It is not that  simple. You don't want to reject  valid postings, and the
signature  files are  growing in  size. LISTSERV  does check  for popular
invalid  syntax  forms  (each  version   being  a  bit  better  than  the
preceding), but  I don't see  any way to  block messages saying  "I would
like to be added to your mailing list" and the like.
 
  Eric