The real  problem is  that 'vacation'  was written in  the early  80s and
never updated since then. All it needs to do is check the return path for
owner-*@* and  not reply  to such messages.  "Precedence: bulk"  does not
just mean "If you're a vacation program, do not reply to this". It mainly
tells unix  (sendmail) systems not to  deliver the mail unless  they have
nothing else to  do. This is the lowest precedence  level besides "junk".
In practice it  can mean anything from no difference  at all to overnight
delivery  or no  delivery at  all (timeout  before the  message gets  the
opportunity  to  be delivered).  All  in  all  this translates  to  lower
service,  not   because  of   a  conscious   decision  by   the  sendmail
administrator but because 90% of people just install the config file that
comes with the code and don't try  to understand the details, seeing as a
sendmail config  file looks like  random character combinations.  So they
get the default behaviour, which severely segretates "bulk" traffic.
 
And that's not all. "Precedence: bulk" tells sendmail never to return the
mail body when it can't  deliver the message (which makes troubleshooting
more difficult),  and on  older versions  of sendmail,  it means  that if
there is an error, no delivery error should be returned at all. The party
line from the current sendmail manual is that you should use "Precedence:
list" to avoid  this misfeature. The old sendmails did  not recognize the
"list" level  and thus  the breakage  will be  bypassed. But,  of course,
'vacation' doesn't know about "list" since  it was introduced in the 90s,
so this would not solve the problem.
 
  Eric