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

What changed

October Windows Server updates introduce a default cap of 512 concurrent RPC Endpoint Lookup contexts. Under Exchange load, this can interrupt OWA, mail delivery and mailbox replication; Microsoft status is Mitigated, not resolved.

Technical details

• Temporary RPC Endpoint Mapper registry mitigation is documented by the Exchange Team; no permanent Windows fix is claimed.

Why it matters

On-premises Exchange 2016, 2019 and Subscription Edition on affected Server 2016, 2019, 2022 or 2025 hosts can lose mail service. This Windows RPC issue does not require the October Exchange security update and is not an Exchange Online incident.

Known limitations

• Windows Server 2016, 2019, 2022 and 2025 hosting Exchange 2016, 2019 or SE can be affected depending on workload.
• Microsoft status is Mitigated, not resolved; monitor RPC memory use after raising the limit.
• This Windows update issue can occur without installing the October Exchange security update.

Additional evidence

• Exchange Team: RPC Endpoint Mapper mitigation ↗

Now check

• Match hosts to KB5126646 (Server 2025), KB5126655 (2022), KB5126651 (2019) or KB5126868 (2016)
• Check OWA for MapiExceptionMdbOffline, hr=0x80004005/ec=1142, Application Event ID 1006, growing mail queues and DAG health
• Follow the Exchange Team mitigation: back up the RPC registry configuration, set HKLM\SOFTWARE\Microsoft\Rpc EpmapSavedContextCap (REG_DWORD) to 65536 and restart under change control
• Do not uninstall the Windows security update as the routine workaround; validate OWA, mail flow, mailbox replication and memory after restart and escalate recurrent issues to Microsoft Support