[exim] Envelope From getting set wrong

Hi folks,

Thanks for the help with the authentication, I'm now a bit further on
but have hit another snag.

It would seem that Exim on my front end servers is re-writing the
envelope from address badly.

Say I send mail using an Exchange server. My email address is
me@???

My exchange server uses privately configured authentication details that
should never be revealed to the world, say relayuser@???.

If my spam platform delivers the message to Exim on my back end servers,
the From and Envelope From all get set to me@???

If my spam platform delivers the message to Exim on the front end
servers I get very different results.

The envelope from address gets changed to this:
<"relayuser@???>
In addition a Sender: header is added with the same value.

This mucks things up mightly as it is not valid, any other instances of
Exim will in fact reject it because of there being a second @ in the
local part! As the envelope from address has now been changed, the
bounce message gets stuck in the queue because relayuser@??? is
only valid for authenticating.

So, which bit of exim.conf should I start with to stop this? There's no
rewrites in there that I can see.

Regards,
Colin.