Microsoft’s October 13, 2026 security updates for Windows Server are causing serious problems for some Microsoft Exchange servers. Reports began surfacing on October 15, with administrators saying that email delivery and access to Outlook on the web can stop working roughly 24 hours after the updates are installed.
Affected servers may show this error:
MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)Although Exchange databases may still appear mounted in the Exchange Admin Center, database services can become unresponsive. Users then lose access to Exchange, and messages can get stuck in the mail queue. Restarting the server has reportedly restored service, but only temporarily.
Microsoft has acknowledged the issue and is investigating it. Reports describe problems on on-premises Exchange servers and Exchange Online.
Administrators have also discussed an unofficial registry change that may prevent Exchange from hanging, though its effect has not been explained. A German-language analysis suggests possible causes and describes additional workarounds, including a PowerShell script. These approaches are not official Microsoft fixes. Microsoft’s Exchange update discussion contains the company’s updates on the issue.