Office 365 - cannot delete shared mailbox alias

Issue

Microsoft Office 365 shared mailbox secondary alias cannot be removed. I tried removing the alias via Office 365 Admin Center, Exchange Admin Center, and PowerShell (Set-Mailbox "mailbox.name" -EmailAddresses @{remove="This email address is being protected from spambots. You need JavaScript enabled to view it.;}). None of these methods works, it initially looks like the operation succeeds, but the alias is never removed.

The shared mailbox in question used to be a user's mailbox, and the alias in question used to be the user's primary email address. I also tried changing the mailbox username / login name, and email alias, but this didn't help either.

Resolution

The solution was to change mailbox's MicrosoftOnlineServicesID property via PowerShell. To do this connect Office 365 tenant to PowerShell and run: Set-Mailbox mailbox.name -MicrosoftOnlineServicesID This email address is being protected from spambots. You need JavaScript enabled to view it.

After this, This email address is being protected from spambots. You need JavaScript enabled to view it. became the new unremovable shared mailbox alias, and the old This email address is being protected from spambots. You need JavaScript enabled to view it. could be removed with no issues.

March 2020
Microsoft Office 365
Exchange Online