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Cannot add email alias in Microsoft 365 admin portal.

Added new domain for email in Microsoft 365. Able to add new domain email accounts as alias in current primary (different) domain. 9 of 10 successful. One gave this error: 

"An Azure Active Directory call was made to keep object in sync between Azure Active Directory and Exchange Online. However, it failed. Detailed error message: Another object with the same value for property proxyAddresses already exists."

I tried sending email to the email address in question and got the standard message "email name not found at domain.com" This would seem to indicate there is no account by that name. I have tried for two days with no difference, same error.

Has anyone a solution for this?

Thank you.

MicrosoftMicrosoft 365Azure

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8/22/2022 - Mon
Vasil Michev (MVP)

Best use PowerShell, as the UI has some "helpful" features such as detecting when the "alias" part of the email address is used by another recipient. Just to be on the safe side, first run this:

Get-Recipient user@domain.com

Open in new window

If no entries are returned, the address is not in use and you can assign it via:

Set-Mailbox blabla -EmailAddresses @{add="user@domain.com"}

Open in new window


I don't think the issue is an existing email address. This morning I tried adding numerous random account names using both domains as an alias to the existing address and all failed with this same error message. This is happening only on the one user account.

Further info: I am able to create the new email alias using a different active user, then I deleted the alias.
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