Demystifying SMTP Error Codes: Understanding Email Delivery Status Notifications

 SMTP (Simple Mail Transfer Protocol) error codes are numerical codes generated by mail servers to indicate the status or outcome of an email delivery attempt. Understanding these codes can help diagnose and troubleshoot email delivery issues. Here's a breakdown of some common SMTP error codes:

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1. 5xx Series (Permanent Failure):

  • 550 - Requested action not taken: The mailbox is unavailable: This error indicates that the recipient's email address does not exist or is invalid. The email cannot be delivered, and the sender should double-check the recipient's address.
  • 554 - Transaction failed: This error is a general failure message indicating that the email delivery transaction has failed for some reason. It could be due to issues such as the recipient's mailbox being full or the sender's IP address being blacklisted.

2. 4xx Series (Transient Failure):

  • 421 - Service not available, closing transmission channel: This error indicates that the mail server is temporarily unavailable or overloaded. The sender should try again later.
  • 450 - Requested mail action not taken: Mailbox unavailable: Similar to the 550 error, this indicates that the recipient's mailbox is temporarily unavailable. The sender should retry the delivery later.

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3. 2xx Series (Success):

  • 250 - Requested mail action okay, completed: This code indicates that the email delivery was successful. The recipient's mail server has accepted the message for delivery.
  • 251 - User not local; will forward to <forward-path>: This code indicates that the recipient's email address is not hosted on the current mail server but will be forwarded to another server for delivery.

4. Authentication and Security Errors:

  • 530 - Authentication required: This error occurs when the sender's email server requires authentication (e.g., username and password) before allowing message submission.
  • 535 - Authentication credentials invalid: This error indicates that the authentication credentials provided by the sender are incorrect or have been rejected by the mail server.

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5. Greylisting and Spam-Related Errors:

  • 451 - Requested action aborted: local error in processing: This error is often encountered in conjunction with greylisting, where the recipient's mail server temporarily rejects the email to discourage spam. The sender should retry the delivery later.
  • 550 - Message identified as SPAM: This error indicates that the recipient's mail server has classified the email as spam and has refused to accept it for delivery.

Understanding these SMTP error codes can help diagnose email delivery issues and take appropriate action to resolve them, such as correcting email addresses, retrying delivery, or addressing authentication problems.

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