Microsoft fixes Exchange sign-in issues caused by Windows Server updates

November updates resolve the problem

Microsoft has fixed Outlook sign-in and mail delivery problems on Exchange Server systems that appeared after the October 2026 Windows Server security updates. According to BleepingComputer, the company confirmed in an update to the Windows release health dashboard that the November 2026 Patch Tuesday updates, KB5131080 and KB5131090, address the issue.

Microsoft advised affected customers to install the November updates on all affected systems. The company first acknowledged the problem in mid-October, after users reported trouble signing in to Outlook or reaching their email.

Why the updates broke mail flow

The fault stems from a change introduced by the October 2026 updates. Microsoft added a new default limit on the amount of memory that the Endpoint Mapper, a Windows component that handles Endpoint Lookup calls, can use.

On busy Exchange servers, that limit could be reached, disrupting sign-ins and mail flow. Microsoft noted that not all servers were affected equally, and that the severity and speed of impact varied with server load. Some servers, it said, might never show a problem.

Keep the October security fixes

Microsoft stressed that this known issue is not linked to the October 2026 Exchange Server security updates. The company urged customers not to uninstall those updates.

This matters because the Exchange updates patch security vulnerabilities. Removing them would leave servers exposed while doing nothing to fix the sign-in problem, which lies in the Windows Server updates instead.

Workaround for admins who cannot update yet

For organisations that cannot immediately deploy the November cumulative updates, Microsoft shared a temporary workaround. It involves modifying a registry key on the affected Exchange Server to raise the maximum number of Endpoint Lookup calls and the memory available to the Endpoint Mapper.

Administrators run the change from an elevated Command Prompt, then restart the server. After the reboot, Microsoft recommends monitoring the Exchange environment and raising the Endpoint Mapper memory limit again if the issue recurs.

The simplest path, however, is to install KB5131080 or KB5131090, which remove the need for the workaround.

What administrators should do now

IT teams running Exchange Server on Windows Server 2019 or later should check whether their systems received the October 2026 security updates. If users reported Outlook sign-in failures or stalled mail delivery, the November Patch Tuesday updates are the permanent fix.

Where deployment must wait, the registry workaround offers relief, but it should be treated as a stopgap. Regular monitoring after any change remains advisable, since Microsoft itself noted that impact varied from server to server.