Microsoft confirms October Windows updates break Exchange Server email services
Windows Server security updates from October 13, 2026, are disrupting Exchange Server mail flow and web access on some on-premises systems.
Key points
- The issue stems from Windows Server updates, not the separate Exchange Server patches released in October.
- Affected systems include Exchange Server 2016, 2019, and Subscription Edition running on impacted Windows versions.
- Microsoft advises administrators not to uninstall the updates and has published a registry-based workaround.
Microsoft confirmed on October 13, 2026, that its Windows Server security updates can disrupt Exchange Server email services, blocking webmail access and preventing message delivery. The software maker stated that the failures originate from changes in the Windows operating system rather than the separate October Exchange Server security updates.
The disruption affects some on-premises systems running Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition. According to Cyber Security News, the issue blocks Outlook on the web sign-ins and leaves messages stuck in the Outbox. Microsoft explicitly advises administrators not to uninstall the Windows updates, noting that the problem is unrelated to the Exchange-specific patches released that same month.
Context and background
The failures stem from a new default limit introduced in the October updates for Endpoint Lookup calls. Microsoft’s Windows release health notice indicates that the updates cap simultaneous calls at 512 to control memory usage by Endpoint Mapper. This component handles remote procedure call communications within Windows.
Heavy Exchange Server workloads can exceed this default allowance, causing service interruptions. BleepingComputer reported that Microsoft described the issue as mitigated rather than resolved, with an investigation still ongoing. The company noted that not all servers will be impacted equally, as severity depends on the specific server load.
Who is affected
Administrators may observe growing mail queues, failed mailbox access during replication, or issues with Database Availability Group members reaching quorum. Users might be unable to sign in to Outlook on the web, while messages sent through desktop Outlook remain undelivered. Restarting an affected server may restore service temporarily, but the underlying limit remains.
One specific error to watch for is MapiExceptionMdbOffline with error code 0x80004005 and ec=1142. Affected systems may also record Event ID 1006 in the Application log. These indicators help administrators determine if an outage matches Microsoft’s report. The affected Windows updates include KB5126646 for Server 2025, KB5126655 for Server 2022, KB5126651 for Server 2019, and KB5126868 for Server 2016.
What should you do?
Microsoft has published a workaround that involves modifying a registry key on affected Exchange Server systems. This change increases the maximum number of simultaneous Endpoint Lookup calls, allowing Endpoint Mapper to use more memory. Administrators must run the command from an elevated Command Prompt and then reboot the device to apply the fix.
An additional mitigation is available through Microsoft Support for affected organizations. Microsoft continues to work on a permanent resolution while monitoring the impact of the workaround. The company urges customers to retain the October 2026 Exchange Server security updates, as they are not related to this known issue.
What to do and how to stay safe: Microsoft
- Monitor Exchange Server application logs for Event ID 1006 and MapiExceptionMdbOffline errors to identify affected systems early.
- Verify if your servers are running the specific Windows Server updates linked to the issue before applying any changes.
- Apply the registry workaround provided by Microsoft to increase the Endpoint Lookup call limit on heavily loaded servers.
- Contact Microsoft Support for additional mitigation options if the standard workaround does not resolve service interruptions.
Step-by-step guide: Mobile Device Security: How to Protect Data Beyond Passwords
General security guidance from the Threat Times newsroom. It is not confirmed advice from the organisations named in this story.
Frequently asked questions
Is this issue caused by the October Exchange Server security updates?
No, Microsoft states the issues are not related to the October 2026 Exchange Server security updates but stem from Windows Server changes.
Which Windows Server updates are linked to the Exchange Server failures?
The linked updates are KB5126646 for Server 2025, KB5126655 for Server 2022, KB5126651 for Server 2019, and KB5126868 for Server 2016.
Should administrators uninstall the Windows updates to fix the problem?
No, Microsoft advises administrators not to uninstall the Windows updates, as the company is working on a permanent fix and has provided a workaround.
