Microsoft fixes Exchange sign-in issues caused by Windows Server updates

Microsoft has fixed Outlook sign-in and mail delivery issues on Exchange Server systems caused by the October 2026 Windows Server security updates. The fix is included in the November 2026 Patch Tuesday updates. The problems, acknowledged in mid-October, stemmed from a new default memory limit on Endpoint Mapper during Endpoint Lookup calls. This affected Windows Server 2019 and later, causing users to be unable to sign in to Outlook on the web or have emails stuck in the Outbox. Microsoft advised against uninstalling the October updates and provided a temporary registry workaround for affected systems. The company noted that not all Exchange servers were equally impacted, with severity varying based on server load.