What is a bounce code?
A bounce code is a numeric error identifier that a sender receives whenever an email they sent out bounces. There are over 60 different bounce codes, and they are as follows:
| Bounce Code | Description | Bounce Type |
| 421 | Happens when the recipient server temporarily defers a message due to too many connections or messages within a short timeframe | Soft |
| 450 | Happens when the recipient mailbox is unavailable | Soft |
| 451 | Happens when there is a processing error | Soft |
| 452 | Happens when there is not enough system storage | Soft |
| 500 | Happens when there is a syntax error so the server is unable to recognise the command | Hard |
| 501 | Happens when there is syntax error in the command arguments | Hard |
| 502 | Happens when a command that is not implemented is used | Hard |
| 503 | Happens when the server encounters a bad sequence of commands | Hard |
| 504 | Happens when a command parameter that is not implemented is used | Hard |
| 550 | Happens when the recipient’s mailbox is unavailable (e.g., not found) | Hard |
| 551 | Happens when the recipient’s mailbox is not local to the server. | Hard |
| 552 | Happens when the action gets aborted because of exceeded storage allocation | Hard |
| 553 | Happens when the command gets aborted due to an invalid mailbox name | Hard |
| 554 | Happens when the transaction fails for an unstated reason | Hard |
| 4.2.2 | Happens when the email account is over its quota | Soft |
| 5.0.0 | Happens when the recipient address does not exist | Hard |
| 5.1.0 | Happens when the recipient mailbox refuses to accept messages from the sender | Hard |
| 5.1.1 | Happens when there is a bad destination mailbox address | Hard |
| 5.1.2 | Happens when there is a bad destination system address | Hard |
| 5.1.3 | Happens when there is a bad destination mailbox address syntax | Hard |
| 5.1.4 | Happens when the destination mailbox address is ambiguous | Hard |
| 5.1.5 | Happens when the email bounces even though the destination mailbox address is valid | Hard |
| 5.1.6 | Happens when the mailbox has moved | Hard |
| 5.1.7 | Happens when the sender’s mailbox has a bad address syntax | Hard |
| 5.1.8 | Happens when the sender has a bad system address | Hard |
| 5.2.0 | Happens when the email bounces due to undefined or other mailbox status | Soft |
| 5.2.1 | Happens when the recipient’s mailbox is disabled and not accepting messages | Soft |
| 5.2.2 | Happens when the recipient’s mailbox is full | Soft |
| 5.2.3 | Happens when the length of the message exceeds the administrative limit | Hard |
| 5.2.4 | Happens when there is a mailing list expansion problem | Hard |
| 5.3.0 | Happens when there is an undefined or other mail system status | Hard |
| 5.3.1 | Happens when the mail system full | Soft |
| 5.3.2 | Happens when the system is not accepting network messages | Hard |
| 5.3.3 | Happens when the system is not capable of selected features | Hard |
| 5.3.4 | Happens when the message is too big for system | Hard |
| 5.4.0 | Happens when there is an undefined or other network or routing status | Hard |
| 5.4.1 | Happens when there is no answer from the host | Hard |
| 5.4.2 | Happens when there is a bad connection | Hard |
| 5.4.3 | Happens when there is a routing server failure | Hard |
| 5.4.4 | Happens when the system is unable to route | Hard |
| 5.4.5 | Happens when there is network congestion | Soft |
| 5.4.6 | Happens when a routing loop is detected | Hard |
| 5.4.7 | Happens when the delivery time expires | Hard |
| 5.5.0 | Happens when there is an undefined or other protocol status | Hard |
| 5.5.1 | Happen when there is an invalid command | Hard |
| 5.5.2 | Happens when there is a syntax error | Hard |
| 5.5.3 | Happens when there are too many recipients | Soft |
| 5.5.4 | Happens when there are invalid command arguments | Hard |
| 5.5.5 | Happens when the wrong protocol version is used | Hard |
| 5.6.0 | Happens when there is an undefined or other media error | Hard |
| 5.6.1 | Happens when the media not supported | Hard |
| 5.6.2 | Happens when conversion is required and prohibited | Hard |
| 5.6.3 | Happens when conversion is required but not supported | Hard |
| 5.6.4 | Happens when conversion with loss is performed | Hard |
| 5.6.5 | Happens when the conversion fails | Hard |
| 5.7.0 | Happens when there is an undefined or other security status | Hard |
| 5.7.1 | Happens when the delivery is not authorized and the message is refused | Hard |
| 5.7.2 | Happens when mailing list expansion is prohibited | Hard |
| 5.7.3 | Happens when security conversion is required but not possible | Hard |
| 5.7.4 | Happens when security features are not supported | Hard |
| 5.7.5 | Happens when there is cryptographic failure | Hard |
| 5.7.6 | Happens when the cryptographic algorithm is not supported | Hard |
| 5.7.7 | Happens when there is message integrity failure | Hard |
| 9.1.1 | Happens when there is a hard bounce with no bounce code found | Hard |