"Precedence:" is a sendmail extension that as far as I know was only ever
implemented  in sendmail  and that  isn't going  to be  standardized. Its
function is to define  the PRIORITY of a message in  terms of how quickly
it  should be  delivered. Kludges  were then  added to  suppress delivery
errors completely or partially for  various precedence levels. The author
of sendmail recommends against using the precedence levels you mentioned,
see the sendmail online manuals.
 
By  the time  AOL et  al implement  this, they  will have  switched to  a
version of  sendmail that generates  bounces LISTSERV can parse,  and you
won't see them at all.
 
  Eric