This article describes how to add UPN suffixes to a forest. Adding these suffixes gives you the ability to use a friendly user-logon name that does not match the domain’s or parent domains’ naming structure.
Adding a UPN Suffix to a Forest
- Open Active Directory Domains and Trusts.
2. Right-click Active Directory Domains and Trusts in the Tree window pane, and then click Properties.
3. On the UPN Suffixes tab, type the new UPN suffix that you would like to add to the forrest.
4. Click Add, and then click OK.
Now when you add users to the forest, you can select the new UPN suffix to complete the user’s logon name.
MS article : https://support.microsoft.com/en-us/kb/243629