Microsoft has confirmed that the October 2026 Windows Server security updates are causing sign-in and mail delivery problems on some on-premises Exchange Server deployments. The issue was disclosed over the weekend via an update to the Windows release health dashboard.

Affected organizations running Windows Server 2019 and later may find users unable to sign in to Outlook on the web (OWA), with email messages sent from Outlook stuck in the Outbox and never delivered. Some administrators are also seeing Exchange fail during mailbox replication, alongside an OWA error reading “MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)” and Event ID 1006 entries in the application log.

Root cause: Endpoint Mapper memory limit

According to Microsoft, the October update introduced a default cap on the memory Endpoint Mapper can use when handling Endpoint Lookup calls. That default limit, 512 simultaneous Endpoint Lookup calls, can be insufficient for Exchange Server workloads under heavy load.

Microsoft stressed that the problem is not tied specifically to the October 2026 Exchange Server security updates themselves. Any version of Exchange running on an affected Windows version could be impacted, and the severity varies by server load, meaning some systems may show no symptoms at all.

Workaround and guidance

Microsoft is urging administrators not to uninstall the October 2026 Exchange security updates. Instead, it has published a temporary registry workaround. From an elevated Command Prompt on the affected Exchange Server, administrators can run:

  • reg add “HKLM\SOFTWARE\Microsoft\Rpc” /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

After applying the change, the server must be rebooted. Microsoft recommends monitoring the environment afterward and raising the Endpoint Mapper memory limit further if the issue recurs.

A permanent fix is still in development. This disruption comes as Microsoft separately confirmed it will end extended security updates for Exchange 2016 and 2019 this month, following a six-month extension to the original ESU program announced in April 2026.