Microsoft: October Windows Server updates break Exchange sign-in, mail flow
Microsoft said a registry workaround can restore mail flow while it investigates October Windows Server updates that affect Exchange systems under heavy load.
- Microsoft confirmed that October 2026 Windows Server updates are causing OWA sign-in and mail delivery issues on Exchange Server systems running Windows Server 2016, 2019, 2022, and 2025.
- The disruption stems from a default 512 memory limit on Endpoint Mapper when handling Endpoint Lookup calls, which proves insufficient for Exchange Server workloads under heavy load.
- Administrators can resolve issues by running a Command Prompt command to modify "HKLM\SOFTWARE\Microsoft\Rpc" and increase the maximum simultaneous Endpoint Lookup calls allowed.
- Microsoft strongly advised customers not to uninstall the October updates, noting that not all Exchange servers are impacted equally as severity varies based on server load.
- While investigating, Microsoft has not committed to an out-of-band update and is concurrently phasing out Extended Security Update support for Exchange 2016 and 2019 this month.
11 Articles
11 Articles
BleepingComputer
IT Security News - cybersecurity, infosecurity newsMicrosoft: October Windows Server updates break Exchange sign-in, mail flow
Microsoft has confirmed that this month's Windows Server security updates are causing OWA sign-in and mail delivery issues on some Exchange Server systems. [...]
After the October patch day, there are more reports of malfunctions on Exchange Server SE. Affected people report that Outlook is no longer available on the web and that e-mails are collected in queues. Microsoft confirms the problem. (Continue reading)
Microsoft acknowledged that Windows Server updates of October 2026 may affect Exchange, with OWA failures and mail delays. The company attributes the problem to a simultaneous call limit and poses a registry adjustment as a temporary solution. *** Problems may affect Exchange Server 2016, 2019 and Subscription Edition on Windows Server 2016 servers, 2019, 2022 and 2025. Microsoft linked the bugs to the default limit of 512 simultaneous Endpoint …
Coverage Details
Bias Distribution
- 100% of the sources are Center
Factuality
To view factuality data please Upgrade to Premium
