[exim] rewriting linux user in Received: header

Hi,

I am a newbie at exim, tried googling and doing rewrite rules to fix
this, cant seem to figure this one out myself.

I am having 2 "Received:" headers (the initial linux user then the IP
of the exim server) appearing in the top of my email (I use evolution
and click show all headers). One of them is showing the linux user
which initiated the mail (this is a proprietary name and I wish to
remove it or change it). I successfully managed to change the
addresses in the envelope-from: and Return-path: headers using
rewriting. However, I cannot see a fix for changing the linux user.
I have a thought that it might be necessary to remove the headers
called Received: from the mail completely, but I do not know if this
makes it tough to get round spam blockers.

Ideally, what I want to do is to change this line in the mail header:

Received: from bob by myhost.mydomain.com with local (Exim
4.63)(envelope-from............

Any idea how I can get rid of bob or change bob to sally?

Rewrites only appear to work for real email addresses and not the
linux user initiating the mail - I did try putting in

bob sally EFTBcfhrst in exim.conf which I thought might have rewrote it.

Can anyone point me in the right direction or tell me if its possible?

TIA

Robert