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"ObjectClass: Unknown" is returned when you use the Get-MailboxStatistics cmdlet
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Exchange Server 2016 Enterprise Edition
Exchange Server 2016 Standard Edition
Exchange Server 2013 Enterprise
Exchange Server 2013 Standard Edition
When you use the Get-MailboxStatistics Windows PowerShell cmdlet to get information about a mailbox in on-premises Exchange Server or Exchange Online in Office 365, the ObjectClass property unexpectedly returns a value of Unknown.
For example, when you run the Get-MailboxStatistics admin | FL *object* command, you receive the following output:
ObjectClass : Unknown ObjectState: Unchanged
In earlier versions of Exchange Server and in Exchange Online in Office 365, the ObjectClass property returns the correct value. For example, this property returns Mailbox for user mailboxes and Mailbox, Disabled for equipment and room mailboxes.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.