http://www.rfc-editor.org/rfc/rfc3886.txt
RFC 3886: An Extensible Message Format for Message Tracking Responses, E. Allman2004-09-01T23:00:00-00:00 This memo defines a MIME content-type for message tracking status in the same spirit as RFC 3464, "An Extensible Message Format for Delivery Status Notifications". It is to be issued upon a request as described in "Message Tracking Query Protocol". This memo defines only the format of the status information. An extension to SMTP to label messages for further tracking and request tracking status is defined in a separate memo. [STANDARDS-TRACK]