Exchange Online mail flow failing with 550 5.6.200 poison-message errors
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MellowCedar42
Is anyone else seeing widespread sending and receiving problems in Exchange Online? I've confirmed the same issue across three unrelated customer tenants. Messages are failing with "550 5.6.200 STOREDRV.Deliver; message is treated as poison," although the error is occasionally blank. The affected mail flow includes messages passing through third-party filtering before Exchange marks them as poison. Is this a known service incident, or could it be caused by a tenant configuration issue?
Answers
One affected client is also seeing messages pass cleanly through Vipre Fusemail and then get flagged as poison in Exchange Online, even with the expected connectors and bypass settings. That makes an upstream Exchange processing issue much more likely than a filtering or connector misconfiguration.
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Seeing the same behavior across several unrelated tenants is a strong sign that the problem is on the service side. A poison-message result means the Store Driver repeatedly failed while processing a message and eventually stopped retrying it. Check message traces and service health, but avoid making configuration changes until Microsoft resolves the incident.
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This appears to be a major Exchange Online service outage rather than a problem with your connectors or mail-flow rules. The incident is tracked under service health issue EX1464935, so check the Microsoft 365 service health dashboard for updates.
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