Microsoft has confirmed that this month’s Windows Server security updates can cause sign-in and mail delivery problems on some on-premises Exchange servers. On the Windows release health dashboard it says users may be unable to sign in to Outlook on the web (OWA), messages sent from Outlook may sit in the Outbox, and Exchange may have trouble accessing mailboxes during mailbox replication. Some servers show the OWA error “MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)” with Event ID 1006 in the application log, and growing mail queues are another reported symptom.
Microsoft’s explanation is that the October 2026 update added a default limit on the memory Endpoint Mapper uses for Endpoint Lookup calls, and the default of 512 simultaneous calls may not be enough for Exchange under heavy load. It stresses that the problem is not caused by the October 2026 Exchange security updates and tells customers not to uninstall them. The dashboard entry for Windows Server 2025 is tied to KB5126646 and lists Windows Server 2025, 2022, 2019 and 2016 as affected platforms; BleepingComputer describes it as Windows Server 2019 and later. Microsoft says not all servers are hit equally and some may never see a problem. It is still investigating.
How to check if you’re affected
Affected products are on-premises Exchange Server systems running on Windows Server with the October 2026 Windows security update installed; Microsoft says any version of Exchange on an impacted Windows version could be affected, and that the severity depends on server load. Check for the OWA error and Event ID 1006 above, growing queues, and Outlook messages stuck in the Outbox.
Microsoft’s temporary workaround, as quoted by BleepingComputer, is to run this from an elevated Command Prompt on the Exchange server and then reboot the device: reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f. Afterwards, monitor the environment and raise the limit further if the symptoms return. Microsoft also offers an additional mitigation through Microsoft Support for business. Review Microsoft’s release health entry and Exchange team blog before changing production servers.
Sources
- Microsoft Windows release health: Users might be unable to sign in to OWA or send emails on Exchange Server
- Microsoft Exchange Team: Exchange servers may become unstable after installation of October 2026 Windows updates
- BleepingComputer: Microsoft: October Windows Server updates break Exchange sign-in, mail flow