SMTP codes
SMTP 503 Bad sequence of commands
Commands came in the wrong order, or the server requires authentication first (“need AUTH”). Classic for a mail client sending without logging in.
- Class
- 5xx · Permanent failure
- Retried?
- No, the message bounces
- Defined in
- RFC 5321
What to do
- Turn on “my server requires authentication” in the mail client.
- Use port 587 with STARTTLS or 465 with TLS and your mailbox login.
How it appears in a bounce
503 Bad sequence of commands
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.