You can force messages destined for a specfic internet Domain to a specific IP address with the use of a Foreign SMTP Domain document.
Create a Foreign SMTP Domai document.
Go to the Routing tab,
Messages Addressed to:
Internet Domain: internet domain.com
Should be Routed to:
Internet host: x.x.x.x
Also, make sure the Configuration document on the server, under the ROUTER/SMTP tab -> Basics tab
SMTP used when sending messages outside of the local internet domain: Enabled