October 2026 Windows Server updates can stall Exchange mail queues

On October 16, 2026, the Exchange Server team published guidance for administrators whose on-premises Exchange servers became unstable after the October 13 Patch Tuesday. Microsoft had first added a warning banner to its October Exchange security update post on October 15. The new post says the cause is a set of Windows Server updates, not the Exchange security updates released the same day.

Which updates Microsoft names

Microsoft lists four Windows Server updates that can cause Exchange failures "after certain time":

• KB5126646 (Windows Server 2025)
• KB5126655 (Windows Server 2022)
• KB5126651 (Windows Server 2019)
• KB5126868 (Windows Server 2016)

Exchange Server 2016, 2019 and Subscription Edition may all be affected, because the trigger is the Windows layer underneath. Microsoft says not every server is hit equally. Severity depends on server load, and some servers might never see a problem.

Symptoms: OWA errors and growing queues

The symptoms Microsoft lists include Outlook on the web failing with MapiExceptionMdbOffline: Unable to make connection to the server (event ID 1006 in the Application log), and Database Availability Group members failing to reach quorum. For senders, the important one is mail flow: Microsoft says email queues "might grow during the day, with email crawling to a halt". A reboot may help only temporarily.

Microsoft's recommended fix

Microsoft explicitly advises against uninstalling the October Windows updates. Instead it recommends setting the EpmapSavedContextCap value under HKLM\SOFTWARE\Microsoft\Rpc to 65536, rebooting, and monitoring. If the issue returns, Microsoft says to raise the limit. It says this has resolved the customer cases it has assisted so far.

What it means if you send to Exchange-hosted domains

This section is our analysis, not Microsoft's. When an Exchange server's queues stall, inbound mail from outside senders can be delayed or temporarily refused. That usually shows up on the sending side as deferrals and 4xx SMTP reply codes rather than hard bounces. A well-configured sending MTA keeps retrying, so mail should arrive late rather than be lost. A spike in deferrals concentrated on a few corporate domains this week is more likely to be this issue than a reputation problem. Check the recipient domain's MX hosts with our MX lookup before changing anything on your side. Exchange Online (Microsoft 365) mailboxes are not named in Microsoft's post; the guidance covers on-premises servers.