| 0 | Unknown/Undefined | NO | Generic category assigned to bounces not classified under other categories. |
| 1 | Bad destination mailbox address | Permanent | The specified email address does not exist. This means that the portion to the left of the @ is invalid. |
| 2 | Bad destination system address | Temporary | The domain specified in the email address is unable to accept emails. |
| 3 | Bad destination mailbox address syntax | Temporary | A syntactic problem was detected by the receiving domain. |
| 4 | Recipient Block Policy | Temporary | The recipient user has created (only at the mailbox level) a personal filter that blocks the sender's address or domain. |
| 5 | Bad sender’s mailbox address syntax | NO | The sender’s address was syntactically invalid. |
| 6 | Bad sender’s system address | NO | The sender’s domain is unable to accept reply emails. |
| 7 | Other or undefined mailbox status | NO | The mailbox was unable to accept emails. |
| 8 | Mailbox disabled, not accepting messages | Temporary | The mailbox exists but does not accept messages. This could be a temporary or permanent error. |
| 9 | Mailbox full | Temporary | The user's mailbox is full because it has reached its maximum capacity. They can free up space by deleting messages. |
| 10 | Message length exceeds administrative limit | Temporary | An administrative limit on the message size was exceeded. This error occurs when the allowed size by the recipient’s mailbox is smaller than the general system limit. |
| 11 | Reputation block | NO | Message rejected due to reputation issues of the sender's IP or domain, caused, for example, by being listed in public blacklists or blocked by anti-spam systems. This type of bounce does not cause the email address to be blacklisted, as the issue can be resolved once the IP or domain is unblocked or de-listed. |
| 12 | No answer from host | NO | The outgoing connection attempt was not accepted either because the system was busy or incapable. |
| 13 | Bad connection | NO | The outgoing connection was established but was unable to process the message, either due to a timeout or poor connection quality. |
| 14 | Routing server failure | NO | The system was unable to forward the message due to unavailable directory servers (e.g., inability to connect to the DNS server). |
| 15 | Unable to route | Temporary | The system cannot find the correct path to reach the destination server. Sometimes this may also depend on the domain not existing. |
| 16 | Routing loop detected | NO | Mail servers check if the same message passes through them multiple times and after a certain number of times (at the postmaster’s discretion) decide that the message will never reach its destination and return it, indicating refusal. |
| 17 | Delivery time expired | NO | Our system, in case the receiving mail server is unreachable (without receiving a specific rejection), makes multiple retries. If after 48 hours the message still has not been delivered, it is considered failed and categorized as a bounce. (The cause may be a temporarily out-of-service server or a domain without MX records). |
| 18 | Other or undefined protocol status | NO | An error occurred in the message transmission protocol. |
| 19 | Invalid command | NO | A protocol command was entered out of sequence or unsupported. |
| 20 | Syntax error | NO | A protocol command was entered that could not be interpreted due to a syntax error or was not recognized. |
| 21 | Delivery not authorized, message refused | NO | The sender is not authorized to send to the recipient (e.g., due to anti-spam filter blocks per host or per recipient) and/or does not meet the required authentication standards (e.g., misalignment of DKIM/SPF/DMARC). |
| 22 | Relaying Denied | NO | During transmission across various servers, one of them was unable to deliver the email. The message is rejected by the server, possibly because the sender is recognized as invalid or unacceptable. |
| 23 | Service not available – access denied | NO | Typically caused by incorrect or outdated email addresses; when trying to deliver mail to a server that is only intended to send mail, not receive it. Sometimes used as a response when emails are rejected because they are considered spam. |
| 24 | Message considered to be spam | NO | The message is considered spam. |