GoKawiilMicrosoft confirms October Windows Server updates break Exchange mail flow, OWA sign-in
Microsoft has acknowledged that its October 2026 Windows Server security updates are causing some on-premises Exchange servers to fail at delivering mail or authenticating Outlook on the web users. Affected systems can show a MapiExceptionMdbOffline error and Event ID 1006, traced to a new default cap on simultaneous Endpoint Mapper lookup calls that proves too low for Exchange under heavy load. Microsoft says the problem stems from the Windows Server update itself, not the separate Exchange security patches, and is advising admins to raise a registry limit rather than uninstall updates.