I fully agree with the interpretation of RFC822 given by Ken Shores:
Delivery error messages should indeed go to the Sender: field in the
header.  I realize this is deadly to the FRECP11 LISTSERV, however,
since there is nothing in the header which can reliably be interpreted
to say "This is an error message, not normal mail", so it is quite
difficult (if not impossible) for LISTSERV to recognize the error.
 
Richard