Microsoft has resolved Outlook sign-in and mail delivery problems on Exchange Server systems that appeared after the October 2026 Windows Server security updates. The issue affected some on-premises Exchange servers, and Microsoft said in mid-October that the impact varied by server load.
Exchange sign-ins and mail flow after October updates
Microsoft said the sign-in and mail delivery failures came from a new default memory limit for Endpoint Mapper when it handled Endpoint Lookup calls. The company tied the issue to the October 2026 updates, which changed that behavior on Windows Server 2019 and later. It said users could have trouble accessing email, signing in to Outlook on the web, or sending mail that stayed in the Outbox without delivery. Microsoft also said mailbox replication could be affected.
The company added that the problem did not hit all Exchange servers in the same way. Some servers could experience a problem quickly, while others might never show one. Microsoft also said the known issue was separate from the October 2026 Exchange Server security updates and told customers not to uninstall those updates.
November patches address the known issue
Microsoft later updated the Windows release health dashboard to say that the November 2026 Patch Tuesday updates, KB5131080 and KB5131090, address the known issue. It advised affected users to install those updates on all affected systems. For users who cannot deploy this month’s cumulative updates right away, Microsoft also shared a temporary workaround for Exchange Server systems.
The workaround requires changing a registry key on affected servers. Administrators must run reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f from an elevated Command Prompt on the Exchange Server, which increases the maximum number of Endpoint Lookup calls and the amount of memory Endpoint Mapper can use. After that, they must restart the device.
Once the system reboots, administrators should monitor the Exchange environment and raise the Endpoint Mapper memory limit if the issue happens again.
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