Microsoft probes Exchange Server outages triggered by October Windows updates

Microsoft has confirmed that its October 2026 Windows Server updates are disrupting on-premises Exchange Server environments. The flaw impacts Exchange Server 2016, 2019, and the Subscription Edition running on Windows Server 2016, 2019, 2022, and 2025. Affected organizations have reported halting mail deliveries, trapped Outbox messages, and Outlook on the web sign-in failures displaying Event ID 1006. [1][2]

The disruptions stem from an operating system change rather than Exchange patches. Microsoft explained that the update introduced a default ceiling of 512 simultaneous Endpoint Lookup calls for the Endpoint Mapper service, a threshold that proves insufficient under heavy Exchange workloads. While Microsoft investigates a permanent fix, it strongly advises administrators against uninstalling the updates. Instead, the company published a temporary workaround that requires administrators to manually edit the Windows registry to increase the Endpoint Mapper memory limit and reboot. [1][2]

The bug drew immediate backlash from systems administrators who spent their weekends restarting servers every few hours to keep mail flowing. Microsoft emphasized that impact varies widely depending on server load, meaning some systems may run without symptoms. The company has not yet committed to releasing an out-of-band emergency patch. [1]