Microsoft says October Windows Server updates break Exchange sign-in and mail flow

BleepingComputer reported on October 19, 2026 that Microsoft has confirmed problems on some on-premises Exchange servers after this month's Windows security updates for Windows Server 2019 and later. Users may be unable to sign in to Outlook on the web, messages can sit in the Outbox without being delivered, and mailbox replication can run into trouble. Some admins also see OWA fail with a MapiExceptionMdbOffline error and Event ID 1006.

The cause is a new default cap in the October update on how much memory Endpoint Mapper can use for Endpoint Lookup calls. The default allows 512 simultaneous calls, which Microsoft says may not be enough for Exchange under heavy load. How badly a server is hit depends on its load, and some may never see the issue. Microsoft stresses that the October Exchange Server security updates are not the cause and should not be removed.

While a fix is in the works, Microsoft's workaround is to raise the limit with a registry change (the EpmapSavedContextCap value under the Rpc key) from an elevated prompt, then reboot and keep monitoring. BleepingComputer also notes that Microsoft is ending extended security updates for Exchange 2016 and 2019 this month.

Why it matters

Email outages hit every team at once, so Exchange admins should check servers after patching and use the workaround instead of rolling back security fixes.

Source: BleepingComputer