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SMTP Response Codes
The categories are:
- 1xx Positive preliminary reply
- 2xx Positive completion reply
- 3xx Positive intermediate reply
- 4xx Temporary failure
- 5xx Permanent failure
The second digit also means stuff:
- x0x Syntax-related response
- x1x Information responses (unlikely to be errors)
- x2x Connection-related response
- x3x Unspecified
- x4x Unspecified
- x5x Mail system-related responses
Both 4xx and 5xx responses mean that the message was not accepted. A 5xx response means it will never be accepted, so you can dispose of the message. A 4xx response means you should try back again.
The common code meanings (from RFC 821/2821) are:
| Code | Meaning |
| 211 | System status, or system help reply |
| 214 | Help message |
| 220 | Service Ready |
| 221 | Service closing transmission channel |
| 250 | Requested mail action okay, completed |
| 251 | User not local; will forward to <forward-path> |
| 252 | Cannot VRFY user, but will accept message and attempt delivery |
| 253 | OK, <n> pending messages for node <x> started. (RFC 1985) |
| 354 | Start mail input; end with <CRLF>.<CRLF> |
| 355 | Octet-offset is the transaction offset (RFC 1845) |
| 421 | Service not available, closing transmission channel |
| 432 | A password transition is needed (RFC 2554) |
| 450 | Requested mail action not taken: mailbox unavailable/busy |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 453 | You have no mail. (RFC 2645) |
| 454 | TLS not available temporarily or authentication failed temporarily. (RFC 2552) |
| 458 | Unable to queue messages for node X. (RFC 1985) |
| 459 | Node <x> not allowed. (RFC 1985) |
| 500 | Syntax error, command unrecognized |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented (usually no different than 500) |
| 503 | Bad sequence of commands |
| 504 | Command parameter not implemented |
| 521 | Machine does not accept mail. (RFC 1846) |
| 530 | Authentication required (RFC 2554) or must issue STARTTLS first (RFC 3207). |
| 534 | Authentication mechanism too weak. (RFC 2554) |
| 538 | Encryption required for requested authentication mechanism. (RFC 2554) |
| 550 | Requested action not taken: mailbox unavailable |
| 551 | User not local; please try <forward-path> |
| 552 | Requested mail action aborted: message too big |
| 553 | Requested action not taken: mailbox name not allowed |
| 554 | Transaction failed |
For further codes, check out RFC 1893. If you're looking for the definition of the "Robustness Principle", it's in RFC 1123.
