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Known update issues

Windows Server: Users might be unable to sign in to OWA or send emails on Exchange Server

WorkaroundKnown issue

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Summary

After the October 2026 Windows security update, some on-premises Exchange servers might have trouble accessing or sending email. The update introduced a default limit on the memory Endpoint Mapper uses for Endpoint Lookup calls, which can be too low for Exchange under heavy load. Microsoft lists the status as mitigated and is still investigating.

Symptoms

  • Users cannot sign in to Outlook on the web (OWA).
  • Emails sent from Outlook stay in the Outbox and are not delivered.
  • OWA might fail with the error MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142), with EventID 1006 in the application log.
  • Email queues grow.
  • Exchange might have trouble accessing mailboxes during mailbox replication.

Affected

  • Windows Server 2025 (KB5126646, OS build 26100.33650)
  • Windows Server 2022 (KB5126655, OS build 20348.5761)
  • Windows Server 2019 (KB5126651, OS build 17763.9376)
  • Windows Server 2016 (KB5126868, OS build 14393.9612)
  • On-premises Exchange servers; installing the October 2026 Exchange security updates is not required for the issue to occur
  • No client versions affected

Workaround

  1. Modify the registry key as described in the Microsoft Exchange team blog post "Exchange servers may become unstable after installation of October 2026 Windows updates". The setting raises the maximum number of simultaneous Endpoint Lookup calls above the default of 512, which increases the memory Endpoint Mapper can use.
  2. Alternatively, contact Microsoft Support for business to obtain an additional mitigation.

Fix

Not fixed yet. Microsoft is investigating and will provide more information.

Recommendation

Check Exchange servers that installed the October 2026 Windows update for the symptoms above, especially growing queues or EventID 1006. If they are affected, apply the registry mitigation from the Microsoft blog post and test it first. Before patching further Exchange servers, consider applying the mitigation in advance or waiting for Microsoft's fix.

Sources

Compiled from the sources listed and translated with AI (Claude); status and dates come from the vendor. The vendor’s statements are authoritative. Updated 1 Nov 2026, 18:19.

Where the information comes from

We continuously read the vendors’ official sources, community reports and trade media and summarise them in our own words, with a link to the source:

  • Microsoft: Windows release health (Windows 10/11, Server 2016 to 2025), Windows message center, known issues of Microsoft 365 Apps (Outlook, Excel, Word, PowerPoint), Edge, Intune, Autopilot, Windows 365 and Entra Connect, Microsoft KB articles with timelines, Exchange and Windows IT Pro blogs, optionally Microsoft 365 service health and message center
  • other vendors: Veeam KB, Citrix (known issues), Ubuntu and Debian (regressions), Broadcom (VMware), Fortinet, Cisco, Mozilla, Let’s Encrypt
  • community (title, short excerpt and link only, without the names of the authors): Microsoft Q&A, Microsoft Tech Community (discussions), Veeam R&D Forums, Eleven Forum
  • targeted web research with AI (Claude), e.g. on the updates of the month – this also finds discussions on Reddit and in forums; such entries are labelled as community reports until the vendor confirms them
  • early warning from trade media (title and link only): Born’s Tech and Windows World, BleepingComputer, AskWoody, Windows Latest, Windows Central, gHacks, The Windows Club, Deskmodder, Dr. Windows, WindowsArea, Frankys Web, heise online

Summary and translation with AI (Claude by Anthropic) – without any data of visitors.

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