(which are frequently used in spamming as a way of determining which spam recipients open (and presumably read) before deleting it) References K. Moore. Simple Mail Transfer Protocol (SMTP) Service Extension for Delivery Status Notifications (DSNs). RFC 3461, January 2003. G. Vaudreuil. The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages. RFC 3462, January 2003. G. Vaudreuil. Enhanced Mail System Status Codes. RFC 3463, January 2003. K. Moore & G. Vaudreuil. An Extensible Message Format for Delivery Status Notifications. RFC 3464, January 2003. A. Melnikov. Message Disposition Notification (MDN) profile for Internet Message Access Protocol (IMAP). RFC 3503, March 2003. T. Hansen & G. Vaudreuil (editors). Message Disposition Notification. RFC 3798, May 2004. Email