Vouch_by_Reference
Vouch by Reference (VBR) is a protocol used in Internet mail systems for implementing sender certification by third-party entities. Independent certification providers vouch for the reputation of senders by verifying the domain name that is associated with transmitted electronic mail. VBR information can be used by a message transfer agent, a mail delivery agent or by an email client.
The protocol is intended to become a standard for email sender certification, and is described in RFC 5518.[1]