Non Delivery Report - Format

Format

The format for the reporting of administrative messages is defined by RFC 6522. A DSN may be a MIME multipart/report message composed of three parts:

1. a human readable explanation;
2. a machine parsable message/delivery-status, a list of "name: type; value" lines that state several possible fields; and
3. the original message, or a portion thereof, as an entity of type message/rfc822.

The second part of a DSN is also quite readable. It is essential to understand which MTA played which role. The Reporting-MTA is responsible for composing and sending the DSN.

When a Remote-MTA rejects a message during an SMTP transaction, a field Diagnostic-Code of type smtp may be used to report that value. Note that beside the numerical 3-digit value, the SMTP response contains itself a human readable part. The information

Remote-MTA: dns; smtp.store.example Diagnostic-Code: smtp; 550 No such user here

is sometimes reported as, e.g.,

while talking to smtp.store.example >>> RCPT TO: <<< 550 No such user here