Microsoft says October Windows Server updates are breaking some Exchange servers
Microsoft says its October Windows Server updates can break some on-premises Exchange email servers. Users may be unable to sign in to Outlook on the web, and mail can get stuck. A registry change is the only fix for now, and Microsoft says not to uninstall the updates.
Microsoft has confirmed that its October 2026 Windows Server updates can disrupt some on-premises Exchange email servers, The Register, BleepingComputer and heise reported.
After installing the updates, some users may be unable to sign in to Outlook on the web, and emails sent from Outlook may sit in the outbox without being delivered, Microsoft said, BleepingComputer reported. Busy servers can see long email queues, "with email crawling to a halt," The Register reported. Microsoft added the warning on Oct. 15, heise reported.
The problem affects Exchange servers running Windows Server 2016, 2019, 2022 and 2025, The Register reported. Heise listed the four updates involved: KB5126646 for Server 2025, KB5126655 for Server 2022, KB5126651 for Server 2019 and KB5126868 for Server 2016.
The cause is a new default limit on memory used by a Windows service called Endpoint Mapper, Microsoft said. "For workloads such as Exchange Server, the default limit of 512 simultaneous Endpoint Lookup calls might not be sufficient under heavy load," the company said. Administrators can change a registry setting to raise the limit.
Rebooting may fix the problem for a while, but the registry edit is the only workaround for now, The Register reported. Heise reported that a Microsoft employee said on Oct. 15 that there was no workaround at that point.
Microsoft said the problem is not caused by the separate October security updates for Exchange itself, and it urged customers not to uninstall them, BleepingComputer reported. It also strongly advised against removing the Windows Server update, The Register reported. "Not all Exchange servers will be impacted by installation of those updates equally," Microsoft said. "Some servers might never see a problem."
Admins were frustrated. "I spent all weekend on my servers because of this," one customer wrote on Microsoft's support page, The Register reported. Another said the problem hit "like every 20h since Wednesday."
Microsoft has not committed to an emergency fix, The Register reported. The company said it is investigating and will share more when it can.
What we know: October Windows Server updates can break Exchange sign-in and mail flow on busy servers.
What we don't: When Microsoft will release a permanent fix.
Where outlets agree The Register, BleepingComputer and heise all report that Microsoft's October Windows Server updates can break Outlook on the web and mail delivery on some Exchange servers.
Where they differ The cited reports agree on the key facts so far.