>  - PMDF on JHUVMS realizes that it cannot deliver mail to user Y and
>    attempts to return the mail.  It picks up the address in the "Sender:"
>    field and voila, Listserv@JHUVM ends up with the mail.
 
What "Sender" field?  When I send mail using LISTSERV's Distribute
function, it doesn't seem to insert one.  Indeed, it wouldn't need to do
so to deliver the mail, since LISTSERV is trusted by its local Mailer.
Is this a change from 1.6d to 1.6e?
                                      John