Microsoft Exchange servers malfunctioning after installing Windows Server October 2026 security updates

Microsoft Exchange Server is Microsoft's messaging and collaboration solution, deployed on Windows Server servers. It relies on the Windows Endpoint Mapper (RPC) service to establish communications between its different components. Microsoft has confirmed that the October 13, 2026 Windows Security Update introduces a default limit on memory used by the Endpoint Mapper for processing Endpoint Lookup calls, set at 512 concurrent calls. On busy Exchange servers, this limit may be insufficient and result in failures to connect to Outlook on the web, messages remaining in the outbox without delivery, growth of message queues, and difficulty accessing mailboxes when they are replicated. In certain configurations, the error “MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)” and event 1006 are logged in the Application log. Microsoft specifies that servers are not affected uniformly, the severity and speed of appearance of the malfunction depends on the load, some servers may never encounter it. This is not a vulnerability but an indirect consequence of applying a security patch. Microsoft says that Exchange security updates released in October 2026 are not the cause of the issue, that their installation is not necessary for it to occur, and recommends against uninstalling the updates. The issue has been in “mitigated” status since October 16, 2026, with a definitive fix still to come.

• Windows Server 2016, 2019, 2022 and 2025 servers that received the October 13, 2026 security update, namely KB5126868 (build 14393.9612), KB5126651 (build 17763.9376), KB5126655 (build 20348.5761) and KB5126646 (build 26100.33650) respectively;
• Microsoft Exchange Server 2016, 2019 and Subscription Edition servers installed on these systems, with Microsoft specifying that any version of Exchange running on an affected version of Windows may be affected;
• No client edition of Windows is affected;

RECOMMENDED ACTIONS:

• Do not uninstall Windows October 2026 Security Updates;
• Apply the temporary workaround recommended by Microsoft to the affected Exchange servers, from a command prompt in administrator mode, then restart the server: reg add “HKLM\SOFTWARE\Microsoft\Rpc” /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f;
• Monitor the operation of Exchange after restart and increase again this t reappears;
• Request Microsoft Support as additional mitigation is available for affected environments;
• Monitor the Windows Release Health Dashboard while awaiting a final investigative fix;