October Windows Server updates break Exchange sign-in and mail flow

Microsoft has acknowledged that October's Windows Server security updates are causing OWA sign-in and mail delivery problems on some Exchange Server systems. The company posted the advisory to its Windows release health dashboard over the weekend.

According to Microsoft, after installing the latest updates on Windows Server 2019 and later, users may be unable to access email or sign in to Outlook on the web. Messages sent from Outlook can remain stuck in the Outbox without being delivered, and Exchange itself may struggle to reach mailboxes during mailbox replication.

In some cases OWA fails with a "MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)" error, accompanied by Event ID 1006 in the application log.

The cause: an Endpoint Mapper memory limit

The root cause, Microsoft says, is that the October update introduced a default cap on the memory Endpoint Mapper uses when handling Endpoint Lookup calls. For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls may not be enough under heavy load.

The company stresses that these issues are not tied to the October Exchange Server security updates and urges customers not to uninstall them. Any version of Exchange running on an affected Windows build could be hit. Not every server will be affected equally, though: the severity and speed of impact depend on load, and some servers may never see a problem at all.

A temporary workaround

While a fix is in the works, Microsoft shared a registry workaround. On the Exchange Server, run the following from an elevated Command Prompt and then reboot:

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

After the restart, administrators are advised to monitor the environment and raise the Endpoint Mapper memory limit further if the issue returns.

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

Source: Bleeping Computer