Do you run split DNS? If you have other MTAs internally, that is how they would route to your exchange server, unless you explicitly reconfigure them.
I am assuming that you can't use the external facing ip on your 10 net.
If your exchange's external inbound ip is reachable by your 10 net, then you just have to configure that one. This requires a separate DMZ, usually.
If only have an external DNS domain, you could structure the MX mail preferences to provide service.
gene