Microsoft has acknowledged that its latest Windows Server security updates, released on October 13, 2026, are causing disruptions in Exchange Server email services. These issues result in blocked webmail access and undelivered messages, affecting many users.
Impact on Exchange Server Systems
The problem affects certain on-premises systems running Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition. Microsoft has issued a workaround, urging administrators to refrain from uninstalling the Windows updates despite the issues.
The root of these failures lies in changes made to Windows Server, independent of the separate Exchange Server security updates released in October. Servers may encounter problems even without the latest Exchange patches, pinpointing the Windows Server update as the core issue.
Details of Affected Updates
Microsoft has identified four specific Windows updates associated with these failures: KB5126646 for Windows Server 2025, KB5126655 for Windows Server 2022, KB5126651 for Windows Server 2019, and KB5126868 for Windows Server 2016. The impact varies based on server workload, with some systems potentially remaining unaffected.
Users may face difficulties signing in to Outlook on the web, and messages sent via Outlook might remain in the Outbox. Administrators might observe increasing mail queues, failed mailbox access during replication, or Database Availability Group member issues. Restarting the server could temporarily resolve these problems.
Mitigation Strategies and Workaround
One error to monitor is MapiExceptionMdbOffline, indicating a server connection issue, often logged as Event ID 1006. Microsoft noted that the October updates set a default limit of 512 simultaneous Endpoint Lookup calls, controlling memory usage by the Endpoint Mapper, a component involved in remote procedure calls. High Exchange workloads may surpass this limit.
Adjusting the limit allows Endpoint Mapper to manage more memory. Microsoft classifies the issue as mitigated, not resolved, with ongoing investigations. Additional mitigation options are available via Microsoft Support for affected users.
Microsoft’s advisory instructs administrators to execute a registry modification through an elevated Command Prompt to address the issue. Following a server restart, administrators should monitor for persistent failures. If problems persist, further limit adjustments may be necessary, although specifics are not provided.
Maintaining current security updates is crucial. As Exchange 2016 and 2019 near the end of their paid update periods, with October 2026 marking their final scheduled security release, the importance of supported patches remains high.
