Admin: 'I spent all weekend on my servers' after Microsoft breaks Exchange Server with October updates

What happened

Microsoft admitted that October 2026 Windows Server updates may cause problems for Exchange Server, the program companies use to run email. Web email (OWA) can have connection errors, and email can pile up in long queues. Microsoft says the Exchange security updates are not the cause.

Key facts

• Affected systems: Windows Server 2016, 2019, 2022, and 2025.
• Affected Exchange versions: Exchange Server 2016, 2019, and the new Subscription Edition (SE).
• Default limit: 512 simultaneous Endpoint Lookup calls.
• Impact varies. Some servers may never have a problem.
• Current workaround: a registry edit (a settings change in Windows) that raises the limit. A server reboot may help for a short time.

Why the limit matters

Endpoint Mapper is a Windows service that helps programs find each other on a network. Microsoft set a default cap on how many of its lookup requests it handles at once. On busy Exchange servers, Microsoft says that cap may be too low, so requests back up and email slows down.

Why it matters

Email is critical for many businesses, so a slowdown can stop work. Admins had to spend weekends on repairs. Some customers are angry that they had to help find the problem. Microsoft has not yet promised a fix outside its normal update schedule, so admins must use the workaround for now.

What to watch

• Whether Microsoft releases an out-of-band update (an urgent fix outside the normal schedule).
• More details from Microsoft's Release Health Dashboard, which it says is still under investigation.
• Whether the registry workaround works long term for busy servers.