MTA's can exist anywhere, and are unimpacted by their location as long as they have connectivity to internal db and to your SMS vendor. From your other post it looks like your architecture is the source of a lot of understandable confusion. I would suggest circling back with your IT dept for delivery issues.

Well this is an interesting diagram. I can't say I understand the app server's existence on the Servers LAN, is it for deployments where IT has to spend excess budget and also doesn't want intranet users to have failover? Are intranet users on a separate LAN, are extranet users just computers that use internet?Subtracting the LAN app server and making assorted corrections- active/passive load balancing for users, resource deployment to app servers, shared storage for app server file upload, etc-...