SMTP Reply Codes

Three-digit reply codes returned by SMTP servers (RFC 5321). The first digit indicates success (2xx), transient failure (4xx), or permanent failure (5xx). The second digit categorises the response; the third gives detail.

38 codes

• 211 System status
• 214 Help message
• 220 Service ready
• 221 Service closing
• 235 Authentication successful
• 250 Requested mail action OK
• 251 User not local; will forward
• 252 Cannot verify user, will attempt delivery
• 334 Server AUTH challenge
• 354 Start mail input
• 421 Service not available
• 422 Mailbox full
• 431 Not enough disk space
• 432 Recipient incoming mail queue paused
• 450 Mailbox unavailable
• 451 Local error in processing
• 452 Insufficient system storage
• 454 TLS not available
• 455 Server unable to accommodate parameters
• 500 Syntax error, command unrecognised
• 501 Syntax error in parameters
• 502 Command not implemented
• 503 Bad sequence of commands
• 504 Command parameter not implemented
• 521 Domain does not accept mail
• 523 Encryption required
• 530 Authentication required
• 534 Authentication mechanism too weak
• 535 Authentication credentials invalid
• 538 Encryption required for authentication
• 541 Recipient address rejected
• 550 Mailbox unavailable
• 551 User not local
• 552 Exceeded storage allocation
• 553 Mailbox name not allowed
• 554 Transaction failed
• 555 MAIL FROM or RCPT TO parameters not recognised
• 556 Domain does not accept mail