Microsoft warned on October 16 that October Windows security updates can cause email problems on some on-premises Exchange Server 2016, 2019 and Subscription Edition installations. Symptoms include Outlook on the web sign-in failures, messages stuck in the Outbox, growing mail queues and mailbox-replication problems.

Microsoft lists Windows Server 2016, 2019, 2022 and 2025 as affected platforms. Installing the October Exchange Server security updates is not required for the issue to occur. Microsoft links the problem to a new limit on simultaneous RPC Endpoint Lookup calls, which can be insufficient for busy Exchange servers.

The issue is listed as mitigated, with investigation continuing. Microsoft documents a temporary administrator-applied registry workaround and offers an additional mitigation through Microsoft Support. Administrators should review the Exchange Team advisory and Windows release-health guidance for their affected servers.