http://www.rfc-editor.org/rfc/rfc6710.txt
RFC 6710: Simple Mail Transfer Protocol Extension for Message Transfer Priorities, A. Melnikov, K. Carlberg2012-08-01T23:00:00-00:00This memo defines an extension to the SMTP (Simple Mail Transfer Protocol) service whereby messages are given a label to indicate preferential handling, to enable mail handling nodes to take this information into account for onward processing. [STANDARDS-TRACK]