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=”email@domain.com”;}). 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 new-mailbox-id@domain.com

After this, new-mailbox-id@domain.com became the new unremovable shared mailbox alias, and the old email@domain.com could be removed with no issues.

March 2020
Microsoft Office 365
Exchange Online