http://www.rfc-editor.org/rfc/rfc6647.txt
RFC 6647: Email Greylisting: An Applicability Statement for SMTP, M. Kucherawy, D. Crocker2012-06-01T23:00:00-00:00Greylisting is an established mechanism deemed essential to the repertoire of current anti-abuse email filtering systems. [STANDARDS-TRACK]