Microsoft warns that the October 2026 Windows Server updates may destabilize on-premises Exchange servers. OWA (Outlook on the Web) logins fail, and mail queues back up. A registry adjustment resolves the issue. Microsoft advises against uninstalling the updates.
Since Patch Tuesday on October 13, Microsoft has been receiving reports from customers whose Exchange servers have become unstable after the monthly updates. The culprit is not the October Exchange update itself. The issue stems from cumulative Windows Server updates. As a result, any version of Exchange running on an affected Windows version may experience problems. This applies to Exchange Server 2016, 2019, and the Subscription Edition (SE).
Four updates are involved: KB5126646 for Windows Server 2025, KB5126655 for Windows Server 2022, KB5126651 for Windows Server 2019, and KB5126868 for Windows Server 2016. The symptoms vary. Outlook on the web may crash with the error message MapiExceptionMdbOffline (ec=1142), causing event ID 1006 to appear in the Application Log. Mail queues may also build up over the day until mail flow virtually comes to a standstill. A restart helps, but only temporarily.
Not every server is affected equally. According to Microsoft, the issue’s severity and rate depend on the load on the Exchange server. Some servers are never affected at all.
Limit on RPC contexts
What exactly is going wrong? According to the Windows release health page, the updates limit the number of concurrently stored RPC Endpoint Mapper contexts to 512 by default. Exchange continues to accumulate these contexts under load. As a result, RPC connections eventually fail.
Microsoft strongly advises against uninstalling the Windows updates. Instead, administrators should use an elevated command prompt to add the registry value `EpmapSavedContextCap` under `HKLMSOFTWAREMicrosoftRpc`, with a value of 65536. This is followed by a restart. If the problem recurs, you must raise the limit further. “This has been shown to solve the problem based on customer reported cases that we have assisted to date,” writes the Exchange team.
Tip: Windows Server 2022 will lose mainstream support next month