smtp
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
2xx Success 8 codes
- 211 System status A status report or system help reply. Typically returned in response to a HELP command or NOOP.
- 214 Help message A response to a HELP command listing supported commands or providing usage information.
- 220 Service ready The server is ready to accept connections. Sent immediately after the TCP connection is established.
- 221 Service closing The server is closing the transmission channel. Sent in response to a QUIT command.
- 235 Authentication successful SMTP AUTH succeeded. The server has accepted the client credentials and the session is authenticated.
- 250 Requested mail action OK The requested action was completed successfully. Returned after EHLO, MAIL FROM, RCPT TO, and DATA (after the message body is accepted).
- 251 User not local; will forward The recipient is not hosted locally but the server will forward the message to the address given in the response.
- 252 Cannot verify user, will attempt delivery The server cannot verify the recipient exists (VRFY is not supported or the user is unverifiable), but will attempt delivery anyway.
3xx Redirection 2 codes
4xx Client error 9 codes
- 421 Service not available The server is temporarily unavailable and is closing the connection. The client should retry later.
- 422 Mailbox full The recipient mailbox has exceeded its storage quota. The message may be retried later if the mailbox is cleared.
- 431 Not enough disk space Insufficient disk space on the server. The client should retry after a delay.
- 432 Recipient incoming mail queue paused The recipient's incoming mail queue has been temporarily suspended. Retry later.
- 450 Mailbox unavailable The requested mailbox action was not taken because it is temporarily unavailable. Retry later.
- 451 Local error in processing The server encountered a local error and could not complete the action. Retry after a delay.
- 452 Insufficient system storage The server does not have enough storage space to accept the message right now. Retry later.
- 454 TLS not available The server cannot begin a TLS negotiation (STARTTLS) at this time. The client may retry the command or use a different security mechanism.
- 455 Server unable to accommodate parameters The server cannot accept the MAIL or RCPT parameters at this time. Retry after a delay.
5xx Server error 19 codes
- 500 Syntax error, command unrecognised The server could not recognise the command. The command line may be too long or contain invalid characters.
- 501 Syntax error in parameters The command was recognised but its arguments are syntactically incorrect.
- 502 Command not implemented The server does not implement the command.
- 503 Bad sequence of commands The command was issued in an incorrect order. For example, DATA before MAIL FROM, or RCPT TO before MAIL FROM.
- 504 Command parameter not implemented A parameter of the command is not implemented by the server, for example an unsupported AUTH mechanism.
- 521 Domain does not accept mail The server does not accept mail for this domain. The connection should be terminated.
- 523 Encryption required The message was rejected because it exceeds the server's size limit or because encryption is required.
- 530 Authentication required The server requires SMTP AUTH before accepting commands. The client must authenticate first.
- 534 Authentication mechanism too weak The server will not accept the requested authentication mechanism. A stronger mechanism is required.
- 535 Authentication credentials invalid SMTP AUTH failed. The username or password is incorrect, or the account is locked.
- 538 Encryption required for authentication The selected authentication mechanism is only available when the session is encrypted with STARTTLS.
- 541 Recipient address rejected The recipient address was rejected due to policy, such as an access control list or spam filtering.
- 550 Mailbox unavailable The recipient mailbox does not exist or the server policy rejects mail to it. Do not retry; bounce the message to the sender.
- 551 User not local The recipient is not local and the server will not forward mail. The response may include the correct address to try.
- 552 Exceeded storage allocation The recipient's mailbox has exceeded its storage limit and cannot accept new messages.
- 553 Mailbox name not allowed The recipient address syntax is invalid or the address is not permitted on this server.
- 554 Transaction failed The transaction failed. A generic permanent failure code; the message text usually gives more detail. Often indicates the message was rejected as spam.
- 555 MAIL FROM or RCPT TO parameters not recognised The server does not recognise or support the extension parameters supplied with MAIL FROM or RCPT TO.
- 556 Domain does not accept mail The domain indicated by the RCPT TO address has a null MX record, meaning it explicitly declares that it does not accept email.