[Exim] from rewrite rule not working when using forwardfile

I'm using Exim version 3.34 #3 built 04-Jan-2002 11:25:07

I have a set of addresses in /etc/exim/mailinglists/us, and the
following in my directors configuration:

lists:
driver = forwardfile
file_transport = address_file
pipe_transport = address_pipe
reply_transport = address_reply
domains = haus
file = /etc/mailinglists/${local_part}
no_check_local_user
forbid_pipe
forbid_file
errors_to = nick-errors@???

I also have a bunch of rewrite rules such as

   *@*.haus  $1@haus
   nicholas@haus           nick@???      Ffrs

so that any machine inside my LAN can easily send mail, with a from
address such as user@???, which is then rewritten to their
correct address. I don't want to rewrite To: addresses, because
user@haus is deliverable, and I'd rather mail not try leave the LAN
only to come back in.

This appears to work just fine. If I send myself a mail via a simple
"echo test | mail nicholas" the mail starts with:

>From nick@??? Sat Apr 27 16:52:26 2002
Return-path: <nick@???>

*However*, if I do "echo test | mail us" in order to send mail to all
the people listed in mailinglists/us, those two addresses stay as
nicholas@haus in the mails people receive:

>From nicholas@haus Sat Apr 27 16:33:23 2002
Return-path: <nicholas@haus>

(The rest get rewritten ok, for example
From: Nicholas Piper <nick@???>
To: us@haus )

Have I forgotten to set something that will cause the return-path
address to be written when using a forwardfile director ? I need it
rewritten, else I get bounces such as :

  xxxxxx@???
    (ultimately generated from us@haus)
    SMTP error from remote mailer after MAIL FROM:<nicholas@haus> SIZE=1697:
    host vimg1.reading.ac.uk [134.225.112.81]: 550 rejected:
    cannot route to sender <nicholas@haus>

Where xxxxxx@??? is either listed in
/etc/exim/mailinglists/us OR is specified in a users .forward file.

Thanks for any help :-)

Nick

--
Part 3 MEng Cybernetics; Reading, UK       http://www.nickpiper.co.uk/
GPG Encrypted mail welcome!                             1024D/3ED8B27F
Choose life. Be Vegan :-) Please reduce needless cruelty + suffering !