Science & Tech · Big Tech & Internet · 1 hr ago
October Windows Server updates disrupt some Exchange systems
Microsoft says October Windows Server security updates are disrupting some Exchange servers 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.
The updates impose a default limit on Endpoint Mapper calls, which may be too low for Exchange servers under heavy load.
Microsoft says the problem is not caused by the October Exchange Server security updates and advises customers not to uninstall them.
Administrators can raise the limit with a registry change on affected servers and then restart them.
Microsoft says the impact varies by server load, and some servers may not encounter the problem.
Microsoft is working on a fix, and advises administrators to monitor their systems after applying the workaround.
Microsoft says October 2026 Windows Server security updates are causing sign-in and email delivery problems on some Exchange Server systems.
Affected users may be unable to access Outlook on the web or send email, and some servers may have trouble accessing mailboxes during replication.
The issue can affect Exchange versions running on impacted Windows Server versions, but Microsoft says the severity depends on server load.
Microsoft says the problem is not linked to the October 2026 Exchange Server security updates and advises customers not to uninstall them.
A temporary workaround changes a registry setting and requires restarting the affected server.
- Who
- Microsoft and administrators of affected on-premises Exchange Server systems.
- What
- October Windows Server updates are causing some Exchange systems to have sign-in, email delivery, and mailbox access problems.
- When
- After installing the October 2026 Windows security update.
- Where
- Systems running Windows Server 2019 and later.
- Why
- The update introduced a default limit of 512 simultaneous Endpoint Lookup calls, which may be insufficient for Exchange under heavy load.
This story does not have two clearly opposing sides.
Some on-premises Exchange servers began experiencing email access, sign-in, delivery, or mailbox replication issues.
Microsoft added the issue to its Windows release health dashboard.
Microsoft described the issue and shared a temporary registry workaround.
- Affected operating systems
- Windows Server 2019 and later
- Endpoint Lookup default limit
- 512 simultaneous calls
- Workaround registry value
- EpmapSavedContextCap set to 65536
- Restart required
- Affected Exchange Server must be rebooted after changing the registry setting
3 sources
What people are saying
Issues reported are not related to installation of October 2026 security updates for Exchange Server.
Not all Exchange servers will be impacted by installation of those updates equally; the severity and speed of impact varies based on Exchange server load.