Add a domain to the Active Directory

How to add a domain to the Active Directory

Login to your domain controller
Open the “Active Directory Domains and Trusts”
Open the Properties of Active Directory Domains and Trusts
Right-click on the top item in the left tree view and select properties
Add the new Domain Name
In the UPN Suffixes dialog, enter the new domain name in the “Alternative UPN Suffixes” field and click on Add
Apply the settings
Click Apply and close the windows. The domain is now added to the domain controller.
(optional) for replication to other domain controllers
If you have multiple domain controllers you can force the replication with the following command in PowerShell / CMD: repadmin /syncall /AdeP

You should now be able to use the new domain name in the Active Directory or in the Exchange Administration Center.