http://www.rfc-editor.org/rfc/rfc7508.txt
RFC 7508: Securing Header Fields with S/MIME, L. Cailleux, C. Bonatti2015-04-01T23:00:00-00:00This document describes how the S/MIME protocol can be extended in order to secure message header fields defined in RFC 5322. This technology provides security services such as data integrity, non-repudiation, and confidentiality. This extension is referred to as 'Secure Headers'.