Windows Server updates disrupt Exchange servers

The latest updates for Windows Server, released on October 13, are causing problems with on-premises Exchange servers. Microsoft has confirmed these issues, which affect versions 2016, 2019, and Subscription Edition. The disruptions are linked to four specific updates: KB5126646, KB5126655, KB5126651, and KB5126868.

Symptoms vary across affected servers

Users are reporting symptoms such as messages getting stuck in Outlook's Outbox, growing message queues, and failures to access mailboxes during replication. Some users also face challenges with database availability groups (DAGs) reaching quorum. However, the symptoms can differ based on server configurations, and some systems may not show any of these issues.

New default limit causes performance issues

Microsoft explained that the October updates introduced a default limit of 512 simultaneous endpoint lookup calls. This setting affects the Endpoint Mapper, a component used in remote procedure call (RPC) operations. High Exchange server workloads can push this limit, leading to the reported disruptions.

Restart and registry change recommended

One affected user found that restarting the Exchange server temporarily resolved the issue, a solution confirmed by Microsoft's Exchange team blog. The team advises against uninstalling the updates, as they address important security vulnerabilities. Instead, they recommend running a specific command in an administrator command prompt to adjust the registry settings.

Microsoft supports Exchange until 2035

Microsoft has reaffirmed its continued support for Exchange Server, with plans to maintain it at least until 2035. The company urges affected users to apply the recommended registry changes and monitor their servers closely to ensure the issue does not return.