ASSP and Other Mail Bounce Back Codes
Introduction
When an email cannot be delivered to its intended recipient, the sending mail server generates a bounce message, also known as a Non-Delivery Report (NDR). These messages contain SMTP reply codes or enhanced status codes that indicate why the email delivery failed.
ASSP (Anti-Spam SMTP Proxy) and other mail servers use these codes to identify issues such as invalid email addresses, full mailboxes, spam filtering, DNS problems, authentication failures, or server connectivity issues.
Understanding these error codes helps administrators quickly diagnose and resolve email delivery problems.
Prerequisites
Before troubleshooting a bounced email, ensure you have:
- Access to the complete bounce message (NDR).
- The SMTP or enhanced status code (for example, 550, 554, or 5.1.1).
- Sender and recipient email addresses.
- Mail server logs (if available).
- DNS records (MX, SPF, DKIM, and DMARC) configured correctly.
- Access to your mail server or hosting control panel.
Understanding NDR Status Codes
Enhanced status codes generally follow the format:
X.Y.Z
Where:
- 4.Y.Z – Temporary failure (Soft Bounce). The message may be delivered after a retry.
- 5.Y.Z – Permanent failure (Hard Bounce). The message will not be delivered unless the issue is resolved.
Examples
| Status Code | Meaning |
|---|---|
| 4.2.2 | Mailbox full (temporary) |
| 4.4.1 | Network connection failure |
| 5.1.1 | Invalid or unknown email address |
| 5.2.2 | Mailbox quota exceeded |
| 5.4.0 | DNS or routing problem |
| 5.7.1 | Permission or relay denied |
Common ASSP and Mail Bounce Codes
Message Rejected
Reason
The sending mail server does not match the sender’s domain or fails spam verification.
Solution
- Verify SPF, DKIM, and DMARC records.
- Ensure the sending server is authorized for the domain.
- Check server hostname and reverse DNS.
450 – Recipient Address Rejected
Reason
- Recipient domain not found
- Temporary DNS issue
- Too many emails sent
- Temporary mail server restriction
Solution
- Verify the recipient domain.
- Wait and retry.
- Check DNS configuration.
- Reduce the sending rate.
500 – Recipient Mailbox Full
Reason
The recipient mailbox has exceeded its storage quota.
Solution
- Ask the recipient to free mailbox space.
- Retry later.
503 – Improper Use of SMTP Command Pipelining
Reason
The sending mail server is using SMTP commands incorrectly.
Solution
- Verify SMTP configuration.
- Update the mail client or SMTP software.
540 – Recipient Address Rejected
Reason
The recipient email account has been suspended or deactivated.
Solution
- Ask the recipient to reactivate their account.
- Confirm the email address is active.
550 – User Unknown
Reason
The destination mailbox does not exist.
Solution
- Verify the recipient email address.
- Confirm the mailbox is active.
- Correct any spelling mistakes.
550 – Too Many Concurrent Connections
Reason
The sending server exceeded the allowed number of simultaneous SMTP connections.
Solution
- Reduce concurrent connections.
- Retry after some time.
554 – Access Denied
Reason
The message was rejected because:
- The sender IP is blacklisted.
- The email resembles spam.
- Reverse DNS is missing.
- SPF or DKIM validation failed.
Solution
- Check blacklist status.
- Configure SPF, DKIM, and DMARC.
- Verify reverse DNS (PTR).
- Modify spam-like subject lines or content.
554 – Relay Access Denied
Reason
The sender is not authorized to use the SMTP relay.
Solution
- Enable SMTP authentication.
- Verify SMTP credentials.
- Use the correct outgoing mail server.
554 – Blocked by Spam Filter
Reason
The sending IP appears on a spam blacklist.
Solution
- Check your IP reputation.
- Request delisting if necessary.
- Secure the server against spam abuse.
Standard SMTP Reply Codes
| Code | Description |
|---|---|
| 211 | System status information |
| 214 | Help message |
| 220 | Service ready |
| 221 | Service closing connection |
| 250 | Requested action completed successfully |
| 251 | User not local, message forwarded |
| 252 | Cannot verify user but will accept message |
| 354 | Start mail input |
| 421 | Service unavailable |
| 450 | Mailbox unavailable (temporary) |
| 451 | Local processing error |
| 452 | Insufficient storage |
| 454 | TLS unavailable temporarily |
| 500 | Command not recognized |
| 501 | Syntax error |
| 502 | Command not implemented |
| 503 | Bad command sequence |
| 504 | Command parameter not implemented |
| 521 | Server does not accept mail |
| 530 | STARTTLS required |
| 550 | Mailbox unavailable |
| 551 | User not local |
| 552 | Storage allocation exceeded |
| 553 | Invalid mailbox name |
| 554 | Transaction failed |
Frequently Encountered Enhanced Status Codes
| Code | Description |
|---|---|
| 4.2.2 | Mailbox full |
| 4.3.1 | Insufficient disk space |
| 4.4.1 | Network timeout |
| 4.4.6 | Too many mail hops |
| 5.1.0 | Invalid recipient address |
| 5.1.1 | User unknown |
| 5.1.2 | Host unknown |
| 5.2.1 | Message too large |
| 5.2.2 | Mailbox quota exceeded |
| 5.3.1 | Mail system full |
| 5.4.0 | DNS or routing failure |
| 5.4.4 | Cannot find next hop |
| 5.5.0 | SMTP protocol error |
| 5.5.5 | Wrong SMTP protocol version |
| 5.7.1 | Permission denied or relay prohibited |
| 5.7.5 | Encryption or cryptographic failure |
Conclusion
Mail bounce messages provide valuable information about why an email failed to reach its destination. By identifying the SMTP reply code or enhanced NDR status code, administrators can quickly determine whether the issue is temporary or permanent and take the appropriate corrective action. Regularly maintaining DNS records, configuring SPF, DKIM, and DMARC, monitoring IP reputation, and following email best practices can significantly reduce bounce rates and improve email deliverability.
