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
Blog Post
We can also confirm the same situation with Exchange 2019.
The update was installed on 10/13/2026 just before 10 PM. Users reported the error on 10/15/2026 just before 8 AM. Outlook Classic connected to Exchange normally, but there was no new mail, and all sent mail remained in the Drafts folder. Outlook Classic did not report any errors. However, the error occurred with OWA users. They were able to log in without any problems, but when it should show the mailbox, they received the error:
X-ClientId: 57FB5E19206A44B3B2291C6889FD202B request-id 41b68bf3-0246-49c8-9749-da1dd7c59029
X-OWA-Error Microsoft.Exchange.Data.Storage.MailboxOfflineException
X-OWA-Version 15.2.1748.39
X-FEServer [ExchSrvName]
X-BEServer [ExchSrvName]
Date:10/15/2026 6:25:33 AM InnerException: Microsoft.Mapi.MapiExceptionMdbOffline"
All databases were mounted and there was no error in the Exchange Admin Center. All Exchange services were up and running. Restarting the server fixed the error, but only temporarily. Today, 10/16/2026, users reported the same issues again. We have now removed update KB5126651 (Windows Server 2019). I will test over the weekend and report back if this resolved the issue.