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 |
Why It Matters
This matters because the choices you make here show up directly in your results. The second digit indicates the category and the third provides detail. When this is handled well it supports engagement, delivery, and the trust subscribers place in your brand; when it is neglected, the effects tend to show up in declining performance and harder-to-fix problems further down the line.
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
Was this useful?
Related Glossary Terms
AI Content Detection
AI content detection refers to the growing ability of email clients, spam filters and consumers to identify machine-generated email copy. It matters for deliverability, trust and engagement in an era of mass-produced AI marketing email.
Batch Sending
Batch sending is the practice of delivering a single email campaign to a large list in groups or waves rather than all at once.
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.
Complaint Rate
Complaint rate is the percentage of delivered emails that recipients mark as spam, a key indicator of sender reputation and list quality.
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.
Frequently Asked Questions
Good practice here means handling Email Delivery Status Notification in a way that is relevant, timely, and honest for your audience. A delivery status notification (DSN) is an automated email message generated by a mail server to inform the sender that delivery has failed, been delayed, or succeeded. DSN messages carry standardised status codes for troubleshooting. Done well, it improves engagement and builds trust; done poorly, it creates friction that costs you results.
Because it touches the parts of email that drive outcomes: relevance, trust, and delivery. Small improvements compound, while repeated mistakes quietly erode the health of your programme.
The most common problems are treating Email Delivery Status Notification as a one-off task, ignoring what the data says, and copying competitors without testing. All three lead to effort that does not translate into better results.
Compare the metrics it should influence — engagement, conversions, and deliverability — before and after you make changes. Trends over time matter far more than any single send.
It supports the same goal as the rest of your email programme: the right message to the right person at the right time. Aligned with segmentation and automation, it reinforces everything else rather than competing with it.