Adding a custom domain after creating users

Question

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!

Answer 1

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"}

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Answer 2

Great, many thanks @AmanpreetSingh-MSFT !