Adding a custom domain after creating users

rddx58 21 Reputation points

Hi all,

I have an Office 365 E3 instance that was set up by someone else. Unfortunately, we have created users with the default domain name (e.g. I now want to set up a custom domain (e.g. but there is a message in the O365 admin centre: "Otherwise, you'll need to update your users' email addresses when you connect your domain."

Can someone describe how this might work? After I set up the custom domain, will I be able to log into O365 admin centre using or will I have to use However this account won't exist in our tenancy so will I effectively lock myself out?

Any help would be greatly appreciated!

Azure Active Directory
Azure Active Directory
An Azure enterprise identity service that provides single sign-on and multi-factor authentication.
14,623 questions
Azure Active Directory Domain Services
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  1. AmanpreetSingh-MSFT 55,521 Reputation points

    Hi @rddx58 • Thank you for reaching out.

    Adding as custom domain will not automatically update the UPN from to You will still be able to sign into O365 Admin center by using

    To update the UPN to after adding the custom domain, you will need to run below cmdlet:

    Set-MsolUserPrincipalName -UserPrincipalName "" -NewUserPrincipalName ""

    To add as secondary email address, you can use below cmdlet:

    Set-Mailbox -Identity "bob" -EmailAddresses @{add=""}


    Please "Accept the answer" if the information helped you. This will help us and others in the community as well.

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  1. rddx58 21 Reputation points

    Great, many thanks @AmanpreetSingh-MSFT !

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