Summary
Microsoft has fixed Outlook sign-in and mail delivery problems on on-premises Exchange Server systems that appeared after the October 2026 Windows Server security updates.
Symptoms
- Users cannot sign in to Outlook on the web (OWA)
- Messages sent from Outlook stay in the Outbox
- Exchange has trouble accessing mailboxes during mailbox replication
Impact varies with server load, and some servers may never be affected. The issue applies to Exchange running on Windows Server 2019 and later.
Cause
The October 2026 updates introduced a new default limit on the memory the Endpoint Mapper uses when handling Endpoint Lookup calls.
Fix
The November 2026 Patch Tuesday updates, KB5131080 and KB5131090, address the issue. Microsoft advises installing them on all affected systems.
Workaround
If you cannot deploy the November updates yet, Microsoft suggests running this from an elevated Command Prompt on the Exchange server, then restarting:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f
This raises the maximum number of Endpoint Lookup calls and the Endpoint Mapper memory limit. Monitor the environment and raise the value again if the problem recurs.
Microsoft says the OWA issue is not linked to the October 2026 Exchange security updates, and advises customers not to uninstall them.