You may want to disable specific users to prevent them accessing email from outside the company.
1. Start the SBS Administrators Console
2. Select Active Directory Users and Computers node
3. Select the View menu and select the Advanced Features option
4. Drill down into the User container under ADUC to the required user
5. Right click the user and select Properties
6. Select the Exchange Advanced tab
7. Select the Protocols button
8. Select HTTP from the protocols list and then the Settings button
9. Remove the tick from “Enable for mailbox” and select OK a few times to get back to the main screen.
10. This user will now not have access to their mailbox via OWA.