[Exim] how do I keep the sender header from being rewritten?

Hi,

I'm using TMDA in conjunction with Exim and it creates an auto-reply
that is handed off to port 25 of the localhost for delivery. It is
configured to use a specified bounce_envelope_sender variable to avoid
infinite loops of confirmation requests <-> bounces.

What's happening though is Exim is superceeding the preference of the
envelope sender (even though it's running as a trusted user with the
"untrusted_set_sender = true" configuration variable) and rewriting the
return-path with the confirmation address; which is a bad thing.

2002-06-21 21:10:05 17LaLl-0005YC-00 return-path spambounce@???
rewritten as tj-confirm-1024711805.21122.d7edc4@??? using Reply-To

I need to turn that function off. I haven't found exactly where to do
that.

How to I preserve the envelope sender as I have specified and not have
Exim rewrite it? (This is 3.3x btw.)

Thanks!
Todd