October 2026 Windows Server Updates Cause Exchange OWA Sign-In and Email Delivery Issues

Microsoft has confirmed that the October 2026 Windows Server security updates are causing Outlook on the web (OWA) sign-in and email delivery problems on some on-premises Exchange Server systems.

Exchange Server users may be unable to access email

In a health dashboard update for new Windows releases, Microsoft said that systems running Windows Server 2019 or later may experience issues after installing the latest security update. Users may be unable to access email, sign in to Outlook on the web, or send messages successfully.

In some cases, messages sent from Outlook remain in the outbox and are not delivered. Exchange Server may also have trouble accessing mailboxes during mailbox replication.

Affected Exchange Server systems may display the following error when OWA fails:

MapiExceptionMdbOffline: Unable to connect to server. (hr=0x80004005, ec=1142)

Administrators may also see Application log event ID 1006.

Microsoft described the issue in its Windows Server release health dashboard:

“After you install the October 2026 Windows Security Updates […] some on-premises Exchange servers can have issues accessing or sending email. Users may not be able to sign in to Outlook on the web (OWA), or email messages sent from Outlook may remain in the outbox and not be delivered. Additionally, Exchange can have problems accessing mailboxes during mailbox replication.”

Endpoint mapper limit is linked to the Exchange issues

Microsoft said the October 2026 update introduced a default limit on the memory used by the endpoint mapper when processing endpoint lookup calls. Exchange Server workloads may exceed the default limit of 512 concurrent endpoint lookup calls during heavy loads.

Administrators can increase the maximum number of concurrent endpoint lookup calls by changing a Windows Registry setting. Microsoft said this increases the amount of memory available to the endpoint mapper.

The company also clarified that the Exchange problems are not related to the October 2026 Exchange Server security update. All Exchange Server versions running on affected Windows Server versions may be impacted, although the severity and speed of the issue will vary based on server load.

“Not all Exchange servers will be affected equally by the installation of these updates. The severity and speed of the impact will vary depending on the load on the Exchange server. Some servers may not experience the issue,” Microsoft said in a separate Exchange Server update.

Microsoft urged customers not to uninstall the October 2026 Exchange Server security update because the reported problems are caused by the Windows Server update rather than the Exchange update.

Microsoft shares temporary Exchange Server workaround

While a permanent fix is in development, Microsoft has provided a temporary workaround for affected Exchange Server systems. Administrators must run the following command from an elevated command prompt and then restart the server:

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

After restarting the system, administrators should monitor the Exchange environment and increase the endpoint mapper memory limit if the issue returns.

Exchange 2016 and 2019 ESU updates

Microsoft also announced that it will stop distributing Extended Security Updates for Exchange 2016 and Exchange 2019 this month. The company previously announced a six-month extension in April 2026 to the original Extended Security Updates (ESU) program.

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