I'm not sure why you'd be getting this.  Apparently someone
(presumably someone malicious) has signed up
[log in to unmask] to your mailing list.  Since there's no
subscription for that address, it's probably signed up with a
forwarding address.

The only method I can think of that would be guaranteed to get rid of
the bad address would be by having all addresses confirm their
subscriptions via an appropriate Renewal= setting.

Here's another thing that could be tried:

1) Issue a "HOLD listname" command.  This will keep messages from
being distributed to the mailing list.

2) Send a message from an email address.  This should not be your
listowner address; perhaps a hotmail account would be the best way to
go.

3) SERVE OFF the address you just sent the message from.  A LISTSERV
maintainer might have to do this.

3) Issue a "FREE listname" command.  The message you sent in step 2
should now be distributed.  When it arrives at LISTSERV LISTSERV will
recognize it as originating from a SERVed OFF address and <should>
bounce it to the LISTSERV site maintainer(s) with the full headers
intact.  The headers will hopefully give enough information that you
will be able to figure out what the subscribed address is that is
causing the problem, or at least enough info to narrow it down.

It's nasty either way, but those are the only ways I can think of to
deal with the problem.

Good luck,
--
Jacob Haller, Technical Support
L-Soft international, Inc
http://www.lsoft.com/