Cannot create email alias in Microsoft 365

Question

We have added a second domain and now adding email aliases to existing users in Microsoft 365 Admin Portal. 10 of 11 alias adds worked but the 1 gives an error. I have found that I cannot add any alias under either of the domains to this particular user. I can add the same rejected alias to a different user, so I know the email address is not in use.
This is the error message when if fails to add the alias to the intended user:

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. ConflictingObject: User_9e52e333-2bae-4b29-9bcf-76ccc63c149f DualWrite (Graph) RequestId: fb70b097-d7f3-475c-97d7-8ff6294e0a3a The issue may be transient and please retry a couple of minutes later. If issue persists, please see exception members for more information.
If the primary email is also their username, then changing the primary email will also change their current username. An alias is another email address that people can use to email **the user

Thank you for any help.

Answer 1

To resolve this issue, determine duplicate values and values that conflict with other AD DS objects. To do this, use one of the following methods.

Method 1: Use the IdFix Microsoft Azure Active Directory Synchronization Tool Error Remediation Tool

Method 2: Map an existing on-premises user to an Azure AD user

Method 3: Determine attribute conflicts that are caused by objects that weren't created in Azure AD through directory synchronization

For more information, you can check out this article - Duplicate or invalid attributes prevent directory synchronization in Office 365

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Answer 2

Are you syncing from on-prem? If so is that account that doesnt work synced from on-prem?

Answer 3

Looking at Azure AD Connect portal, it shows that an Azure AD Connect sync has never run.

Answer 4

@Galaxy Technical Services

Based on your description, you could add this email address to anther account. It mean this email address isn't used. I would suggest you double check the "Directory synced" for this account from Azure AD.

If the "Directory synced" is false, I guess there may exist issue with this account, you could open a service request to let Microsoft check whether is this account damaged.

If the "Directory synced" is true, you may need to enable AAD Connect and manage this account from local AD.

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