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.
To do so, the receiver queries a TXT resource record for the name composed: The returned data, if any, is a space-delimited list of all the types that the service vouches, given as lowercase ASCII.
The result of the authentication can be saved in a new header field, according to RFC 6212, like so: OpenDKIM and MDaemon Messaging Server by Alt-N Technologies[2] have been among the first software implementations of VBR.
Roaring Penguin Software's CanIt anti-spam filter supports VBR as of version 7.0.8 released on 2010-11-09.