Microsoft has confirmed that its October 2026 Windows Server security updates are disrupting Exchange Server sign-in and mail delivery on some on-premises deployments, a problem that stems from a new memory cap in the Endpoint Mapper, not from the Exchange-specific patches themselves. BleepingComputer reported the issue following a Windows release health dashboard update over the weekend.
According to Microsoft, systems running Windows Server 2019 and later that have installed the October 2026 update may find that users cannot sign in to Outlook on the web (OWA), that outbound messages remain stuck in the Outbox, and that Exchange encounters problems accessing mailboxes during replication. In some cases, administrators are also seeing Exchange OWA fail with a “MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)” error, accompanied by Event ID 1006 in the application log.
What is causing the problem
Microsoft explained that the October 2026 Windows security update introduced a default cap on the memory Endpoint Mapper can use when handling Endpoint Lookup calls. The default limit of 512 simultaneous Endpoint Lookup calls can be insufficient for workloads such as Exchange Server under heavy load, Microsoft said.
The company was clear that the fault lies with the Windows Server update, not with the Exchange Server security patches released at the same time. ‘Issues reported are not related to installation of October 2026 security updates for Exchange Server. This is not a “known issue” related to October 2026 Exchange update. Any version of Exchange that runs on impacted versions of Windows could be impacted,’ Microsoft wrote in a Microsoft Community Hub post from the Exchange Team. Microsoft also noted that impact varies by server load: ‘Not all Exchange servers will be impacted by installation of those updates equally; the severity and speed of impact varies based on Exchange server load. Some servers might never see a problem.’
Customer reports of instability began emerging from 13 October 2026, according to the Microsoft Community Hub post.
Workaround and what administrators should do
While a permanent fix is in development, Microsoft has published a registry-based workaround. Running the following command from an elevated Command Prompt on the affected Exchange Server, then rebooting, raises the Endpoint Mapper memory limit:
reg add “HKLM\SOFTWARE\Microsoft\Rpc” /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f
After the restart, Microsoft advises administrators to monitor the Exchange environment and raise the limit further if issues recur. Crucially (and the company was emphatic on this point) administrators should not uninstall the October 2026 Windows security updates while waiting for a fix.
Separately, Microsoft had previously announced that Extended Security Updates for Exchange 2016 and 2019 would end in October 2026, after a six-month extension announced in April 2026 to the original Extended Security Update programme. Administrators still running those versions have further reason to review their migration timelines alongside applying the workaround above.
Microsoft’s stated position is that it is working on a fix; in the meantime, the registry workaround and the guidance on the Windows release health dashboard represent the current recommended path for affected deployments.
