Microsoft’s October 2026 Windows Server updates are breaking Outlook on the web sign-ins and stalling mail flow on some on-premises Exchange Server deployments — including Exchange Server 2016, 2019, and later, running on Windows Server 2019 or newer. The issue is confirmed by Microsoft and documented in its Windows Release Health dashboard and a dedicated support blog post published Saturday, October 18, 2026. It affects customers globally, not just in the US or India, and has no link to the separate October 2026 Exchange Server security updates. Those remain safe to install. This is strictly a Windows Server compatibility problem. And it’s not universal: Microsoft says severity depends on server load, and some installations won’t see any impact at all. But for those that do, symptoms include failed OWA logins, emails stuck in Outlook’s Outbox, and mailbox replication failures. One telltale error is MapiExceptionMdbOffline (hr=0x80004005, ec=1142), alongside Event ID 1006 in the application log. The root cause? A new default memory limit imposed by the update on the Windows Endpoint Mapper service, specifically, capping simultaneous Endpoint Lookup calls at 512. Exchange Server, under heavy load, can exceed that. Microsoft’s workaround is registry-based: administrators must manually raise that cap via an elevated Command Prompt, then reboot the server. The command isn’t public in the source material. Microsoft only says it exists and requires a restart. No timeline for a permanent fix has been announced. Meanwhile, Exchange 2016 and 2019 are now officially out of extended support: Microsoft ended ESUs for both this month, following a six-month extension announced in April 2026. That means no more patches, security or otherwise. For those versions after October 2026. Administrators still running them should treat this as a hard deadline to migrate. The next scheduled Windows Server patch Tuesday is November 11, 2026.