> Sorry - I am still confused.
>
> I understand that *bounces* and bounce errors can be from spam,
> spoofed-address posts, and the like.
>
> This was not that type of error - I get hundreds of those and ignore
> them. This was from the Daily Error Monitoring Report, which I thought was
> the method Listserv uses to verify *subscribed* addresses. Am I just
> totally misunderstanding that?  And I have never seen this before. I am
> concerned how this could even be happening for an internal, undocumented
> list.
>
> Sorry to be thick here...

I'll say again in more detail and I will use LIST2  as the name of your
list:

Spammer sends to LIST2  from a non-existent address. This also could be a
virus generated message, it really makes no difference for this situation.
LISTSERV sends that address a message that they are not allowed to post to
LIST2. This is because your list is set to something other than
Send=Public.

When LISTSERV sends a message which relates to a particular list it
generates a special Return-Path address. A Return-Path address is used by
all mailers to send bounces to. For your list this special address is:
owner-LIST2@your domain.com.  Since the Spam or virus message had a non-
existent From: address, the message bounced back to LISTSERV. When this
message arrived at LISTSERV addressed to  owner-LIST2@your domain.com,
LISTSERV says; Ah a bounce for the LIST2 list, I will place this on
tonights Daily Monitor report.

This is only one scenario on how a non-user gets on this report. LISTSERV
uses the owner-listname@  return-path for many various command responses.
All it takes is a non-existent address to send a post or a command to your
list or LISTSERV for this to happen.

The two most common causes are the one above and this one below.

An existing subscriber has their mail forwarded to another account. The
other account quits working or bounces list mail. That bounce reaches
listserv addressed From: the non-existing account and To: the special
return-path address. LISTSERV doesn't care if that address is subscribed
or not, it bounced and was related to your list so it goes on your report.

I hope this is a bit more clear.