[exim] Router to handle message based on MX

Hi all,

First time post, please be nice :-)

I have a requirement to loook at the headers of email coming into our Exim
server and make decisions based on where it was from, and what MX says. Our
email is scanned by an external company for spam/viruses so MX is set to
them.

If an email is generated locally however it stays local and is delivered
without having been scanned.

What I want to do is as follows:

check MX for incoming email
if primary MX is AV/AS Company AND AS/AV company in headers then deliver the
email locally
else
send out to AS/AV company

This will halt a problem we currently have whereby some locally generated
email stays local without ever going out to be scanned.

It is an Exim 4 server.

Many thanks in advance for any suggestions:

Doug