Microsoft Confirms October 2026 Patch Tuesday Updates Are Breaking Exchange Servers

Microsoft has confirmed that the October 2026 Windows Patch Tuesday updates are causing stability problems on some Exchange Server deployments, leading to Outlook on the web (OWA) failures, growing email queues, and database availability group (DAG) quorum issues.

According to a Microsoft advisory, the problems affect Exchange Server 2016, 2019, and Subscription Edition (SE). Microsoft believes the issues relate to Windows Server updates rather than Exchange Server security updates.

Which Windows Server updates are affected?

Microsoft has identified four October 2026 security updates associated with Exchange Server instability:

• KB5126646: Windows Server 2025
• KB5126655: Windows Server 2022
• KB5126651: Windows Server 2019
• KB5126868: Windows Server 2016

Affected organizations may experience OWA failures with the following error message:

MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)

Administrators may also find Event ID 1006 in the Windows Application Log, notice email queues growing unexpectedly, or encounter DAG members failing to establish quorum.

Restarting affected servers can temporarily restore functionality, but the problems may return.

Microsoft provides a temporary workaround

Microsoft recommends keeping the October 2026 Windows Server security updates installed rather than uninstalling them.

To address the instability, administrators should open an elevated Command Prompt on affected Windows Servers and run the following command:

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

After applying the registry change, administrators should restart the affected environment and monitor Exchange Server stability.

If the problems continue, Microsoft advises increasing the registry value further based on the server workload.

The company has not yet identified the exact underlying cause or announced a permanent fix.

Meanwhile, Microsoft’s October updates have also brought changes and problems for Windows 11 users. The company fixed game and app crashes with KB5127648, although the same update has also triggered installation error 0x800f0845 and other issues on some systems.

Via Neowin