Symptoms
After you install the October 2026 updates for Windows Server, servers that run Microsoft Exchange Server become unstable and exhibit the following symptoms:
- Outlook on the Web (OWA) fails and returns the following error message:
MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142) - Exchange Server mail queues grow progressively throughout the day. This condition causes delayed email delivery. Eventually, the mail flow slows down or stops.
- Database Availability Group (DAG) members can't communicate or establish a quorum.
Note
Any version of Exchange Server that runs on affected versions of Windows Server could be affected. The following Windows Server updates can cause the listed failures:
- KB5126646 (Windows Server 2025)
- KB5126655 (Windows Server 2022)
- KB5126651 (Windows Server 2019)
- KB5126868 (Windows Server 2016)
The impact of the October updates varies across Exchange servers, depending on the server workload. The severity and rate of issue progression differs from server to server. Some servers might not experience any of these symptoms.
Workaround
Do not uninstall the October 2026 Windows Server updates.
Run the following command in an elevated Command Prompt window on each affected Exchange server:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /fRestart the server.
After you restart the server, monitor the environment to determine whether the issue is resolved. If the issue reoccurs, increase the limit.
More information
For more information about Windows Server known issues and notifications, see Users might be unable to sign in to OWA or send emails on Exchange Server.
Microsoft is investigating this issue and will update this article as more information becomes available.