You can create a user principal name (UPN) suffix (e.g., @DivisionA.com) at the top level of the AD forest, as I demonstrated in "Exchange 2000 Hosting: The ASP Model, Part 1," November 2001. However, you might want to create the UPN suffix as a property of an organizational unit (OU) instead. If you do so, new users that you create within that OU will receive that UPN suffix as the default.
To create a UPN suffix as an OU property, you must use the ADSI Edit utility. Open the domain naming context (domain NC) and expand the containers until you find the OU. Open the OU properties, select the UPN suffix attribute, and enter @DivisionA.com just as you would at the forest level.
An easy way to find the UPN suffix attribute is to select the Select a property to view drop-down list and press the letter U until you see the uPNSuffixes attribute. Enter the UPN suffix and click Add. After you create a UPN suffix as an OU property, when you add a user to the OU, you'll see the UPN suffix as the default.