The  vast   majority  of   mailing  lists  have   a  return   address  of
[log in to unmask] This is  very easy to key on and  no ordinary private mail
would ever use this address. That's all there is to it. For a description
of why  setting 'Precedence: bulk' is  a bad idea, see  my recent posting
and/or the sendmail installation guide.
 
  Eric