Some on-premises Exchange servers may lose Outlook on the web access or see email queues grow after installing October 2026 updates for Windows Server, Microsoft’s Exchange team says. The company stresses that this is associated with the Windows Server updates, not with the October Exchange Server security update. Installing that Exchange update is not required for the problem to occur.
Which systems and symptoms are involved?
Microsoft names KB5126646 for Windows Server 2025, KB5126655 for Server 2022, KB5126651 for Server 2019, and KB5126868 for Server 2016. Exchange Server 2016, 2019, and Subscription Edition can be affected when running on an impacted Windows Server version. Not every deployment will experience the issue, and its severity can vary with server load.
Reported symptoms include Outlook on the web failing with MapiExceptionMdbOffline and event ID 1006, mail accumulating in queues or remaining in the Outlook Outbox, and database availability group members failing to reach quorum. A restart may temporarily improve service, but Microsoft says administrators should watch for recurrence. The Windows Server 2025 update’s known-issues page also documents the OWA and mail-delivery symptoms.
Microsoft’s recommended response
Microsoft links the behavior to a new default limit on memory used by Endpoint Mapper for Endpoint Lookup calls. On a busy Exchange server, the default cap of 512 simultaneous calls may be insufficient. Its published workaround raises the EpmapSavedContextCap registry value under HKLM\SOFTWARE\Microsoft\Rpc to 65536, followed by a reboot and monitoring. Administrators should follow the official instructions for the exact command and escalation path.
Microsoft explicitly advises against uninstalling the October Windows Server security updates as the first response. Its support documentation says an additional mitigation may be available through Microsoft Support for affected devices. The company was still investigating the issue in the latest advisory reviewed for this report.
For operations teams, the immediate check is to compare Windows update history with OWA availability, mail queues, and Exchange event logs. A server that received the update but shows no symptoms does not necessarily need an emergency intervention. An affected server needs the documented mitigation, a controlled reboot, and continued monitoring for a return of the problem.