Here is how to add a new email domain (UPN suffix) to Active Directory
- Log in to your domain controller
- Open “Active Directory Domain and Trusts”
- On the top left corner right-click Active Directory Domain and Trusts and select Properties
- Type the new domain under Alternative UPN suffixes and click Add
That’s it! Now the new domain will be available under “Active Directory Users and Computers”. You can all change the user UPN from the on-prem exchange. Once the UPN is applied, the users will need to use the new domain to login to Office 365.