[Exim] Setting up an SMTP Internet Gateway using Exim

Help! I'm trying to set up a SMTP Internet gateway system using Exim,
and am running into a problem. I have a system setup with a fresh
install of Debian 3.0 and Exim behind a firewall. The firewall is
configured to pass port 25 traffic through to the Exim box. I want
relay mail through the Exim box to our internal email server (eventually
also using Mailscanner, SpamAssassin and Sophos). Outgoing email from
the internal email server should also pass through the Exim box, and
then out to the Internet. There are no local accounts on the Exim box.

I've been able to set up Exim to relay in both directions, but I need to
be able to stop forged From headers with my domain from coming into our
network (ie, anything not coming from 192.168.1.0/24 that is from
example.com should be blocked). The system as it is now fails abuse.net
relay test #6 (the Exim server is off-line now until it is fixed). How
can I do this?

Thanks in advance,

Ken McCord