Email address change for a user - .onmicrosoft.com address not updating

Question

Hi, I changed a user's email address AD on-prem. I added their previous email address as a proxy address, and things synced up fine in O365. The new email address became the primary email address, the old is in there as an alias. I did notice that the user's @<domain>.onmicrosoft.com address remained the same though. I'm unable to edit it in Exchange Admin Center as it's being synced from on-prem.

I noticed a potential solution of moving the user to an un-synced OU so the user is deleted from O365, then restore the deleted user in O365 so it's not being managed by on-prem AD and is "cloud only", then edit the .onmicrosoft address, and then move the user back to a synced OU.

Instead of doing this, is it ok to just leave the .onmicrosoft address as the old address? Will this cause any potential issues?

Thank you!

Answer 1

Hi @Blake Lawhorne ,

Thank you for posting your question in the Microsoft Q&A forum.

When we sign up for Microsoft 365, Microsoft provides a fallback domain <domain>.onmicrosoft.com for us. The fallback domain is used by default in email addresses, it’s OK to keep the <domain>.onmicrosoft.com email address for your mailbox.

Also, you can create other new onmicrosoft.com domain but it’s limited to five onmicrosoft.com domains in your Microsoft 365 environment, and you can’t rename or remove an existing onmicrosoft.com domain.

You can check these articles for more information about onmicrosoft.com domain:

Add and replace your onmicrosoft.com fallback domain in Microsoft 365 - Microsoft 365 admin | Microsoft Learn,

Domains Frequently Asked Questions | Microsoft Learn 

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Answer 2

There should be no issues, you only ever need to change the onmicrosoft alias if you have some naming policy you need to adhere to. And as with any other alias change, it's best to leave the old one, just in case.

If you do need to update it, here's a easier to use workaround:

Set-Mailbox ******@domain.com -WindowsEmailAddress ******@domain.onmicrosoft.com

The above cmdlet will add ******@domain.onmicrosoft.com as the primary SMTP address for the user, while preserving any existing aliases. Thus, you can use it to ensure that a new onmicrosoft alias is added, after which you can rerun the same cmdlet, but with the actual desired Primary SMTP address (i.e. the one you added on-premises). This will work even for synced users, and saves you the trouble of having to disable sync, or use workarounds such as the one you detailed above (not a supported method btw).