Microsoft Says October Windows Server Updates Can Disrupt Exchange Sign-In and Email
Summary
Microsoft says October 2026 Windows Server security updates can disrupt Exchange sign-in, email delivery, and mailbox replication on some systems. It advises against uninstalling the updates and provides a registry workaround while developing a fix.
Key points
- The issue may affect Exchange Server systems running Windows Server 2019 or later, with impact varying by server load.
- Reported symptoms include failed Outlook on the web sign-ins, undelivered messages stuck in the Outbox, and mailbox replication problems.
- Microsoft attributes the issue to a new default limit of 512 simultaneous Endpoint Mapper lookup calls, which may be insufficient under heavy Exchange workloads.
- Microsoft says the issue is not caused by the October Exchange Server security updates and advises customers not to uninstall them.
- The temporary workaround raises the Endpoint Mapper limit through a registry change and requires restarting the affected server; Microsoft is working on a fix.
Article Details
- Event Type
- Windows Server security update-related Exchange Server service disruption
- Impact
- Microsoft confirmed that October 2026 Windows Server security updates can cause sign-in and mail-flow problems on some on-premises Exchange Server systems running Windows Server 2019 or later. Affected users may be unable to sign in to Outlook on the web, send email, or access mailboxes during replication. Severity varies with server load. Microsoft said the issue is unrelated to the October Exchange Server security updates, advised customers not to uninstall those updates, and provided a registry-setting workaround while it works on a fix.
Vendors
Products
Microsoft Exchange ServerMicrosoft has confirmed that this month's Windows Server security updates are causing OWA sign-in and mail delivery issues on some Exchange Server systems.Outlookupdate over the weekend, Microsoft said users may have trouble accessing email or signing in to Outlook after installing the latest updates on systems running Windows Server 2019 and later.Windows ServerMicrosoft: October Windows Server updates break Exchange sign-in, mail flow