Microsoft Issues Mitigation for Exchange Server Issues Caused by October Windows Updates
Petri, Monday, October 19th, 2026
October Windows Server updates destabilized on-premises Exchange, and Microsoft offers a registry fix that raises Endpoint Mapper limits.
After installing October 2026 Windows Server updates, organizations running on-premises Exchange Server reported Outlook on the web failures, growing mail queues, inconsistent access and database availability group problems.
Microsoft traced the issue to a limit on simultaneous Endpoint Mapper requests, set by default at 512, which heavier Exchange workloads can exceed.
It has published a registry-based mitigation that raises capacity while it works on a permanent fix.
The bug affects Exchange on Windows Server 2016, 2019, 2022 and 2025.