Microsoft has fixed Outlook sign-in and email delivery problems affecting some on-premises Exchange servers after the October 2026 Windows Server security updates, BleepingComputer reported on November 11. The November cumulative updates address the issue, giving administrators a fix for failures that can prevent users from accessing or sending email.
The problem affects Exchange systems running on Windows Server 2019 and later with the October Windows security updates installed. Microsoft acknowledged it in mid-October. The impact depends on server load: the severity and speed at which problems appear vary, and some servers may never encounter them.
Users can be unable to sign in to Outlook on the web, also called OWA. Messages sent from Outlook can remain in the Outbox without being delivered, while Exchange can also have trouble accessing mailboxes during mailbox replication, the process of copying mailbox data.
Why the October updates disrupted email
Microsoft traced the failures to a new default memory limit introduced by the October 2026 Windows Server updates. The limit applies to Endpoint Mapper, a Windows component that helps software locate services, when it handles requests called Endpoint Lookup calls.
That change can interfere with Exchange operations on affected servers. For users, a visible symptom is an email that appears to have been sent but stays in Outlook’s Outbox.
Microsoft said the Outlook on the web issue is unrelated to the October 2026 Exchange Server security updates and urged customers not to uninstall those updates. The distinction matters when administrators decide which updates to deploy or troubleshoot.
Install the November fix or apply the workaround
In an update to its Windows release health dashboard, Microsoft identified the November 2026 Patch Tuesday updates KB5131080 and KB5131090 as resolving the issue. It advised installing them on all affected systems.
For administrators who cannot immediately deploy the cumulative updates, Microsoft provided a temporary registry workaround. On each affected Exchange server, run this command from an elevated Command Prompt, then restart the device:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f
The change increases the maximum number of Endpoint Lookup calls and the memory Endpoint Mapper can use. After restarting, administrators should monitor the Exchange environment and, if the problem recurs, raise the Endpoint Mapper memory limit, Microsoft advised.
