Microsoft says October 2026 Windows Server security updates are causing some on-premises Exchange servers to fail at Outlook on the web sign-in and to stop delivering email. The issue affects Exchange running on Windows Server 2019 and later and is tied to a new default limit in the RPC Endpoint Mapper, which Exchange can exceed under heavy load. Microsoft said this is not caused by the October 2026 Exchange security updates and published a temporary registry workaround: set HKLM\SOFTWARE\Microsoft\Rpc\EpmapSavedContextCap to 65536 and reboot.
Why it matters: Organizations running on-premises Exchange may see users locked out of webmail and messages stuck in the Outbox after applying October Windows Server patches. Admins should not remove the Exchange security updates, but should monitor Exchange closely and apply Microsoft's registry-based workaround if affected.
Sergiu Gatlan
2026.10.19
100% relevant
This article establishes a distinct Microsoft patch-regression story: October 2026 Windows Server security updates, not Exchange updates, are causing OWA sign-in, mailbox access, and mail-flow failures on some on-premises Exchange deployments.
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