# Windows Server security updates disrupt some Exchange email systems

_Published Monday, October 19, 2026 at 5:06 AM EDT · Infrastructure, Products · Latest · Tier 2 — Notable_

![Windows Server security updates disrupt some Exchange email systems — Primary](https://www.bleepstatic.com/content/hl-images/2026/10/19/Microsoft-Exchange-Server.jpg)

Microsoft confirmed that October 2026 Windows Server security updates can disrupt sign-in and email delivery on some on-premises Exchange systems. Users may be unable to access Outlook on the web, messages may remain in Outlook's Outbox, and mailbox replication may encounter access problems.

The updates introduced a default limit of 512 simultaneous Endpoint Lookup calls, which Microsoft said may be insufficient for Exchange under heavy load. While developing a fix, Microsoft provided a registry workaround to increase the limit, followed by a reboot. Impact varies with server load, and some servers may never encounter the problem. Microsoft distinguished the issue from October's Exchange Server security updates.

## Sources

- [BleepingComputer](https://www.bleepingcomputer.com/news/microsoft/microsoft-october-windows-server-updates-cause-exchange-issues/)

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Canonical: https://techandbusiness.org/newswire/MHvwdQtXl2lS5XthE4wHvo
Published: 2026-10-19T09:06:41.169Z
Story chronology: 2026-10-19T08:58:41.000Z
Retrieved: 2026-10-19T12:10:31.238Z
Publisher: Tech & Business (techandbusiness.org)