SMTP Error Codes and Messages

• 100: Continue: This means the server is ready to serve you the request, indicating that the request has been received and the process can continue.
• 101: Switching Protocols
  • The server is switching protocols, which is uncommon in SMTP communications.

2XX – Successful responses

• 220: “Service ready” 
  • The server is prepared to start the conversation.
• 250: “Requested mail action okay, completed” 
  • The action was successfully completed, and the session is progressing smoothly.
• 221: “Service closing transmission channel” 
  • The server is ending the conversation, usually after successfully sending the email.
• 200: “OK”
  • A general acknowledgment that the server has received and processed the request.

3XX – Persistent Transient Negative Completion Replies

• 354: Start mail input; end with <CRLF>.<CRLF>
  The server is asking for the body of the message to be sent. It expects the email content to follow, and the message should end with a period on a new line.

4XX – Temporary failure (retryable errors)

• • 421: Service not available, closing transmission channel
    The server is temporarily unavailable. This could be due to heavy load or maintenance. The connection will be closed.
  • 450: Requested mail action not taken: mailbox unavailable
    The recipient’s mailbox is temporarily unavailable, often due to being full or locked.

• SMTP 450 Email Error Variants:

• • • 450 4.1.0 “Recipient Address Rejected” – The recipient’s email server has temporarily rejected the address. Try resending later or verify the email address.
    • 450 4.2.0 “Mailbox Unavailable” – The recipient’s mailbox is either full or unavailable. Attempt delivery again after some time.
    • 450 4.7.1 “Client Host Rejected” – The receiving server has flagged your server or IP due to reputation issues or policy settings. Check your sender reputation or contact your email provider.
    • 450 4.4.1 “Connection Timed Out” – There was a connection issue with the receiving server. Resend the email or check network settings.
• 451: Requested action aborted: local error in processing
  There was a temporary issue with the mail server, such as a processing error, and the request couldn’t be completed.
  • SMTP 451 Email Error Variants:
    • 451 4.7.1: Access denied, often due to IP reputation or policy settings.
    • 451 4.3.2: Server overload or system not accepting network messages.
    • 451 4.7.0: Temporary server error, suggesting a retry might succeed.
    • 451 4.4.4: Routing failure, indicating problems with email delivery paths.
    • 451 4.3.0: Internal mail system error, a generic issue within the server.
    • 451 4.3.5: Server configuration issue, related to improper setup affecting delivery.
• 452: Sorry, we’re a bit too crowded at the moment
  • The server is unable to process the request due to a lack of available storage.
• 455: Server unable to accommodate parameters
  The server cannot process the request with the given parameters.

5XX – Permanent failure (non-retryable errors)

• 500: Syntax error, command unrecognized
  The server has received an unrecognized command or the syntax of the command is incorrect.
• SMTP 500 Email Error Variants:

• • • 500.0 – Internal server error
      A generic server error indicating an issue with the mail server’s internal configuration or resources.
    • 500 5.1.1 – Mailbox unavailable
      The recipient’s email address does not exist or is incorrectly entered.
    • 500 5.7.1 – Permission denied
      The server rejected the email due to authentication issues or because the sender is not authorized to send emails through that server.
    • 500 5.4.1 – Domain not found
      The server could not resolve the recipient’s domain.
• 501: Syntax error in parameters or arguments” or “Invalid address”
  The server has detected an error in the command parameters or arguments.
• 502: Command not implemented; Bad Command Sequence; or Command Not Supported
  The server does not support the requested command.
• 503: Oops, don’t forget about the “503 Valid RCPT command must precede DATA” message.

The client has sent commands in the wrong order, and the server is unable to process them correctly.

• 504: Command parameter not implemented or 504 Gateway Timeout

The server does not support the given parameter for the requested command.

• 550: Requested action not taken: mailbox unavailable
  The recipient’s mailbox is unavailable or the address doesn’t exist. It can also occur if the message is rejected by a spam filter.
• 551: User not local; please try <forward-path>
  The recipient is not local to the server, and the server suggests a forwarding address.
• 552: Requested mail action aborted: exceeded storage allocation
  The recipient’s mailbox has exceeded its storage limit.
• 553: Requested action not taken: mailbox name not allowed
  The recipient’s mailbox name is invalid or the email address has a syntax error.
• 554: Transaction failed
  The server rejected the message, often because it was identified as spam or violated policy rules.

Knowing what the error message pertains to, can help you understand the actions you can take to resolve it. In most cases, receiving an SMTP error message may have something to do with the reputation of your email domain, to ensure that this is not the case, then you can utilize Warmy.io and its free email deliverability test.

Along with its email deliverability test, Warmy.io also wants to ensure that you can have a reputable email domain with SPF and DMARC generators that will provide a positive standing with different Internet Service Providers (ISP).