Security · Updated 19 Oct, 02:28 pm IST
October Windows Server update breaks Exchange sign‑in and mail flow
Why it matters for readers: This explains why a Windows update might suddenly stop email and that Microsoft has described a way to fix it.
- Microsoft confirmed that October Windows Server updates can prevent users signing in to Outlook on the web and can stop email from being delivered on some on‑premises Exchange servers.1
- Affected systems are those running Windows Server 2019 and later, per Microsoft's Windows release health dashboard update.1
- Some customers see OWA errors such as MapiExceptionMdbOffline with Event ID 1006 and messages stuck in Outbox.1
- Microsoft says the October update introduced a default limit on memory used by Endpoint Mapper, limiting simultaneous Endpoint Lookup calls to 512 and causing issues under heavy load.1
- Administrators can change a registry setting to increase the maximum number of simultaneous Endpoint Lookup calls to mitigate the problem.1
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