Adding a custom domain after creating users

rddx58 21 Reputation points
2021-09-23T12:14:31.367+00:00

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. bob@bluecorp.onmicrosoft.com). I now want to set up a custom domain (e.g. bluecorp.com) 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 bob@bluecorp.onmicrosoft.com or will I have to use bob@bluecorp.com? 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
0 comments No comments
{count} votes

Accepted answer
  1. AmanpreetSingh-MSFT 55,521 Reputation points
    2021-09-23T12:41:06.303+00:00

    Hi @rddx58 • Thank you for reaching out.

    Adding bluecorp.com as custom domain will not automatically update the UPN from bob@bluecorp.onmicrosoft.com to bob@bluecorp.com. You will still be able to sign into O365 Admin center by using bob@bluecorp.onmicrosoft.com.

    To update the UPN to bob@bluecorp.com after adding the custom domain, you will need to run below cmdlet:

    Set-MsolUserPrincipalName -UserPrincipalName " bob@bluecorp.onmicrosoft.com" -NewUserPrincipalName "bob@bluecorp.com"

    To add bob@bluecorp.com as secondary email address, you can use below cmdlet:

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

    -----------------------------------------------------------------------------------------------------------

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

    0 comments No comments

1 additional answer

Sort by: Most helpful
  1. rddx58 21 Reputation points
    2021-09-26T07:57:07.747+00:00

    Great, many thanks @AmanpreetSingh-MSFT !

    0 comments No comments