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In your Exchange System Manager, create two SMTP Virtual Servers, one is an External server (no relay, no external dns configured, bind to external nic) and one is an Internal server (relay allowed, and external dns configured, bind to internal nic). This serves two purposes, 1) it doesn't allow mail relay unless the source is from within the organization 2) you can restrict access to those that can use the Internal server (which can relay and has external dns).

Right click on your SMTP Internal server, select properties. Under the Access tab, click on connection (under the connection control frame), select All except the list below, and add the ip address of those computers that you want to restrict to internal mail only. I have tried this and it works in our organization.

hope this helps.

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I don't understand why Microsoft didn't include a feature to allow Exchange administrator filtering users through exchange system administration MMC to limit some of them to internal mail only.
I'm sure that I'm not the only one who needs a featurelike that for certain users. Strange...