http://www.rfc-editor.org/rfc/rfc5518.txt
RFC 5518: Vouch By Reference, P. Hoffman, J. Levine, A. Hathcock2009-04-01T23:00:00-00:00This document describes the Vouch By Reference (VBR) protocol. VBR is a protocol for adding third-party certification to email. It permits independent third parties to certify the owner of a domain name that is associated with received mail. [STANDARDS-TRACK]