Windows Server 2025 build 26100.33674 (KB5134137) — release notes, known issues

Windows Server · 2025 · Release · Out-of-band update

October 26, 2026—KB5134137 (OS Build 26100.33674) Out-of-band

Windows Server 2025: LTSC mainstream support through 2029-11-13

All 2025 builds JSON — this build from the builds API

Microsoft's known issues — KB5134137

Issue Known issue After installing this 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. In some affected configurations, Exchange Server OWA might fail with an error MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142) with Event ID 1006 logged in the application log. Another reported symptom is growing email queues. Installing the October 2026 Exchange server security updates isn’t requir Workaround: This issue can be temporarily mitigated by modifying the registry key as described in Exchange servers may become unstable after installation of October 2026 Windows updates . 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 Support: An additional mitigation is available for affe