through the log files, and perhaps the mailer logs the list server uses, for some clues.

>>> [log in to unmask] 04/20/01 10:20AM >>>
Greetings. I'm a new list member, and have been lurking for a bit. I
own about 16 lists, ranging from one of about 250 members, to several
quite smaller. I am the Computer Coordinator for the Department of
Philosophy of York University, so I run the dept lists. Most of the
department lists are private and composed of different groups, e.g.,
the graduate faculty, or undergraduate instructors, etc.

My question is this. I periodically receive a notice that there is an
error on an account that is not and, to the best of my knowledge, has
never been on the list. There are two concerns: security of the list,
and the annoyance of the error messages.

Can anyone help?  The message is below. It is for [log in to unmask],
and came as: GRADPHIL: Daily error monitoring report

Thanks,
Michael

The following 1 subscriber is currently being monitored:

Err First Last  Address
--- ----- ----- -------
  3 04/17 04/18 [log in to unmask] 
                Last error: User unknown

Err=   Number of delivery errors received thus far
First= Date first delivery error was received (mm/dd)
Last=  Date of most current delivery error (mm/dd)