Exact response pattern
503 5.5.1 Bad sequence of commandsPermanentGenericProtocol
What it means
SMTP commands were sent in the wrong order.
Likely causes
- DATA before RCPT.
- AUTH before EHLO where not allowed.
- Session state lost.
Recommended response
- Review the full transcript.
- Correct the client state machine.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.