http://www.rfc-editor.org/rfc/rfc7410.txt
RFC 7410: A Property Types Registry for the Authentication-Results Header Field, M. Kucherawy2014-12-01T23:00:00-00:00This document updates RFC 7001 by creating a registry for property types in the Authentication-Results header field, used in email authentication work, rather than limiting participants to using the original, small set of fixed values.