In order to block incoming Internet mail traffic for a specific groups follow these procedure:

1.    Go to "Start", "Programs", "Administrative Tools", and then click "Active Directory Users and Computers".

2. Right-click the distribution group and click "Properties".

3. Click the "Exchange General" tab.

4. Under "Message restrictions", click to select the "From authenticated users only" check box.

If you want to block it to user only.

1.     Go to "Active directory users and computers"

2.     Right-click the user account, click "Properties".

3.     Click "exchange general" tab.

4.     Click "delivery restrictions"

5.     Under "Message restrictions", click to select the "From authenticated users only" check box.