Three Exchange servers on the same network, diff domains, can't send mail

We run a small IT shop.  In the building, we have our own exchange server, a hosted mail server for customer mail and a test mail server.

On the WAN side of the router, we have Comcast inbound with 15 static IPs and ATT inbound with 10 static IPs.  On the router, the Comcast is the primary with ATT as secondary.

Exchange1 has mail coming in from public IP xx.xx.xx.138 (Comcast)
Exchange2 has mail coming in from public IP xx.xx.xx.105 (ATT)
Exchange3 has mail coming in from public IP xx.xx.xx.109 (ATT)

Our problem was that mail coming in on 109 (ATT) was going out on 138 (Comcast) and that was giving us some grief for obvious reasons.  So, we wrote some policy rules in our firewall that forced traffic coming in on a public IP to go out on the same public IP.  However, when that rule is in place, Exchange3 cannot send mail to Exchange1.

Remove the rule and Exchange3 can send mail to Exchange1, but it goes out on the wrong public IP.  With the rule in place, everything is good, except Exchange3 cannot send mail to Exchange1.  We need to rule to make sure traffic leaves on the same IP it entered on.

Remember, all three Exchange servers are on the same 10.50.0.x network and we suspect that has something to do with it, but we are striking out homing in on the specific problem.

Ideas?

Thanks

Cliff