MICROSOFT warns that Exchange Server instability observed after October 2026 Patch Tuesday is linked to Windows updates, not an Exchange patch. Since 13 October 2026, some on‑premises Exchange servers under heavy load have begun experiencing failures, with symptoms such as OWA errors (for example, MapiExceptionMdbOffline: Unable to make connection to the server) and Event ID 1006 in the Application Log, alongside growing mail queues and, on some DAG members, failure to reach quorum. The issue affects Exchange Server 2016, 2019, and Subscription Edition, though reboot can provide only a temporary reprieve.
The root cause, per Microsoft’s Exchange Team advisory, is a new default memory cap on Endpoint Mapper introduced by the October 2026 Windows updates (KB5126646, KB5126655, KB5126651, KB5126868). The cap of 512 simultaneous lookup calls may be insufficient under heavy load, explaining why busy servers are hit hardest while others remain unaffected. The guidance is not to uninstall the Windows updates.
Instead, administrators are advised to apply a registry change from an elevated prompt: reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f, then reboot and monitor; if instability recurs, raise the limit further. Microsoft offers an additional mitigation through Microsoft Support for business. The article emphasises keeping security patches in place and acting promptly to preserve mail flow.