October updates take down Exchange servers

Installing Windows Server’s October updates can cause outages on Exchange servers. Microsoft reports that Outlook on the web (OWA) becomes inaccessible and email queues grow. The Exchange Server security updates released on the same Patch Tuesday do not need to be installed for this to happen. Microsoft is investigating the connection to the Windows Server updates; the company has not yet identified a confirmed cause.

Microsoft has identified four Windows Server updates that may be affected: KB5126646 for Windows Server 2025, KB5126655 for Server 2022, KB5126651 for Server 2019 and KB5126868 for Server 2016. The updates were released as part of Microsoft’s October Patch Tuesday.

OWA errors and email queues

In its notice about the October security updates, Microsoft’s Exchange team describes the OWA error as MapiExceptionMdbOffline: Unable to make connection to the server with the codes hr=0x80004005, ec=1142. Microsoft says event ID 1006 also appears in the application log. At the same time, emails can pile up in the queues. Microsoft added the warning on Oct. 15, 2026.

A Microsoft employee said in the comments on Oct. 15 that there was no workaround at that point. Microsoft was continuing to investigate the issue.

In addition to the Windows Server updates, Microsoft released security updates for Exchange Server SE, 2019 and 2016 in October. They address vulnerabilities in Exchange itself. Exchange Server 2016 and 2019 require the second phase of the paid ESU program.

(fo)