Interpretation
Use timestamps, queue IDs, remote responses, source IP, VMTA, sender domain and message class to establish the failure boundary. Avoid reacting to one isolated delivery attempt.
Protocol evidence
Capture the stage of rejection: connection, MAIL FROM, RCPT TO, DATA or end-of-data. This often identifies the policy layer responsible.
Recovery
- Freeze the affected stream.
- Compare a minimal clean test message.
- Validate authentication and identity.
- Review links, headers and content construction.
- Resume only after acceptance is stable.
Operational rule: collect evidence before changing policy, make one controlled change, verify the result, and retain a rollback path.