Admin: 'I spent all weekend on my servers' after Microsoft breaks Exchange Server with October updates
OWA borked and lengthy email queues reported on busy servers after Microsoft tweaks a Server default
Microsoft has revealed that their October 2026 Windows Server updates may have caused issues with Exchange Server. The problem stems from a default memory limit on Endpoint Mapper when handling Endpoint Lookup calls. This bug can affect Exchange Servers running Windows Server 2016, 2019, 2022, and 2025. The instability may not be immediate and can result in connection glitches or lengthy email queues.
Rebooting the server might provide a temporary fix, but a registry edit is recommended for a permanent solution. Microsoft strongly advises against uninstalling the updates, as the severity and speed of impact vary depending on the Exchange server load. Some servers may never experience any problems. Microsoft is investigating the issue and plans to provide more information as it becomes available.
Customers have expressed frustration, stating that the workaround only temporarily solved the issue and that Microsoft should have tested the updates before releasing them.
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