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SMTP response codes are three-digit numbers that indicate the status of an email transaction. The first digit defines the response category:
  • 1xx – Informational (not commonly used in SMTP)
  • 2xx – Success (The request was successfully processed)
  • 3xx – Redirection (Additional action required)
  • 4xx – Temporary failure (The request failed, but retry may succeed)
  • 5xx – Permanent failure (The request failed, do not retry)
Here’s a breakdown of common SMTP status codes:

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2xx – Success Codes

These indicate that the email transaction was successfully processed.
  • 200 – Nonstandard success response (rarely used).
  • 211 – System status or help reply (provides server info).
  • 214 – Help message (returns helpful information).
  • 220 – SMTP server ready (indicates the server is ready to process requests).
  • 221 – SMTP server closing connection (indicates the session is ending).
  • 250 – Requested action completed successfully (most common success response).
  • 251 – Recipient not local, will forward (email will be relayed to another domain).

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3xx – Intermediate/Redirection Codes

These codes indicate that the server expects further input from the client before completing the request.
  • 334 – Server response to an authentication request (challenges the client for credentials).
  • 354 – Start mail input (indicates the server is ready to receive the email body).

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4xx – Temporary Failures

These errors indicate a transient issue that may resolve itself. The sender should retry later.
  • 400 – Generic temporary error.
  • 421 – Service not available (server is shutting down or overloaded).
  • 450 – Requested action not taken because the recipient’s mailbox is unavailable (e.g., mailbox full).
  • 451 – Requested action aborted due to a server error (e.g., system overload).
  • 452 – Insufficient system storage (server cannot process the email due to lack of resources).

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5xx – Permanent Failures

These indicate a permanent error, meaning the request cannot be fulfilled. The sender should not retry.
  • 500 – Syntax error (server did not recognize the command).
  • 501 – Syntax error in parameters (invalid email address or malformed command).
  • 502 – Command not implemented (server does not support the requested command).
  • 503 – Bad sequence of commands (commands sent in the wrong order).
  • 504 – Command parameter not implemented (server does not recognize a specific parameter).
  • 550 – Mailbox unavailable (e.g., recipient does not exist).
  • 551 – User not local (server cannot relay the message).
  • 552 – Mailbox full (message storage limit exceeded).
  • 553 – Invalid recipient address (email address incorrect or malformed).
  • 554 – Transaction failed (email rejected, often due to spam filters or security policies).

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Example SMTP Transaction with Status Codes

Get more indepth information on all SMTP codes here: https://smtp.codes/