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Exchange Server: October 2026 Windows Server Updates Break OWA and Email Delivery
Be careful if you run a Microsoft Exchange mail server! After installing the October 2026 Windows Server updates, your users may no longer be able to sign in to OWA, while their emails get stuck in the Outbox. Microsoft has confirmed the issue... Here’s what you need to know.
After the Remote Desktop Services bug in September, October brings another one, this time affecting email. Microsoft has added a new known issue to its Windows dashboard. It affects the cumulative updates released on October 13, as part of the October 2026 Patch Tuesday and its 1,407 fixed vulnerabilities.
Four Windows Server versions are affected:
- Windows Server 2025 : KB5126646 (build 26100.33650)
- Windows Server 2022 : KB5126655 (build 20348.5761)
- Windows Server 2019 : KB5126651 (build 17763.9376)
- Windows Server 2016 : KB5126868 (build 14393.9612)
These are the updates I covered in my article about what’s new in the October 2026 updates for Windows Server. Once installed on a server hosting Exchange Server 2016, Exchange Server 2019, or Exchange Server SE, they can cause several issues:
- Unable to connect to OWA: Outlook on the web may fail with the error MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142), associated with event 1006 in the Application log (visible in Event Viewer).
- Emails stuck: messages sent from Outlook remain in the Outbox and are not delivered. Direct consequence: message queues grow.
- Mailbox replication: Exchange may have trouble accessing mailboxes during replication operations.
The culprit: a new limit on the RPC Endpoint Mapper
The October update introduces a default limit on the memory used by the RPC Endpoint Mapper component when it handles Endpoint Lookup calls. This service, which listens on port 135, tells RPC clients which port to use to reach a given service. And Exchange relies heavily on RPC: this limit creates problems for Exchange.
"For workloads like Exchange Server, the default limit of 512 concurrent calls may not be enough under heavy load", Microsoft explains. Not all servers are affected in the same way: the severity and how quickly symptoms appear depend on the load on the Exchange server. Failures may occur after some time, not necessarily right after a reboot. "Some servers may never experience a problem", the Exchange team notes in a dedicated post.
What you need to understand is that the Exchange security update released the same day is not responsible. It is not even necessary for it to be installed to be affected. This is clearly an issue linked to the Windows Server updates, and Windows Server is the platform Exchange Server runs on.
Microsoft does not explain why this limit was introduced, but the consequences are very real. One possible clue: this Patch Tuesday fixes CVE-2026-96300, a denial-of-service flaw in the RPC Endpoint Mapper service, related to uncontrolled resource consumption (CVSS score: 5.9). The connection is tempting... but it remains only a hypothesis.
How can you work around the issue?
Above all, do not uninstall the October Windows Server updates. The workaround is to raise the limit through the Registry on each affected Exchange server:
- Open an elevated Command Prompt on the Exchange server.
- Run the following command, which creates the
EpmapSavedContextCapvalue underHKLM\SOFTWARE\Microsoft\Rpc:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f- Restart the server so the setting takes effect.
- Then monitor your Exchange environment: if the issue comes back, the limit should be increased further.
Microsoft also says an additional mitigation is available through its support team for businesses.
Have you run into this issue?
