Microsoft Exchange Server Glitch Linked to October 2026 Windows Updates
Microsoft confirms October 2026 Windows Server updates can destabilize Exchange Server, causing OWA failures and email delays, with a registry workaround available.
Key points
- Microsoft acknowledged that October 2026 Windows Server updates may destabilize Exchange Server environments.
- The issue stems from a default memory limit on Endpoint Mapper, causing connection glitches and email queue delays.
- Administrators are advised to apply a registry edit rather than uninstalling the updates, as Microsoft investigates a permanent fix.
Microsoft has acknowledged that the October 2026 Windows Server updates may be causing stability issues for Exchange Server deployments. According to The Register, the software giant confirmed the problem affects multiple server versions and is unrelated to the specific security updates released for Exchange Server itself.
The destabilization is linked to a list of Windows Server updates released in October 2026. Microsoft stated that the glitch can impact Exchange Servers running on Windows Server 2016, 2019, 2022, and 2025. The company clarified that these are not the October 2026 security patches designed specifically for Exchange Server, but rather general operating system updates.
Context and Background
The root cause is a default limit on memory usage by Endpoint Mapper when handling Endpoint Lookup calls. Microsoft’s Release Health Dashboard explains that the default limit of 512 simultaneous Endpoint Lookup calls may be insufficient for heavy workloads like Exchange Server. This constraint leads to connection glitches in Outlook Web Access (OWA) and significant delays in email processing.
Administrators reported that email queues grew lengthy, with some servers experiencing a complete halt in email flow. Rebooting the affected server may provide temporary relief, but the issue typically returns. Microsoft noted that the severity and speed of the impact vary based on the Exchange server load, meaning some installations might not encounter any problems at all.
Who Is Affected
Exchange Server 2016, 2019, and the new Subscription Edition (SE) are all susceptible to this destabilization issue. The problem is not immediate for all users; it depends heavily on the server's workload and the specific Windows Server version it runs on. Microsoft emphasized that not all Exchange servers will be impacted equally by the installation of these updates.
Microsoft stated it is investigating the issue and will provide more information when available. The company strongly advised customers not to uninstall the October 2026 Windows Server updates, noting that the severity and speed of impact varies based on Exchange server load.
What should you do?
Microsoft strongly advised customers not to uninstall the October 2026 Windows Server updates. Instead, the company recommends applying a registry edit to increase the maximum number of simultaneous Endpoint Lookup calls. This adjustment addresses the memory limit causing the bottleneck. Administrators should monitor their server loads closely, as the impact varies significantly between environments.
What to do and how to stay safe: Microsoft
- Check if your Exchange Server is running on Windows Server 2016, 2019, 2022, or 2025 and has received the October 2026 updates.
- Monitor Outlook Web Access (OWA) for connection glitches and check email queues for unusual delays or halts.
- Apply the registry edit recommended by Microsoft to increase the Endpoint Lookup call limit if you experience performance issues.
- Avoid uninstalling the October 2026 Windows Server updates, as Microsoft advises against this action while investigating a permanent fix.
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 Exchange Server security updates?
No, Microsoft stated the problem is related to general Windows Server updates, not the specific October 2026 security updates for Exchange Server.
Which Windows Server versions are affected?
The issue can impact Exchange Servers running on Windows Server 2016, 2019, 2022, and 2025.
What is the recommended workaround?
Microsoft advises applying a registry edit to increase the memory limit for Endpoint Mapper, rather than uninstalling the updates.
