Eric, you might wish to propose off list that you will treat as usenet
traffic any mail containing the string on a line by itself:
 
   this document is usenet traffic and should be processed as such
 
Then it is up to the usenet folks whether they want to insert that at
the gateway.  And up to them to propose what it should mean in various
contexts.   Not that you would necessarily implement that, but at least
the discussion about how you know something is Usenet traffic would be over.
 
Peace and best wishes.   Dave
 
Dave Gomberg, role model for those who don't ask much in their fantasy lives.
GOMBERG@UCSFVM  Internet node UCSFVM.UCSF.EDU             fax-> (415)731-7797