Admin: 'I spent all weekend on my servers' after Microsoft breaks Exchange Server with October updates

OWA borked and lengthy email queues reported on busy servers after Microsoft tweaks a Server default

Microsoft has acknowledged that the October 2026 Windows Server updates may be destabilizing Exchange Server.

The problems are related to this list of Windows Server updates (see here) and Microsoft makes it clear they are not related to the October 2026 security updates for Exchange Server.

The software giant says the bug can hit Exchange Servers running Windows Server 2016, 2019, 2022, and 2025. The glitch might not be immediate and could impact Exchange Server OWA, which can throw up connection glitches or cause lengthy email queues, "with email crawling to a halt."

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Rebooting the server might make the problem go away temporarily, but a more permanent workaround is a registry edit.

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The destabilization issue could impact Exchange Server 2016, 2019, and the new Subscription Edition (SE), says the Windows maker.

Microsoft strongly advised that customers should not uninstall the October 2026 Windows Server update and stated, "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."

However, for those administrators who found their weekend swallowed by borked Exchange Servers, the registry edit is currently the only workaround.

The cause of the issue is a default limit on the amount of memory used by Endpoint Mapper when handling Endpoint Lookup calls.

In Microsoft's Release Health Dashboard for Windows Server 2025 (Server 2022, 2019, and 2016 are also affected), the Microsoft said: "For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls might not be sufficient under heavy load." Hence, the instruction to use a registry setting to increase the maximum.

It added, "We are investigating this issue and will provide more information when it is available."

Customers were understandably frustrated. On Microsoft's support page, one wrote, "We have had the problem like every 20h since Wednesday, when we installed the windows updates for October and the exchange su. Reboot only solution," before noting that the registry setting appeared to have improved things.

Another fumed, "Also please release your updates tested and without problems. Without giving us the job to test for you. We're already paying for the software and you want to make tests with us?! I spent all weekend on my servers because of this."

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The issue is bad form on the part of Microsoft and appears to be the result of a poorly thought-through default applied in the operating system which impacted Exchange Server.

Microsoft has yet to commit to issuing an out-of-band update to resolve the issue, after all, not every customer is affected, and the problem depends heavily on how busy a server is.

However, at least there's a registry workaround that might do the trick while Microsoft ponders its next move. ®