Microsoft has acknowledged that the latest Windows Server security updates are causing sign-in and mail delivery issues for certain Exchange Server systems, particularly affecting users of Windows Server 2019 and beyond. Affected customers report encountering a "MapiExceptionMdbOffline" error and Event ID 1006 in application logs. The October 2026 Windows security update has introduced a default limit on memory allocated to Endpoint Mapper, which may be insufficient under heavy load for Exchange Server workloads. Microsoft clarified that the reported issues are not related to the October 2026 Exchange server security updates and advised against uninstalling them. A temporary workaround involves modifying a registry key to increase the Endpoint Mapper memory limit. Additionally, Microsoft announced it would cease shipping extended security updates for Exchange 2016 and 2019 this month.