http://www.rfc-editor.org/rfc/rfc1891.txt
RFC 1891: SMTP Service Extension for Delivery Status Notifications, K. Moore1996-01-01T23:00:00-00:00This memo defines an extension to the SMTP service, which allows an SMTP client to specify (a) that delivery status notifications (DSNs) should be generated under certain conditions, (b) whether such notifications should return the contents of the message, and (c) additional information, to be returned with a DSN, that allows the sender to identify both the recipient(s) for which the DSN was issued, and the transaction in which the original message was sent. [STANDARDS-TRACK]