http://www.rfc-editor.org/rfc/rfc6758.txt
RFC 6758: Tunneling of SMTP Message Transfer Priorities, A. Melnikov, K. Carlberg2012-10-01T23:00:00-00:00This memo defines a mechanism for tunneling of SMTP (Simple Mail Transfer Protocol) Message Transfer Priority values through MTAs (Message Transfer Agents) that don't support the MT-PRIORITY SMTP extension. This document is not an Internet Standards Track specification; it is published for informational purposes.