>4. Analyze the *cause* of each of the "delivery-error" messages,
>   and either politely inform the administrators of the system
>   which generated the "delivery-error", and set the specific ID to 'NOMAIL'
>   status, or ruthlessly delete the offender's ID from your mailing-list.
 
Melvin offers great suggestions.  I'm starting to think differently
though about 4. Experience has shown that those users then send messages
back to the LIST instead of to the list owner asking why they aren't
receiving such mail.  Now on a large/private list, this is a waste of
resource/time.  If they were unsubed instead, they'd get a nastygram
when they tried to port.  Or on a an editted list (by (list)), it would
go to the editor who could deal with it much more effectively.
 
You list/users may be different.
 
/P