SMTP 550 5.7.64. — Relay / holding attribution.
Domain: Microsoft 365.Priority: High.
What does this error mean?
Exchange Online refuses the message because the stream is not correctly assigned or allowed.
Probable causes
- incorrect configuration or dependency.
- service, network or storage unavailable.
- recent change or component failure.
Verification and resolution
- Keep the message complete and the time of the, error.
- Identify the exact component that returns the code.
- Control dependencies before any global repair.
- Apply a targeted correction and then retester.
Precaution:::.:. Avoid generic repair scripts or irreversible removals before having confirmed the cause.
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