Update October 16, 2026: For more information related to installation of October 2026 updates for Windows Server and the possible impact on Exchange Server, please see Exchange servers may become uns...
Updated Oct 16, 2026
Version 7.0
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I set up a controlled lab environment with Exchange Server SE RTM with September SU10 V2 on Windows Server 2022, initially patched to the August 2026 CU (KB5120242).
I then installed the October Windows Server update (KB5126655)
Approximately 12 hours later, I haven't experienced any of the reported issues. OWA, Outlook MAPI/HTTP, database access, and mail flow are working normally. No MapiExceptionMdbOffline errors or related MRS Event ID 1006 have been observed.
The server was installed with English (en-US) Windows and initially used en-US system locale. I also tested with de-DE system locale, without reproducing the issue.
I'm wondering whether OS language, system locale, or Exchange language settings could be contributing factors.
Another possibility worth investigating is third-party antimalware or EDR software. I've seen cases in customer environments where security products interfered with software update installation or uninstallation processes.
Could something similar be happening here, potentially affecting how Windows updates are installed or how Exchange behaves afterward? It might be worth reviewing update installation logs and comparing the security products running in affected environments.
I'd be interested to hear what others are seeing, particularly regarding language settings and installed security software.
I have a 3 node DAG with the issue, I installed all 3 updates (CU, .NET, Exchange SU). Not running any software on the servers other than exchange, and just using default windows defender. Check the mail queue on your servers, I found even before the db offline error the queue starts rising. Rebooted yesterday morning, same issue recurred today.
ceyhunkirmizitas
Have you also installed the .Net 4.8 update as well (2026-10 .NET Framework Security Update (KB5127690))? Or just the Windows patch?I installed .NET Framework 4.8.1 during the initial server setup, but I haven't installed the October 2026 .NET cumulative update (KB5128928). It is currently listed as pending installation in Windows Update.
Only the October Windows Server 2022 update (KB5126655) was installed.
Thanks for the answer.
