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Office 365 - Allow a User's Mobile Device using PowerShell
Let's assume a scenario where a user's mobile device was blocked or quarantined in Office 365 and you want to Allow it. Sometimes Office 365 - Exchange Admin Center (EAC) takes forever to show user's mobile device when you click on View details under Mobile Devices.

It's must quicker and easier to user Exchange Online PowerShell to view the list of devices and allow it if it's blocked or quarantined.
Follow the below steps to Allow the device.
Connect to Exchange Online PowerShell
Run below cmdlet to get the list of devices for the user​
Get-MobileDevice -Mailbox "user's display name or UPN" | fl FriendlyName, Identity, DeviceAccessState, DeviceID
Make note of the DeviceID. If the DeviceAccessState is Blocked or Quarantined, run below cmdlet to Allow it
To Allow it, run
Set-CASMailbox -Identity "user's display name or UPN" -ActiveSyncAllowedDeviceIDs @{add='type DeviceID here - you made note in step 3 above'}
Now run the cmdlet in second step above, the DeviceAccessState should now be set to Allowed
PowerShell is there to give us hand when EAC fails ;-)