What is a Non-Delivery Report (NDR)?

A Non-Delivery Report (NDR), also called a Delivery Status Notification (DSN) or bounce message, notifies senders that their email failed to deliver.

Standard components:

Human-readable explanation: Description of why delivery failed. Machine-readable status codes: SMTP codes and enhanced status codes. Diagnostic information: Server responses, timestamps, routing details. Original message: Headers or full content of the failed message.

NDR format is defined in RFC 3464 and uses MIME multipart structure with specific content types.

Delivery to:

NDRs are sent to the Return-Path address, not the From address. This is why bounce handling addresses should be monitored.

NDRs are the official failure reports of email. They tell you what went wrong, when, and often why.