[Exim] how can i whack an "envelope from" header?

folks -

i'm trying to write an exim 3.x filter that will run as a director and,
under certain conditions, pipe a message to my cyrus 1.x deliver
program. unfortunately, "deliver" chokes if the message it gets on stdin
contains an envelope from, as in:

From dunham@??? Wed Jul 03 16:56:24 2002
Return-path: <dunham@???>
Envelope-to: dunham@???
...

if i zap that first line before passing it to "deliver" everything works
a-ok. is there any way i can prevent exim from adding this line?

in case it's something dumb i'm doing in my director or filter file,
i've included these below.

thanks in advance....

=====

director:

   spam_filter_director:
     driver = forwardfile
     file = /usr/local/services/exim/conf/spam_filter
     filter
     no_verify
     check_local_user
     owners = mail
     user = mail
     group = mail
     allow_system_actions
     pipe_transport = address_pipe_transport

filter file:

# Exim filter

   if
     $h_X-Spam-Status: contains "Yes"
     or
     "${if def:h_X-Spam-Flag {def}{undef}}" is "def"
   then
     pipe "/usr/local/services/cyrus_imapd/sbin/deliver
           -m junk-mail ${local_part}"
     seen finish
   endif

=====

--
||||||||||| Matthew E. Dunham
||||| Instructional Computing
|||||||||||| UC Santa Barbara
|||||||||||||||| 805 893 7687