Dear Befuddled:
 
A list subscriber is either automatically forwarding or RESENDing
list messages to another userid which in turn is bouncing.
Because of the headers generated and the mail system bouncing it,
it is being returned to the original sender (you).
 
The list-owner (or you) should contact the list subscriber and/or
the postmaster of the subscriber (if it is an autoforward) to
stop doing this until the target user's e-mail system is fixed.
 
OR, turn off the distribution of mail to the subscriber.
 
/Pete Weiss at Penn State