Confirm the reason
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.
Checks
- Encoded MIME size, not only source attachment size
- Provider and organizational limits
- Duplicate or malformed MIME parts
- Campaign template expansion
Correction
Reduce the message, host large assets externally, and retest with a representative recipient before restarting volume.
Operational rule: collect evidence before changing policy, make one controlled change, verify the result, and retain a rollback path.