I'm seeing the same email problem across three separate customer tenants. Messages are failing to send or arrive, sometimes with no useful error details, and other times with: "550 5.6.200 STOREDRV.Deliver; message is treated as poison." Has anyone else encountered this, and is there a known service incident or recommended troubleshooting step?
3 Answers
Confirmed—one client is seeing messages pass through its filtering service and then get marked as poison despite valid connectors and bypass rules. The best option is to monitor the incident and service health updates until Microsoft resolves it.
Yes, this is part of a major Exchange Online outage. Microsoft identified it under incident EX1464935. Check the Microsoft 365 service health dashboard and message traces before changing connectors or mail-flow settings.
The fact that several unrelated tenants are affected points to a Microsoft-side service problem rather than a local configuration issue. A poison-message error means the Store Driver repeatedly failed while processing a message, so Exchange stopped retrying it. Messages may also be flagged this way after passing through third-party filtering services.
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