In an Exchange Server 2013 environment, ActiveSync devices cannot be removed from a mailbox when there's a forward slash in the DeviceType property. This issue occurs when you use the Remove-MobileDevice or Remove-ActiveSyncDevice commands, or when you remove the device by using the Exchange Control Panel (ECP).
When you try to remove the device, you may not receive an error message, and the Get-MobileDevice cmdlet may no longer show the device. However, the Get-MobileDeviceStatistics cmdlet will show it.
When you try to remove the device, you may not receive an error message, and the Get-MobileDevice cmdlet may no longer show the device. However, the Get-MobileDeviceStatistics cmdlet will show it.