Bounced emails are disappointing; it means some of your recipients are not receiving the email campaigns you’re sending them. And if the bounce rate is high, it will impact your deliverability.
To minimize this problem, you should manage your bounces in a proactive way by applying some helpful tactics. So what must be done first?
The answer is: Identifying the bounce codes which are responses from the receiving mail server, specifying why the email was not successfully sent.
| Code | Summary | Description |
|---|---|---|
| 2.XXX.YYY | Success | Success specifies that the DSN is reporting a positive delivery action. Detail sub-codes may provide notification of transformations required for delivery. |
| 4.XXX.YYY | Persistent Transient Failure | A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future *may be successful*. In this case, the bounce will be classified as a soft-bounce. |
| 5.XXX.YYY | Permanent Failure | A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery. |
How does VBOUT handle bounce codes (messages)?
Once we get an email reply (bounce message) from the mail server, we use the numeric bounce code in order to identify if the bounce type is a soft bounce (temporary failure) or hard bounce (permanent failure). VBOUT will automatically unsubscribe email recipients who hard bounce. Also, if a recipient has more than one soft bounce, the system will consider him a hard bounce.
You can find below a list of two types of bounce codes. The 5.X Bounce Codes (Enhanced SMTP bounce responses) which are composed of 3 numbers separated by decimal points i.e. (5.1.0) and the traditional SMTP Status Codes which have 3-digit codes without decimal points i.e. (504). You may find both in the bounce reasons for a particular email campaign.
5.X Bounce Codes (Enhanced SMTP Bounce Responses - Permanent Failures)
| Code | Description |
|---|---|
| 5.0.0 | Address does not exist |
| 5.1.0 | Other address status |
| 5.1.1 | Bad destination mailbox address |
| 5.1.2 | Bad destination system address |
| 5.1.3 | Bad destination mailbox address syntax |
| 5.1.4 | Destination mailbox address ambiguous |
| 5.1.5 | Destination mailbox address valid |
| 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 |
| 5.3.2 | System not accepting network messages |
| 5.3.3 | System not capable of selected features |
| 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 – the address is invalid |
| 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 |
| 5.5.5 | Wrong protocol version |
| 5.6.0 | Other or undefined media error |
| 5.6.1 | Media not supported |
| 5.6.2 | Conversion required and prohibited |
| 5.6.3 | Conversion required but not supported |
| 5.6.4 | Conversion with loss performed |
| 5.6.5 | Conversion failed |
| 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 |
Traditional SMTP Status Codes
The table below is not a definitive list of all available status codes, only a representation of the most frequent ones.
| Status Code Group | Group Name | Description |
|---|---|---|
| 1xx | Informational | Request received, continuing process |
| 2xx | Success | The action was successfully received, understood, and accepted |
| 3xx | Redirection | Further action must be taken in order to complete the request |
| 4xx | Client Error | The request contains bad syntax or cannot be fulfilled |
| 5xx | Server Error | The server failed to fulfill an apparently valid request |
| Code | Description |
|---|---|
| 421 | 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 |
| 500 | The server could not recognize the command due to a syntax error. |
| 501 | A syntax error was encountered in command arguments. |
| 502 | This command is not implemented. |
| 503 | The server has encountered a bad sequence of commands. |
| 504 | A command parameter is not implemented. |
| 550 | User’s mailbox address is invalid |
| 551 | The recipient is not local to the server. |
| 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. |
Hope this list of codes will help you determine all the reasons to better reduce your email bounce rates.
Want to learn the best practices that improve your email deliverability? Check out this article.
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