[exim] Change From: address

Greetings all

A question or two if I may regarding authenticated relay and accountability.

After much sweat and frustration gotten my config pretty much where I
need it but have one thing I can't seem to get sorted other than one or two
small things which can wait...

What I am desperately trying to do is change the sender email address
(From: "Name" <email@addie>) to the $authenticated_id
which in my case is also the senders email address - I use localpart and
domain to form a unique username. I am wanting to implement this in
order to stop sender address spoofing. I may want to change the
"Reply-To:" as well if it is set.

Basically what I am wanting to do is change :
From: "John Doe" <john@???> to
From: "John Doe" <john@???>
where "john@???" is the spoofed address and "john@???" is
the true email address which is also the $authenticated_id. I also
need to account for situations where the name is not given but only
the email addie; eg "From: <john@???>".

I have figured out how to remove the from header and then recreate it but
I can't seem to figure out how to retain the "John Doe" name part.

Any assistance in getting this working would be greatly appreciated
by a mere exim n00b like me. I can provide what ever part or whole of my
config you might want.

It may be nice to take the original From: and add it as a X-OrigFrom: for
problem determination.

Many thanx for lending me your ears.... or at least your experience
and expertise.

Regards
Michael L Griffin