Old Versions of Exchange ActiveSync Clients Get the Bullet

Microsoft to Customers: Use Exchange ActiveSync 16.1 or Later

The December 15, 2025 announcement (also posted as MC1197103) that Exchange Online will stop servicing Exchange ActiveSync (EAS) connections from email clients running a version older than 16.1 from March 1, 2026 is hardly unexpected. Microsoft has been steadily eliminating old components from Microsoft 365 workloads, and deciding to concentrate on one supported version (16.1, released in 2016) makes sense. You might notice higher protocol revision numbers for Exchange ActiveSync in Microsoft documentation, but server-side protocol updates don’t affect the client version.

Microsoft doesn’t control how or when companies that license the EAS protocol update their email clients. Apple has used version EAS 16.1 since iOS 10, and Microsoft says that Google and Samsung are updating their email clients to use EAS 16.1, but there are many older EAS clients still in use. The clients work today because Microsoft doesn’t currently enforce a minimum protocol requirement. However, once the requirement for 16,1 comes into force on March 1, 2026, the clients running an older version will be unable to download or send email or synchronize calendars.

Checking for Older Versions of Exchange ActiveSync

Because no one likes surprises, it’s wise to run the PowerShell command included in Microsoft’s blog to identify clients using older versions of EAS to connect to Exchange Online. One thing I noticed is that older clients can return a value like

EURPR04A002.prod.outlook.com/Microsoft Exchange Hosted Organizat

for the UserDisplayName property. This is obviously an artifact of some previous attempt to update the property with the display name of the device owner that didn’t quite work. However, if you check the Identity or DistinguishedName properties for the device, you’ll find the owner’s name there. For example, here’s what the identity for a device looks like:

james ryan\ExchangeActiveSyncDevices\iPhone§J3LGEB1EA55MPCMKJDBHQMELQS

Alternatively, use this version of the command:

Get-MobileDevice -ResultSize Unlimited | Where-Object {($_.ClientType -eq 'EAS' -or $_.ClientType -match 'ActiveSync') -and $_.ClientVersion -and ([version]$_.ClientVersion -lt [version]'16.1')} | Sort-Object UserDisplayName | Select-Object UserDisplayName, Identity, DeviceId, DeviceModel | Format-List

Hopefully, any devices you find haven’t been used in quite some time and can safely be removed. Exchange Online does not remove obsolete device partnerships after users have moved on to new phones, so it’s a good idea to clean up obsolete mobile devices from Exchange Online periodically.

The Case for Outlook Mobile

The great thing about EAS is its simplicity and robustness. The downside is that EAS is a tad too simple to handle many scenarios. Microsoft’s interest in EAS faded quickly along with their OWA for Devices strategy for mobile device support for Exchange once they decided to buy the Acompli mobile client in November 2014.

Microsoft quickly rebranded Acompli as Outlook Mobile and adopted key aspects of the new client such the preprocessing of data on the server that made features like the focused inbox possible. The preprocessing originally used Amazon Web Services before Microsoft moved it to Azure in 2016. Preprocessing is core to the ability of the new Outlook for Windows client to deliver features to users whose mailboxes are on antiquated POP3 and IMAP4 servers. The synchronization of data from POP3 and IMAP4 servers for processing on Azure caused a lot of negative commentary some years ago, mostly from people who didn’t realize why functionality depended on the preprocessing. Outlook Mobile doesn’t use EAS, so it’s unaffected by the requirement for 16.1.

Given the availability of highly usable Outlook mobile clients for iOS and Android, apart from some edge cases, I don’t understand why Exchange Online users continue to use EAS clients when they can use Outlook Mobile.

I guess it’s important to facilitate choice and some are very committed to the Apple mail client (for example) and simply don’t want to change, even when they know that Microsoft will never invest in EAS to bring its support for functionality anywhere close to Outlook Mobile. And even if Microsoft dedicated the necessary engineering effort to create a new version of EAS, no commitment exists from EAS licensees to upgrade their email clients to take advantage of new features.

The Last Hurrah for Exchange ActiveSync

If Microsoft doesn’t create a new version of EAS, this might be the last round of email client updates for Exchange ActiveSync. EAS has had a good run, spanning from the days of BlackBerry dominance to the introduction of the iPhone to the massive expansion of smartphone and tablet usage. Over that period, successive versions of the EAS protocol have served users well. That audience has declined and continues to decline. How long it will last is anyone’s guess.

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So much change, all the time. It’s a challenge to stay abreast of all the updates Microsoft makes across the Microsoft 365 ecosystem. Subscribe to the Office 365 for IT Pros eBook to receive insights updated monthly into what happens within Microsoft 365, why it happens, and what new features and capabilities mean for your tenant.