Understanding why your emails bounce can help you improve your email deliverability and overall campaign performance. When emails bounce, the receiving server usually provides a
bounce code that explains the reason for the failure. Bounce codes can be divided into two main categories: traditional bounce codes and enhanced bounce codes.
Traditional Bounce Codes
Traditional bounce codes are numeric codes that provide basic information about the bounce. They are divided into two main ranges:400 Range: Generally indicates soft bounces. Soft bounces are temporary issues that may resolve over time, such as a full mailbox or a temporary server issue.
500 Range: Generally indicates hard bounces. Hard bounces are permanent issues that prevent the email from being delivered, such as an invalid email address.
Here are some common traditional bounce codes:
| Code | Explanation |
| 421 | <domain> Service not available, closing transmission channel. |
| 450 | Requested mail action not taken: mailbox unavailable (e.g., mailbox busy). |
| 451 | Requested action aborted: error in processing. |
| 452 | Requested action not taken: insufficient system storage. |
| 550 | User’s mailbox was unavailable (such as not found). |
| 552 | The action was aborted due to exceeded storage allocation |
| 553 | The command was aborted because the mailbox name is invalid. |
| 554 | The transaction failed for some unstated reason. |
Enhanced Bounce Codes
Enhanced bounce codes provide more detailed information about the bounce reason. These codes are typically a combination of three numbers separated by dots, such as 5.1.1.
Here are some common enhanced bounce codes:
| Code | Explanation |
| 5.0.0 | Address does not exist. |
| 5.1.1 | Bad destination mailbox address. |
| 5.1.6 | Mailbox has moved. |
| 5.1.7 | Bad sender’s mailbox address syntax. |
| 5.1.8 | Bad sender’s system address. |
| 5.2.0 | Other or undefined mailbox status. |
| 5.2.1 |
Mailbox disabled, not accepting messages. |
| 5.2.2 | Mailbox full. |
| 5.2.3 | Message length exceeds administrative limit. |
| 5.2.4 | Mailing list expansion problem. |
| 5.3.0 | Other or undefined mail system status. |
| 5.3.1 | Mail system full. |
| 5.3.4 |
Message too big for system. |
| 5.4.0 | Other or undefined network or routing status. |
| 5.4.1 | No answer from host. |
| 5.4.2 | Bad connection. |
| 5.4.3 | Routing server failure. |
| 5.4.4 | Unable to route. |
| 5.4.5 | Network congestion. |
| 5.4.6 |
Routing loop detected. |
| 5.4.7 | Delivery time expired. |
| 5.5.0 | Other or undefined protocol status. |
| 5.5.1 | Invalid command. |
| 5.5.2 | Syntax error. |
| 5.5.3 | Too many recipients. |
| 5.5.4 | Invalid command arguments. |
| 5.6.0 |
Other or undefined media error. |
| 5.7.0 |
Other or undefined security status. |
| 5.7.1 | Delivery not authorized, message refused. |
| 5.7.2 | Mailing list expansion prohibited. |
| 5.7.3 | Security conversion required but not possible. |
| 5.7.4 | Security features not supported. |
| 5.7.5 |
Cryptographic failure. |
| 5.7.6 | Cryptographic algorithm not supported. |
| 5.7.7 | Message integrity failure. |
Custom Messages
Mail server administrators can create custom messages that accompany bounce codes. Therefore, you might see variations or additional text along with the bounce codes provided above. It's essential to review these custom messages carefully to understand the specific reason for the bounce and take appropriate action.
Conclusion
By understanding and interpreting email bounce codes, you can identify and resolve issues with your email campaigns more effectively. If you have any questions or need further assistance, feel free to reach out to our support team.
