[Exim] How to hid internal network?

I have configured exim on a firewall host.
When on my internal network clients send email using the exim host as the
SMTP gateway. I see internal clients host name and IP address on the outbound message.

E.g.

Received: from internal_host1([192.168.1.11] helo=internal_hostl)
        by firewall.mydomain.com with smtp (Exim 3.31 #1 (Debian))
        id 15frSZ-0001Kh-00
        for <test@???>; Sat, 08 Sep 2001 17:24:23 -0600

As you can see the sent message will identify the internal host my IP and name.
I would like to hide the internal network with all mail appearing to come from

either mydomain.com or mail.mydomain.com.

I have scanned the FAQs and mail archives and have note found a solution.

Does anyone have a suggestion?

Thanks for your help and time.

Please CC me on any response.

Anthony
.