Cleaning Up Obsolete Mobile Devices is Always a Good Idea

After publishing the article about the removal of mobile device management settings for users from OWA and the new Outlook, I was contacted by several people to ask should they clean up the obsolete devices that exist for so many mailboxes.

Generally speaking, I am all for removing clutter. Cleaning out unwanted data is a good thing. Just ask any of the Copilot tenants who struggle with digital debris (old, unwanted, inaccurate, and misleading information stored in Microsoft 365) that corrupt Copilot responses to user prompts. Removing some old device partnerships is not in the same category as cleaning up a bunch of old files from SharePoint Online and OneDrive for Business, but the same principle applies.

Mobile Device Partnerships with Mailboxes

When a mobile device begins to synchronize information from Exchange Online, it creates a partnership between the device and the target mailbox. The Get-MobileDevice cmdlet lists active partnerships. If you run the cmdlet without any parameters, it returns all the partnerships in the tenant. That’s not a great idea in anything other than a demo tenant, but I’ve seen people advising others that this is the way to go.

It’s better to run Get-MobileDevice to check the partnerships for a mailbox (which is what happens to populate the set of devices shown by OWA and the new Outlook). This is the command I use:

[array]$Devices = Get-MobileDevice -Mailbox James.Ryan@office365itpros.com

The information returned by the cmdlet reveals a lot about the device. Here’s a snippet:

FriendlyName            :
DeviceId                : C05A87A5DE4F468DA2399763634D4686
DeviceImei              :
DeviceMobileOperator    :
DeviceOS                : iOS 18.6 22G86
DeviceOSLanguage        :
DeviceTelephoneNumber   :
DeviceType              : Outlook
DeviceUserAgent         : Outlook-iOS/2.0
DeviceModel             : Outlook for iOS and Android
FirstSyncTime           : 14/12/2022 17:01:58
UserDisplayName         : Tony Redmond
DeviceAccessState       : Allowed
DeviceAccessStateReason : DeviceRule
DeviceAccessControlRule : Outlook for iOS and Android (DeviceModel)
ClientVersion           : 1.0
ClientType              : Outlook

Some properties don’t have values. While I can’t be certain why this is so, it’s likely because of the changing focus within Microsoft to device management. Intune is the preferred solution now, so Exchange mobile device management is limited to what’s needed for Exchange and devices to synchronize. However, there’s enough here to confirm that it is an iOS device running Outlook for iOS and that the user first connected the device to their mailbox in December 2022.

Getting Device Statistics

What’s missing from the partnership information is any indication of activity. That information comes from the Get-ExoMobileDeviceStatistics cmdlet (use Get-MobileDeviceStatistics for on-premises mailboxes). Device statistics show the last time the device synchronized with the mailbox:

LastPolicyUpdateTime               : 15/08/2025 02:52:24
LastSuccessSync                    : 15/08/2025 10:32:27
LastSyncAttemptTime                : 15/08/2025 10:32:27
MailboxLogReport                   :
NumberOfFoldersSynced              : 0
Status                             : DeviceOk

Again, there are many properties that are not populated or don’t contain useful information. For instance, the Status property reflects the state of the device as known to Exchange the last time synchronization occurred. The device could be lost, stolen, or wrecked. The important property is LastSuccessSync, the timestamp for the last successful synchronization. It’s reasonable to use the timestamp as an indicator of activity. If a device hasn’t synchronized in a year (or less), it’s likely an obsolete mobile device. In many cases, people replace devices and never remove the old device, leaving an accumulation of obsolete devices to build up.

Cleaning up the Mess with PowerShell

When users don’t clean up, it’s up to administrators to swing into action. Some PowerShell will do the trick. To illustrate the point, I wrote a script (downloadable from the Office 365 for IT Pros GitHub repository) that:

• Finds all user mailboxes.
• Checks each mailbox for device partnerships.
• Checks each device for the last synchronization time.
• If the device hasn’t synchronized in over a year (customizable threshold), note it as an obsolete device.
• Report what’s been found and ask the administrator if the obsolete devices should be removed. As you can see from Figure 1, some of the devices haven’t synchronized in years. The entry for Outlook for Mac is there because that client uses Microsoft synchronization technology to fetch information from Exchange Online, just like Outlook for iOS and Android do.
• Clean up the old devices with the Remove-MobileDevice cmdlet.

The script isn’t very complicated and took less than an hour to write in front of the TV (my excuse for any errors). Feel free to amend the code to suit your purposes. The script should run with on-premises Exchange if you replace Get-ExoMailbox with Get-Mailbox and Get-ExoMobileDeviceStatistics with Get-MobileDeviceStatistics.

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