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Common Bounce Codes
A bounce code is a reply from the recipient’s mail server that describes why an email wasn’t delivered. Mail server administrators can customize the messages that accompany codes, so messages can be different from server to server. Account Engagement doesn’t generate bounce codes, so contact the recipient’s mail server administrator if you need clarification about a message.
Your bounce reports can show both traditional and enhanced codes. Traditional SMTP bounce codes are three digits. Enhanced SMTP bounce codes are three digits separated by decimal points. Popular services such as Gmail and Outlook.com often have their own bounce responses, so make sure to check the service’s documentation for more.
Traditional Bounce Codes
| Code | Explanation |
|---|---|
| 421 | <domain> Service not available, closing transmission channel. |
| 450 | Requested mail action not taken: mailbox unavailable (for example: mailbox busy). |
| 451 | Requested action aborted: error in processing. |
| 452 | Requested action not taken: insufficient system storage |
| 500 | The server couldn’t recognize the command due to a syntax error. |
| 501 | A syntax error was encountered in command arguments. |
| 502 | This command isn’t implemented. |
| 503 | The server has encountered a bad sequence of commands. |
| 504 | A command parameter isn’t implemented. |
| 550 | User’s mailbox was unavailable (such as not found). |
| 551 | The recipient isn’t 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. |
Enhanced Bounce Codes
| Code | Explanation |
|---|---|
| 5.0.0 | Unknown issue |
| 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 |
| 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 |
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