DSN (Delivery Status Notification)
A DSN (Delivery Status Notification) is the standardized, machine-readable report a mail server generates to tell a sender what happened to a message it couldn't deliver, or in some cases confirm one it could -- defined by RFC 3461's SMTP extension for requesting them and RFC 3464's format for the report itself. A DSN includes a status code identifying the failure class (permanent versus temporary), the recipient address that failed, and which server reported the problem: machine-parseable fields a sender's own systems can act on automatically, separate from the human-readable bounce text a mailbox owner actually reads. RFC 5321 requires a DSN for a failed delivery to be sent with a null envelope sender (MAIL FROM:<>), specifically so a DSN about an undeliverable message can never itself generate another DSN and loop forever -- the same rule that, when a message's original envelope sender was forged, concentrates every resulting DSN onto whichever address was forged rather than the real sender.
By AdminKit team, Netbase JSC · Updated