SMTP error codes explained
What each SMTP enhanced status code means per RFC 3463, and how to fix the bounce or deferral.
- 550 5.1.1SMTP 550 5.1.1: recipient address rejected (user unknown)
- 550 5.4.1SMTP 550 5.4.1: no answer from host (and the Microsoft meaning)
- 550 5.7.1SMTP 550 5.7.1: delivery not authorized (message refused by policy)
- 550 5.1.0SMTP 550 5.1.0: sender address rejected (bad from or return-path)
- 552 5.2.2SMTP 552 5.2.2: mailbox full (recipient over quota)
- 550 5.2.0SMTP 550 5.2.0: mailbox unavailable (other mailbox status)
- 554 5.7.5SMTP 554 5.7.5: cryptographic failure (TLS or signature verification)
- 550 5.4.14SMTP 550 5.4.14: hop count exceeded (mail loop)
- 552 5.3.4SMTP 552 5.3.4: message too big for system (over size limit)
- 421 4.7.0SMTP 421 4.7.0: try again later (temporary security or policy defer)
- 451 4.4.1SMTP 451 4.4.1: no answer from host (connection timed out)
- 450 4.2.0SMTP 450 4.2.0: mailbox unavailable (temporary, often greylisting)
- 452 4.2.2SMTP 452 4.2.2: recipient over quota (temporary mailbox full)
- 421 4.3.2SMTP 421 4.3.2: system not accepting network messages (server busy)