Microsoft Exchange Server experiences outages following Windows updates

Microsoft has confirmed that Windows Server security updates released in October cause significant malfunctions in local Exchange Server environments. The issue affects Exchange Server 2016, 2019, and the Subscription Edition running on Windows Server 2016, 2019, 2022, and 2025.

Symptoms include login failures for Outlook on the Web (OWA) with the error 'MapiExceptionMdbOffline' (Event ID 1006) and mail queues that eventually halt all message flow. The root cause is a new limitation introduced by the updates that restricts the RPC Endpoint Mapper to a maximum of 512 simultaneous contexts. Under regular workloads, Exchange Server quickly exceeds this limit.

Microsoft has not yet provided a timeline for a corrective update. As a temporary workaround, administrators are advised to manually adjust the registry. Setting the 'EpmapSavedContextCount' value to 65536 under the path 'HKLM\SOFTWARE\Microsoft\Rpc' and restarting the server is recommended to restore stability.