[exim] Changing Return-Path on SMTP relay

Hello,

I have a question about header rewriting and for you to better understand
what I'm trying to accomplish I will explain the background of the problem.

What I'm trying to do, is to rewrite the "Return-Path:" on my Exim4 smtp
relay server. The server accepts mail from an Exchange 2007 server and
relays it to the internet via ISP smarthost. The reason I'm trying to
rewrite this specific header is, that per some RFC Exchange is creating
automated out of office e-mail responses with this header present, but not
populated, so it looks like this "Return-Path:<>". Our ISP smarthost doesn't
accept any e-mail messages if the Return-Path is empty. I can't set Exchange
to deliver via DNS.

Can someone just answer me, if what I'm trying to accomplish can somehow be
done? I read a hole lot of material on the web and still can't seem to
figure it out. I also read in the Exim4 documentation that there are some
header_remove and return_path options, but they didn't do nothing or perhaps
I have inserted it in some wrong file, I'm pretty new to Exim4 or Linux
based e-mailers in general.

Thanks for your time and help.

Best regards,

Uros