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 |
| 499 | Unable to connect to MX servers (e.g., wrong email address or destination email server is down) |
| 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 was unavailable (such as not found) |
| 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. |