Updates for Microsoft Office disrupt ActiveSync connectivity of Outlook

February 6, 2024

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Categories: blog, Featured, Z-Push
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Updates for Microsoft Office disrupt ActiveSync connectivity of Outlook

With this blog post we want to inform you about an issue, which apparently has been introduced to some variants of Outlook with recent updates for Microsoft Office.

Currently we are receiving reports through various channels stating Outlook when using Exchange ActiveSync as protocol is no longer syncing any new mails, after the latest updates by Microsoft have been installed.

Although the Microsoft Office changelog does not explicitly mention such a specific change, we could pinpoint the issue starts appearing with version 2401 (Build 17231.20182), which was released one day earlier than Build 17231.20194, which appears to be the version most people start noticing the issue as they update to this version in one step.

It is quite possible that this issue is not an intentional change and might also be caused by a software bug.

Which variants of Outlook are affected?

So far we have received information of the following Microsoft Office variants and it’s included Outlook versions to be affected:

The build numbers mentioned above apply to the following Outlook variants:

– Microsoft Office 365

– Retail variants of Microsoft Office 2021, Microsoft Office 2019 and Microsoft Office 2016 C2R

So far we did not get any reports on variants matching build numbers shipping under volume license terms.

Which Kopano product is affected?

Users of Kopano Groupware are affected. Kopano Groupware relies on ActiveSync (via Z-Push) and the Kopano Outlook Extension for Outlook connectivity and collaboration features.

Users of Kopano Cloud based products and services are not affected. Kopano Cloud relies on MAPI over HTTP for Outlook connectivity, without any extensions for collaboration features.

Are other products or services affected?

Yes, this is the case. At the time of writing we are aware of at least four other products / services facing the same issue:


MDaemon Email Server

Zimbra (with and without Z-Push)

Zoho Mail ; 2nd case in the Microsoft forums

This emphasizes that the problem is on the client side (Outlook).

What can you do?

If you are able to hold back the latest Office updates, we suggest you do. According to the Microsoft Office changelog no security related fixes were included – so you won’t miss anything of real importance.

If you are already affected by the issue, we suggest you downgrade your installation to a known to be working version 2312 (Build 16.0.17126.20132). The downgrade procedure is documented here.

If working with Outlook is of utmost importance to you, you might want to consider to take a look on the various forms of which Kopano Cloud is available.

Kopano Cloud is not affected by the issue at hand and provides a better user experience with Outlook overall as it uses a MAPI over HTTP instead of ActiveSync.

What can Kopano do?

Kopano as a company has no influence on any changes Microsoft introduces with updates to their products – be it intentional or otherwise.

Of course we will try to understand the issue as best as we can and we will monitor any activities Microsoft is going to take regarding this.

We’ll update this blog post if there is new relevant information available and / or if the issue has been solved.

Update 1 ( February 9, 2024 )

KerioConnect is also among the affected prodcuts / services.

Microsoft has officially started an investigation of the issue and recommends the downgrade of Outlook as stated in this blog post.

Update 2 ( February 16, 2024 )

Microsoft has issued a new version of Outlook (Build 17231.20236). Although the Microsoft Office changelog does not mention a fix regarding this issue and the status of the issue is still set to investigating, we have received first reports of this version not to be affected by the issue anymore.

Although we could replicate those findings, we advise caution as the issues has not officially been solved yet!

Hint: Apparently the update is not yet offered to all users of Microsoft Office. This might be related to user circles, load balancing or availability on local update servers – we do not know the exact reason. So it is quite possible you are also not yet offered to install the update. However an installation of this specific version can be forced by using the same methods as described in the downgrade procedure.