503 5.5.1: bad sequence of commands
Your software sent SMTP commands in the wrong order, for example sending a message before authenticating.
Permanent failure (hard bounce)
What 503 5.5.1 means
SMTP requires commands in a fixed order (hello, authenticate, sender, recipient, data). A server returns 503 when one arrives too early, for example MAIL FROM before authentication, or DATA before any recipient is accepted.
Common causes
- Authentication was skipped or failed earlier
- A script sending commands out of order
- Every recipient was rejected before DATA
How to fix it
- Make sure you authenticate before sending
- Check the earlier lines of the SMTP log for the first failure
What to do with the address
Unrelated to the recipient address on its own.
Check an address now with the free email checker, or look at a domain's mail setup with the email domain checker, MX lookup and SPF checker.
Related error codes
Frequently asked questions
- What does 503 5.5.1 mean?
- Your software sent SMTP commands in the wrong order, for example sending a message before authenticating. SMTP requires commands in a fixed order (hello, authenticate, sender, recipient, data). A server returns 503 when one arrives too early, for example MAIL FROM before authentication, or DATA before any recipient is accepted.
- Is 503 5.5.1 a hard bounce or a soft bounce?
- Permanent failure (hard bounce). The address will keep failing, so remove it.
- What should I do with the address after 503 5.5.1?
- Unrelated to the recipient address on its own.