Sendmail Relay, Exchange and domain names

[Neily]

We have set up a Sendmail Relay (sendmail.domain.com) to do our SPAM filtering. This then relays mail on to our Exchange server (exchange.domain.com). mail.domain.com is the web interface/alias.

This is all working fine, but we've been asked to sort the headers.

Basically, the headers have the following:

Received: from sendmail.domain.com (mail.domain.com [1.2.3.4])
by mx.gmail.com with ESMTP id d7si33596wra.2005.06.06.04.06.39;
Mon, 06 Jun 2005 04:06:39 -0700 (PDT)
Received-SPF: softfail (gmail.com: domain of transitioning me@domain.com does not designate 1.2.3.4 as permitted sender)
Received: from mail.domain.com (computer.office.domain.com [192.168.0.1])
by sendmail.domain.com (8.13.1/8.13.1) with SMTP id j56B5UjL015397
for <me@gmail.com>; Mon, 6 Jun 2005 12:06:37 +0100
Date: Mon, 6 Jun 2005 12:06:37 +0100
Message-Id: <200506061106.j56B5UjL015397@sendmail.domain.com>

We'd like to stop the headers showing sendmail.domain.com and just show mail.domain.com as the origin of our email. Searching showed up MASQUERADING which sounded correct, but didn't work.

We are also getting our email, which should be user@domain.com appearing as user@exchange.domain.com

Can anyone shed some light?

THANKS