Microsoft is warning admins about Exchange Server instability after the October 2026 Patch Tuesday cycle. Since October 13, some on-premises servers have started failing under load. However, the culprit is not an Exchange patch. Instead, the problem comes from the October 2026 Windows updates. What Admins Are Seeing The symptoms vary. For example, OWA may fail with the error "MapiExceptionMdbOffline: Unable to make connection to the server." Event ID 1006 then shows up in the Application Log.