SMTP enhanced status codes
This page lists all enhanced SMTP status codes returned by the SMTP API.
You typically need to explicitly handle:
- 4xx - Temporary failures (retry recommended)
- 5xx - Permanent failures (request must be corrected before retrying)
Codes in the 2xx and 3xx range represent normal protocol states and are generally handled automatically by SMTP client libraries.
2xx - Success
| Status code | SMTP response message | Command | Description |
|---|---|---|---|
220 2.0.0 | Ready to start TLS | STARTTLS | TLS handshake is ready to begin. The server accepted the STARTTLS command and the client should now initiate TLS negotiation. |
221 2.0.0 | Bye | QUIT | Session ended normally. The server acknowledged the client's QUIT command to close the connection. |
235 2.7.0 | Authentication successful | AUTH | The client provided valid credentials via one of AUTH command mechanism and is now authenticated. |
250 2.0.0 | OK | RSET | Transaction state has been reset by RSET command. Any in-progress (MAIL FROM / RCPT TO / DATA) state is cleared. |
250 2.0.0 | OK | NOOP | Keep-alive acknowledged by NOOP command. The server is still active and the connection is healthy. |
250 2.1.0 | Sender <address> OK | MAIL FROM | The sender address was accepted by MAIL FROM command. The server validated the email address format and is ready to accept recipients. |
250 2.1.5 | Recipient <address> OK | RCPT TO | The recipient address was accepted by RCPT TO command. The server validated the email address format and added it to the recipient list. |
250 2.6.0 | Message queued as <message_id> | DATA | The message body was successfully received and processed after DATA command. Server returned UUID by which message can be tracked in the system. |
Note
The <message_id> is a unique identifier assigned by the Infobip SMTP API to each accepted message. When retrieving delivery reports, you can use the <message_id> to identify which report belongs to which original message.
3xx - Intermediate
| Status code | SMTP response message | Command | Description |
|---|---|---|---|
354 | End data with <CR><LF>.<CR><LF> | DATA | The server is ready to receive the message body on DATA command. The client should send the email content and terminate it with a line containing a single dot. |
4xx - Temporary failures
Temporary failures indicate that the request was valid but could not be processed at this time.
| Status code | SMTP response message | Command | Description |
|---|---|---|---|
421 4.3.0 | Mail system failure, closing transmission channel | Any | A temporary internal error occurred. The system is protecting itself by closing the connection. The client should retry after a delay. |
421 4.3.2 | Too many connections, try again later | Connection | The server has reached its maximum total connection limit. The client should retry after a delay. |
421 4.3.2 | Client connection limit reached | Connection | The client's IP address has too many simultaneous connections open. The client should reuse the opened connections or retry after a delay. |
421 4.3.2 | Service not available | DATA | The downstream processing service is temporarily unavailable. The client should retry after a delay. |
421 4.4.0 | Problem attempting to execute commands. Please try again later. | Any | An I/O error occurred while the server was processing the command. The connection will be closed. The client should retry after a delay. |
421 4.4.2 | Timeout waiting for the client | Any | The client took too long to send the next command. The server timed out and is closing the connection. The client should ensure commands are sent promptly. |
421 4.4.2 | Timeout during DATA | DATA | The client took too long to transmit the message body. The server timed out and is closing the connection. The client should ensure commands are sent promptly. |
421 4.4.2 | Connection reset during DATA | DATA | The TCP connection was unexpectedly reset by the client while the server was reading the message body. The client should verify connection settings and timeouts. |
452 4.5.3 | Too many recipients | RCPT TO | The message has exceeded the maximum number of allowed recipients per transaction. |
5xx - Permanent failures
Permanent failures indicate that the request cannot succeed without modification.
| Status code | SMTP response message | Command | Description |
|---|---|---|---|
501 5.1.7 | Bad sender address <address> | MAIL FROM | The email address provided in the MAIL FROM command is not a valid email address. Check the sender address format (for example, [email protected]). |
501 5.1.7 | Bad recipient address <address> | RCPT TO | The email address provided in the RCPT TO command is not a valid email address. Check the recipient address format (for example, [email protected]). |
501 5.5.2 | Syntax: EHLO <hostname> | EHLO | The EHLO command was sent without the required parameter. Correct syntax: EHLO mail.example.com. |
501 5.5.2 | Syntax: HELO <hostname> | HELO | The HELO command was sent without the required parameter. Correct syntax: HELO mail.example.com. |
501 5.5.2 | Syntax: STARTTLS (no parameters allowed) | STARTTLS | The STARTTLS command was sent with extra parameters. The command does not accept any arguments. Correct syntax: STARTTLS. |
501 5.5.2 | Syntax: RCPT TO: <address> <parameters> | RCPT TO | The RCPT TO command has invalid syntax. Correct syntax: RCPT TO:<[email protected]>. |
501 5.5.2 | Syntax: AUTH <type> <initial-response> | AUTH | The AUTH command has invalid syntax. Correct syntax: AUTH PLAIN <base64-credentials> or AUTH LOGIN. |
501 5.5.4 | Syntax: MAIL FROM: <address> <parameters> | MAIL FROM | The MAIL FROM command has invalid syntax. |
501 5.5.4 | Not a valid Base64 string | AUTH | The credentials sent during AUTH are not valid Base64-encoded text. Verify the client's AUTH encoding. |
501 5.5.4 | Does not contain NUL (\0) | AUTH PLAIN | Invalid AUTH format. Decoded string must follow the format: <NUL>username<NUL>password. |
501 5.5.4 | Does not contain the second NUL (\0) | AUTH PLAIN | The AUTH PLAIN credential string is malformed. The decoded Base64 value must follow the format: \0username\0password. |
501 5.7.0 | Authentication canceled by client | AUTH | The client sent * during the AUTH challenge-response exchange, which cancels authentication. Re-issue the AUTH command to try again. |
502 5.5.1 | VRFY command has been disabled | VRFY | The VRFY (verify) command is disabled on this server for policy reasons. |
502 5.5.1 | EXPN command has been disabled | EXPN | The EXPN (expand mailing list) command is disabled on this server for policy reasons. |
502 5.5.1 | Unrecognized command | Any | The command sent by the client is not recognized by the server. Check for typos or unsupported SMTP extensions. |
503 5.5.1 | Already authenticated | AUTH | The client is already authenticated in this session. There is no need to authenticate again. |
503 5.5.1 | Sender already specified | MAIL FROM | A MAIL FROM command has already been issued in this transaction. Use RSET to reset the transaction before specifying a new sender. |
503 5.5.1 | Need MAIL command | DATA / RCPT TO | The client sent DATA or RCPT TO before issuing MAIL FROM. The correct command sequence is: MAIL FROM, then RCPT TO, then DATA. |
503 5.5.1 | Need RCPT command | DATA | The client sent DATA without specifying any recipients. Issue at least one RCPT TO command before DATA. |
503 5.5.1 | TLS already active | STARTTLS | The client issued STARTTLS, but a TLS session is already established on this connection. TLS negotiation cannot be performed twice. |
504 5.5.4 | Unsupported authentication type <mechanism> | AUTH | The authentication mechanism requested is not supported by this server. Use one of the mechanisms advertised in the EHLO response (for example, PLAIN or LOGIN). |
504 5.5.4 | AUTH mechanism mismatch | AUTH | The authentication mechanism specified in the AUTH command does not match the handler processing it. |
504 5.5.4 | Unrecognized command parameter <args> | HELP | The HELP command was issued with an unrecognized parameter. Issue HELP without arguments to see available topics. |
530 5.7.0 | Must issue a STARTTLS command first | MAIL FROM / RCPT TO / DATA | The server requires TLS encryption before accepting mail commands. Issue STARTTLS and complete TLS negotiation first. |
530 5.7.0 | Authentication required | MAIL FROM / RCPT TO / DATA | The server requires authentication before accepting mail commands. Issue AUTH with valid credentials first. |
535 5.7.8 | Authentication credentials invalid | AUTH | The provided username/password or API key failed verification. The credentials are incorrect or the account does not exist. |
550 5.3.0 | Server error | DATA | An unexpected internal error occurred while processing the message. Contact support if the issue persists. |
550 5.7.1 | Unauthorized access (account blocked by policy) | AUTH | Authentication succeeded, but the account or user is blocked from sending traffic due to an administrative policy (for example, blocked account). |
550 5.7.1 | Empty sender not allowed | MAIL FROM | The MAIL FROM address is empty (null sender or bounce address). This server does not accept null senders. |
550 5.7.1 | Sender not allowed | MAIL FROM | The sender address provided in MAIL FROM is not authorized for this account. The domain or address is not registered or verified. |
550 5.7.1 | Header from domain does not align with envelope from domain | DATA | The domain in the message's From header does not match the domain used in the MAIL FROM envelope sender. This is an alignment check to prevent spoofing. |
552 5.3.4 | Message size exceeds fixed limit | MAIL FROM | The SIZE parameter in MAIL FROM exceeds the server's maximum allowed message size. Reduce the message size or split into smaller messages. |
552 5.3.4 | Message size exceeds fixed limit | DATA | The total message size (body and attachments) exceeds the maximum allowed size configured on the server. Reduce the message size or split into smaller messages. |
554 5.6.0 | Pre-mature end of <CRLF>.<CRLF> | DATA | The DATA stream ended before the proper SMTP end-of-data sequence (CRLF.CRLF) was received. The message transmission was incomplete. |
554 5.6.0 | Invalid or corrupt MIME multipart | DATA | The MIME structure of the message could not be parsed. The multipart boundaries or encoding are malformed. Verify the email client's MIME formatting. |
554 5.6.0 | Could not read the message. <e.g Local address ends with dot> | DATA | An email address in the message headers (To, Cc, Bcc, From) has invalid syntax and could not be parsed. |
554 5.6.0 | Attachment is empty | DATA | Policy rejection. One or more attachments in the message have zero bytes of content. Remove empty attachments or provide actual content. |
554 5.6.0 | Inline image is empty | DATA | Policy rejection. One or more inline images (Content-ID referenced) have zero bytes of content. Remove empty inline images or provide actual content. |
554 5.6.0 | Subject not present | DATA | Policy rejection. Missing Subject header. |
554 5.6.0 | Subject too long | DATA | Policy rejection. Subject header too long. |
554 5.6.0 | Body not present | DATA | Policy rejection. The message has no text body, no HTML body, and no attachments. At least one form of content is required. |