How To: Permanently Delete All User Mailboxes in Microsoft Exchange Server 2016

When you permanently delete active user mailboxes and disconnected mailboxes, all mailbox contents are removed from the Exchange mailbox database and the associated active directory account is also deleted. In this case, data loss is permanent and you cannot recover the data.  

To get started, open PowerShell with administrative privileges and execute the following command.

Get-Mailbox | Remove-Mailbox -Permanent $true

To know how this worked,  execute Get-Mailbox which won’t return you any user mailbox.