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
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- Type the new domain under Alternative UPN suffixes and click Add
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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.