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Cannot remove email alias for account in Office 365

Chris Krueger
I am trying to add/remove aliases to an account in Microsoft 365 and get this error:

The operation on mailbox "Blane Smith" failed because it's out of the current user's write scope. The action 'Set-Mailbox', 'EmailAddresses', can't be performed on the object 'Blane Smith' because the object is being synchronized from your on-premises organization. This action should be performed on the object in your on-premises organization.
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Server engineer
You cannot add/remove aliases to an account in Microsoft 365 which is "Synced with AD".

Your office 365 account is "Synced with AD" so you need to do the change from you local AD.

Go to properties of the user in local AD and go to attribute editor.

Search for "proxyaddress" attribute on the user properties in attribute editor and there you need to add an alias.
Saif ShaikhServer engineer

If you have an exchange server in the onpremise local AD then you can do it from Exchange management console for exchange 2010 server or you can use EAC for exchange 2013 server.

If you don't have any exchange servers in your local AD then probably you need to do it manually from local AD as described above.