Microsoft has fixed the Outlook sign-in and mail delivery problems that struck on-premises Exchange Server systems after the October 2026 Windows Server security updates. The company said on 11 November 2026 that the November Patch Tuesday updates, KB5131080 and KB5131090, resolve the known issue and advised affected organisations to install them on all affected systems. ## What went wrong Microsoft first acknowledged the problem in mid-October 2026, confirming that users had trouble signing in to Outlook on the web and sending email after installing the October updates on systems running Windows Server 2019 and later. Affected users could not sign in to Outlook on the web, which returned a MapiExceptionMdbOffline error, while sent messages stayed in the Outbox and mail queues grew through the day until mail flow slowed or stopped. Microsoft traced the fault to a new default limit on the amount of memory the Endpoint Mapper uses when handling Endpoint Lookup calls, introduced by the October updates. The severity varied with server load, so not every Exchange server was hit equally. ## The fix and the workaround The permanent fix is to install the November 2026 update for Windows Server. Microsoft also published a temporary workaround for organisations that cannot deploy the updates immediately: administrators can raise the Endpoint Mapper memory limit with a registry change, setting the EpmapSavedContextCap value, and then restart the server. Microsoft said the registry key can stay in place after the November updates are installed, but it is no longer required. The company also clarified that the Outlook on the web issue was not linked to the October Exchange Server security updates themselves, and it urged customers not to uninstall those updates. [BleepingComputer](https://www.bleepingcomputer.com/news/microsoft/microsoft-fixes-exchange-sign-ins-broken-by-october-windows-server-updates/) and [Microsoft Support](https://support.microsoft.com/en-us/servicing/exchange/server/update/2026/5133432)