October Windows Server patches disrupt Exchange email access and sign-in

Microsoft confirmed through its Windows release health dashboard that the most recent Windows Server security patches are causing problems with Exchange Server email access and authentication on systems running Windows Server 2019 and later versions.

Impacted users report being unable to sign into Outlook on the web (OWA) or send messages, which remain stuck in the Outbox. Some systems are displaying the error "MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)" alongside EventID 1006 entries in the application event log.

After installing the October 2026 Windows security update [..], some on-premises Exchange servers might experience issues accessing or sending email. Users might be unable to sign in to Outlook on the web (OWA), or email messages sent from Outlook might remain in the Outbox and not be delivered. Additionally, Exchange might experience issues accessing mailboxes during mailbox replication.

Microsoft

Root cause and scope

The October 2026 update introduced a new default constraint limiting the memory available to Endpoint Mapper during Endpoint Lookup operations. For Exchange Server workloads operating under substantial load, the preset ceiling of 512 concurrent Endpoint Lookup calls proves insufficient.

The October 2026 update introduced a default limit on the amount of memory used by Endpoint Mapper when handling Endpoint Lookup calls. For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls might not be sufficient under heavy load. Administrators can use this registry setting to increase the maximum number of simultaneous Endpoint Lookup calls, which increases the amount of memory that Endpoint Mapper can use.

Microsoft

Microsoft clarified that the problems stem from the Windows Server patches, not from the October 2026 Exchange Server security updates themselves, and advised customers against removing either update.

Issues reported are not related to installation of October 2026 security updates for Exchange Server. This is not a 'known issue' related to October 2026 Exchange update. Any version of Exchange that runs on impacted versions of Windows could be impacted.

Microsoft

The impact varies significantly across deployments. Severity and onset depend on individual Exchange server load characteristics, with some systems potentially experiencing no problems whatsoever.

Not all Exchange servers will be impacted by installation of those updates equally; the severity and speed of impact varies based on Exchange server load. Some servers might never see a problem.

Microsoft

Temporary workaround

While developing a permanent resolution, Microsoft released a temporary fix involving a registry modification on affected Exchange Server systems. Administrators should execute the following command from an elevated Command Prompt and then restart the server:

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

Following the reboot, administrators should monitor their Exchange environment closely and increase the Endpoint Mapper memory limit further if the issue resurfaces.

Extended support timeline

Separately, Microsoft announced in July that extended security updates for Exchange 2016 and 2019 will conclude this month. The company had previously granted a six-month extension to the Extended Security Update program in April 2026 beyond the original termination date.