Microsoft fixes Exchange sign-in and mail delivery failures caused by October Windows Server updates
Microsoft said on November 11 that its November 2026 Patch Tuesday updates fix sign-in and mail delivery problems that affected some on-premises Exchange Server systems after the October Windows Server security updates. The issue affected systems running Windows Server 2019 and later, but Microsoft said that not every Exchange server would be affected.
The October updates introduced a new default memory limit for Endpoint Mapper when it handled Endpoint Lookup calls, Microsoft said. On affected servers, users could be unable to sign in to Outlook on the web, messages could remain in Outlook's Outbox, and Exchange could have trouble accessing mailboxes during replication.
Microsoft identified KB5131080 and KB5131090 as the November updates that address the problem. The company advised administrators to install them on all affected systems and said they should not uninstall the October Exchange Server security updates because the Outlook on the web issue was not linked to those Exchange updates.
Administrators who cannot deploy the cumulative updates immediately can apply Microsoft's temporary workaround from an elevated Command Prompt on the Exchange Server. The change raises the Endpoint Mapper limit and requires a restart. Microsoft said administrators should monitor the Exchange environment afterward and raise the limit again if the problem returns.
The workaround command is: reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f