DSN
A Delivery Status Notification: the standard machine-readable report (RFC 3464) on what happened to a message, used for bounces, delays and success notices.
What it means
A DSN is a multipart/report message with report-type delivery-status. Its first part is a human-readable explanation; its second is a message/delivery-status part with fields for the reporting server and, for each recipient, the Final-Recipient, an Action (failed, delayed, delivered, relayed or expanded), a Status holding an enhanced status code, and usually a Diagnostic-Code quoting the remote server's reply. A third part may return the original message or only its header.
Because the format is standard, bounce processing can read the Action and Status fields instead of guessing from free text. Senders can ask for DSNs on success, delay or failure through the DSN SMTP extension; failure reports are sent by default.
Example
Reporting-MTA: dns; mx.example.net
Final-Recipient: rfc822; old.user@example.net
Action: failed
Status: 5.1.1
Diagnostic-Code: smtp; 550 5.1.1 User unknownRelated terms
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Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes