Perhaps the Listserv could detect a mail sent by a mailer and see if it
was a mail delivery error, then take the apropiate actions to delete the
user, inform the list owner, or report it to the listserv manager. We have
a local server here and this seems to work well. And why not to have listser
to define it's sender tags with:  Listserv 1.6 for list "LSTSRV-L",
or with the list owner address? After all the sender is the one who sends the
mail, or I'm wrong?. Well this is only a comment.
 
-Alejandro
 
Disclaimer: A bunch of characters to fill the bandwith.