SMTP Bounce Codes

What is it?

What is it?

SMTP Bounce Codes are standardized error codes generated by mail servers to indicate the status or reason for the failure of email delivery attempts. When an email message cannot be successfully delivered to its intended recipient, the recipient's mail server generates an SMTP bounce code, which is included in a bounce message returned to the sender's email server. Understanding SMTP bounce codes helps email senders diagnose delivery issues and take appropriate action to address them.

Key points to remember

Key points to remember

  • Error Classification: SMTP bounce codes are categorized into different classes, including soft and hard bounces, based on the nature and severity of the delivery failure.

  • Soft Bounces: Soft bounce codes indicate temporary delivery failures caused by issues such as the recipient mailbox being total, temporary server errors, or message size limits being exceeded. Soft bounces typically suggest that delivery may be retried later.


  • Hard Bounces: Hard bounce codes indicate permanent delivery failures that result from issues such as invalid recipient addresses, domain not found, or recipient email account does not exist. Hard bounces indicate that further delivery attempts are unlikely to succeed and may suggest that the recipient's address should be removed from the mailing list.


  • Diagnostic Information: SMTP bounce codes often include additional diagnostic information or error messages that provide details about the specific reason for the delivery failure, helping senders troubleshoot and resolve issues more effectively.


  • Handling Bounce Messages: Email senders can configure their email systems to process bounce messages automatically, categorize bounce types, and take appropriate actions, such as removing invalid addresses, updating contact lists, or adjusting email content or delivery practices.


  • Bounce Management: Effective bounce management practices, including monitoring bounce rates, analyzing bounce patterns, and maintaining clean mailing lists, are essential for maintaining good email deliverability and sender reputation.

Example of Use

Example of Use

  1. Bounce Handling System: An email marketing platform automatically processes bounce messages received from mail servers, categorizes bounce types based on SMTP bounce codes, and updates subscriber lists accordingly, removing invalid or undeliverable email addresses to improve deliverability.


  2. Diagnostic Troubleshooting: A system administrator reviews SMTP bounce codes reported in bounce messages to identify recurring delivery issues, diagnose underlying causes, and implement corrective measures, such as adjusting email server settings or contacting the recipient's mail provider for resolution.

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