I have similar situation.
One list is sending out the error notice of
* You  are not  authorized to  send mail  to the  FRAN-L list  from
your
* [log in to unmask] account. You might be ..."

But, then sends out this error noitce to all subscribers.
Here is log of posting,
  "Sent information mail to [log in to unmask]
6 Jun 2006 08:34:09 Processing file 3769074 from [log in to unmask]
6 Jun 2006 08:34:09 Processing mail from [log in to unmask] for
FRAN-L
6 Jun 2006 08:34:09 Distributing mail ("FRAN-L") from owner-fran-l@LIST.UC.EDU..."


Any suggestions to stop owner-fran-l from sending bogus email to every
subscriber?
Thanks.

On Wed, 17 May 2006, Francoise Becker wrote:

> On 17 May 2006 at 9:04, David Phillips wrote:
>
> > We've been getting a bunch of messages from listserv with the subject
> > "Problem processing mail from from bogusaddress"
> >
> > The body reads "An error occurred while processing file 171804 from
> > [log in to unmask]: "Mail has been  received in the  LISTSERV mailbox
> > from a  user that had  been served out".
> >
> > That's followed by a forwarded message.  What is very puzzling to me
> > is that the to field in the forwarded message is
> > [log in to unmask]
>
> What you see on the To: field is not necessarily the only recipient
> of the message. It's sort of like sticking a letter addressed to
> George Bush into an envelope addressed to Bill Clinton. Where the
> message gets delivered is the address on the envelope, not the
> address on the letter inside the envelope.
>
> It's why you are able to receive email messages where the To: is
> [log in to unmask] rather than your own email address.
> Your email address is on the envelope, but not in the enclosure.
>
> >
> > Two questions come up.
> > 1.  Why is OUR listserv getting this email and sending it onto our list owners?
>
> That address is probably being used to spam a lot of addresses,
> including your listserv@domain address, hence it got served off due
> to too many invalid commands. The copy of the email that was
> addressed (on the envelope) to your list was intercepted because it
> was served off and LISTSERV wants the list owner to know about it in
> case there is something to be done about it.
>
> Normal subscribers can get served off if they send too many bad
> commands to the LISTSERV command address, for example by sending a
> message to listserv@domain instead of [log in to unmask] In that case,
> this message to the list owner is helpful because then the list owner
> can serve the subscriber back on and let them know what happened.
>
> Most of the served off notices I get these days are from spammers who
> try to send to one of my lists after they've been served off.
>
> > 2.  Can we stop it?
>
> I recommend you serve off the address even more and send the command:
>
> SERVE [log in to unmask] OFF DROP
>
> That tells LISTSERV: don't process any mail from this address and
> don't even tell me about it.
>
> --
> Francoise Becker <[log in to unmask]>
>
> Knowledge is just a click away: http://www.lsoft.com/optin.html
>