Microsoft has fixed a known issue that disrupted Outlook sign-in and mail delivery on on-premises Exchange Server systems after the October 2026 Windows Server security updates were installed. The problem affected systems running Windows Server 2019 and later.

What broke

Microsoft attributed the issue to a new default limit on the memory that Endpoint Mapper uses when handling Endpoint Lookup calls, introduced by the October updates. Affected administrators reported that some users could not sign in to Outlook on the web (OWA), that messages sent from Outlook remained in the Outbox without being delivered, and that Exchange had trouble accessing mailboxes during mailbox replication.

Microsoft noted that impact varied with Exchange server load, and that some servers might never experience the problem. The company also said the OWA issue was not linked to the October Exchange server security updates, and it advised customers not to uninstall them.

Resolution

Microsoft’s update to the Windows release health dashboard states that the November 2026 Patch Tuesday updates, KB5131080 and KB5131090, address the known issue. Administrators are advised to install them on all affected systems.

For organizations that cannot deploy the November cumulative updates immediately, Microsoft published a temporary workaround that involves modifying a registry key on affected Exchange Server systems. Administrators 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

This command raises the maximum number of Endpoint Lookup calls and the memory Endpoint Mapper can use. After the reboot, administrators should monitor the Exchange environment and raise the Endpoint Mapper memory limit again if the problem recurs.

Takeaway for administrators

Teams running on-premises Exchange should confirm whether their October Windows Server patches are installed, check for Outlook on the web sign-in failures or stuck outgoing mail, and prioritize the November cumulative updates. Where patching must wait, the registry change offers a documented stopgap.