> Could someone explain how the TAG information is placed on
> mail from listserv lists?
 
Yes, it is set by RSCS when a file is received, and, at that time, the
fourth and fifth fields indicate the creator of the spool file that was
sent.  If the receiving host runs VM Mailer (and has it registered in
XMAILER NAMES), the spool file is handed by RSCS to MAILER, which makes
a whole new spool file for the recipient.  MAILER assigns a TAG to
indicate the sender of the mail specified in the mail header after
having checked that the *actual* sender was either a trusted mailer or
the specified originator.  MAILER R1 didn't do this, and I don't
remember exactly when this behavior was introduced, but I think it was
very early in R2.
                                     John