Microsoft Confirms October Windows Server Updates Disrupt Exchange Functionality

Microsoft has acknowledged that the recent Windows Server security updates are causing issues with OWA sign-in and email delivery on certain Exchange Server systems.

In an update to the Windows release health dashboard over the weekend, Microsoft stated that users may experience difficulties accessing email or signing in to Outlook after applying the latest updates on systems running Windows Server 2019 and later.

Affected customers are also reporting that Exchange Server OWA fails with a “MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)” error and Event ID 1006 in the application log.

According to Microsoft, “After installing the October 2026 Windows security update, some on-premises Exchange servers might experience issues accessing or sending email. Users might be unable to sign in to Outlook on the web (OWA), or email messages sent from Outlook might remain in the Outbox and not be delivered. Additionally, Exchange might experience issues accessing mailboxes during mailbox replication.”

The company explained that the October 2026 update introduced a default limit on the amount of memory used by Endpoint Mapper when handling Endpoint Lookup calls. For workloads like Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls may be insufficient under heavy load. Administrators can adjust a registry setting to increase the maximum number of simultaneous Endpoint Lookup calls, thereby allowing more memory for Endpoint Mapper.

However, Microsoft clarified that these known issues are not related to the October 2026 Exchange Server security updates and advised customers against uninstalling them. “Issues reported are not related to the installation of October 2026 security updates for Exchange Server. This is not a ‘known issue’ related to the October 2026 Exchange update. Any version of Exchange that runs on impacted versions of Windows could be affected,” Microsoft stated in a separate blog post.

It added, “Not all Exchange servers will be impacted by the installation of those updates equally; the severity and speed of impact varies based on Exchange server load. Some servers might never see a problem.”

While working on a permanent fix, Microsoft has provided a temporary workaround that involves modifying a registry key on affected Exchange Server systems. To implement this, users need to execute the following command from an elevated Command Prompt on the Exchange Server and then reboot the device:

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

After the system restarts, customers should monitor the Exchange environment and consider raising the Endpoint Mapper memory limit if issues persist.

In July, Microsoft announced that it would cease shipping extended security updates for Exchange 2016 and 2019 this month, following a six-month extension that was announced in April 2026 to the original Extended Security Update (ESU) program.

Source: BleepingComputer