OWA errors and growing mail queues

The October 2026 Windows Server security updates can disrupt on-premises Exchange servers. Symptoms include MapiExceptionMdbOffline with ec=1142 in Outlook on the Web, delayed mail delivery, and communication or quorum problems in Database Availability Groups (DAGs). Some servers remain unaffected; workload and time in operation influence when symptoms appear.

Exchange Server SE, 2019 and 2016 on the respective Windows Server versions can be affected. Microsoft identifies KB5126646 (2025), KB5126655 (2022), KB5126651 (2019) and KB5126868 (2016) as triggers. The October Exchange security update itself does not have to be installed for this issue to occur.

Apply the temporary RPC limit and restart

The Windows updates cap simultaneous endpoint lookup calls in the RPC Endpoint Mapper at 512 by default. This can be insufficient for Exchange. The documented workaround raises that limit and the potential memory consumption:

  • Under HKLM\SOFTWARE\Microsoft\Rpc, configure EpmapSavedContextCap as REG_DWORD, decimal 65536. Record the existing configuration first; do not blindly reduce an already higher value.
  • Restart the affected Exchange server. For DAG members, move workloads in a controlled manner and work through members one at a time.
  • Then check OWA, mail delivery, queues and DAG health. Also monitor event 1006 in the Application log; this event alone does not confirm the cause.

Keep security updates installed

Do not uninstall or defer Windows security updates because of this issue. The registry value can be prepared before installing the update. As of October 26, 2026, Microsoft lists the issue as mitigated, not resolved. Contact Microsoft Support if disruptions persist. Document the exception and reassess it when a permanent fix becomes available.