The list you are talking about is probably a Mail-Via= Distribute
in the header.  Since you have a backbone listserv at your site,
it will not cause additional network overhead.  If you look at
where the file originates, it is probably [log in to unmask]  If you
check your Listserv console spool at the time of delivery, it
should have information indentifying that it was "distributing"
and also whom it distributed to.
 
/ahw