[Exim] Newbie: Return-Path

Hi,

I have written a web-based email type program, All mail is sent by httpd
which is run as nobody. Problem is that is the mail is sent to a non
existing user it gets returned to nobody. In the message I am sending to
exim I set the the lines 'Return-Path' and 'Errors-To', but exim seems to
strip these. I have tried setting return_path_add to false, and some other
things from the manual, but I don't know if I'm even typing these lines in
the right place in the config. Nothing I do in the config file seems to make
much difference. I am running Redhat 6

Cheers
Tyrone