With Exchange 2007 you could only configure some specific things on the Edge server, in Exchange 2010 RC some of these functions are also available on the HUB server. One of those things is block messages to specific domains. In Exchange 2007 this could only be configured on the Edge server, in Exchange 2010 RC this can also be done on the HUB server.
As you can see in the screenshot above you configure that internal users can’t send messages to the gmail.com domain. Besides this options a lot of other options are added to the transport rules in Exchange 2010 RC. I will write a tutorial about it soon which will describe these options and will give some examples what you can do with these options.