Exchange 2010 - Restrict users to be able to send to only a specific group of users

With Exchange 2010, is it possible to restrict a user's ability to send to certain INTERNAL users ?  

We have a small group of users that should only be able to send to a small specific subset of users and I am not sure how to make this restriction.  We do not want these users to be able to email OUTSIDE, or to RECEIVE EMAIL from outside, and we only want them to be able to send (and receive) email from 10 people.  We can place these people into a DISTRIBUTION GROUP and I know that we can set the permissions easily on the Distribution group, but how can I limit these users to ONLY be able to send to that distribution group (and it's members individually) ?

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Create a distribution group for both the users who are to be restricted and the users that they can send to.
In EMC, go to Organization -> Hub Transport.
Go to the Transport Rules tab.
Right Click -> New Transport Rule.
Give it a name and press next.
Click, "From a member of a distribution list" and select the group to be restricted.
Press Next.
Tick "Send rejection message to sender with enhanced status code".
Click on "Rejection message" in the bottom pane and type a descriptive message to be sent to the users.
You'll also need to click on "Enhanced Status Code" and enter a number in there. Just use 5.7.1 as this really doesn't matter.
Press Next.
Tick "Except when a message is sent to a member of a distribution list".
Select the distribution list containing the users that they're allowed to email.
Press Next.
Press New.

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