Scenario: Your existing SMTP domain is sbsfaq.com and you also want to recieve email as microsoft.com for all users.
1. Start the SBS Administrator Console
2. Navigate down the tree to Exchange > Recipients > Recipient Policies
3. Right click the Default Policy and go to properties
4. Select the E-Mail Addresses tab
5. Click NEW and select SMTP Address.
6. Enter the secondary SMTP domain into this address ie @microsoft.com, and then select OK
7. A message box will ask if you want to update all email addresses to which you press OK.
All users will now be able to recieve email via the secondary domain name. By default when the send email out it will go out as the primary domain.