When you try to delete a shared mailbox, which is placed in Office 365 (Exchange Online), and you are running Hyrbid, you may see this:
The following error occurred during validation in agent ‘Windows LiveId Agent’: ‘Unable to perform the save operation. ‘TESTDELT’ is not within a valid server write scope.’
Then when you try to delete it from the ECP on the on-premise Exchange server:
… “isn’t a mailbox user.”
Use powershell on the on-premise Exchange:
Remove-remotemailbox –identity firstname.lastname@example.org
You could also delete the user account, for the shared mailbox, and force a AAD Connect delta sync.