Definition
A delivery status notification, often called a bounce message or non-delivery report (NDR), is an automated response from a receiving mail server that communicates a delivery outcome back to the sender. DSN messages follow the RFC 3464 standard and contain structured information including the original recipient address, the delivery status code and, in many cases, a human-readable explanation.
DSN codes use a three-number system. The first digit indicates delivery class: 2 for success, 4 for temporary failure and 5 for permanent failure. The second digit indicates the category and the third provides detail.
Common DSN Codes
| Code | Class | Meaning |
|---|---|---|
| 250 | Success | Message accepted for delivery |
| 421 | Temporary | Service not available, retry later |
| 450 | Temporary | Recipient mailbox unavailable |
| 452 | Temporary | Recipient mailbox over quota |
| 550 | Permanent | Recipient address does not exist |
| 551 | Permanent | User not local, try a different path |
| 552 | Permanent | Message exceeds size limit |
| 554 | Permanent | Transaction failed, possible spam detection |
Best Practices
- Process DSN messages automatically to update your bounce list
- Investigate spikes in 5xx codes immediately — they indicate permanent delivery failures
- Monitor trends in 4xx codes by recipient domain to catch early reputation issues
- Remove addresses that produce repeated permanent failures after the first occurrence
Related Glossary Terms
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.
DMARC Alignment
DMARC identifier alignment determines whether the domain in the From header matches the domains used in SPF and DKIM authentication. Strict or relaxed.
Acceptance Rate
Acceptance rate is the percentage of emails submitted for delivery that are accepted by the recipient's mail server, representing the first deliverability checkpoint before inbox placement is determined.
Email Acceptance Rate
Email acceptance rate measures the percentage of emails accepted by recipient mail servers versus total sent. It is a core deliverability metric that reveals whether your sending infrastructure is trusted by receiving servers.