Definition: Vouch By Reference (VBR) lets email senders specify third parties vouching for their mail to be legitimate.
In an extra header field of outgoing mail, the sender can list so-called domain certification services that they believe will certify their integrity and good standing. The recipient's software can look up VBR vouchers in the domain name service. If it finds trusted services do attest the sender to be reputable, any spam filtering can be bypassed.
Vouch By Reference uses the domain name (what follows the '@' sign in an email address) in the From: line to determine a sender. To prevent forgery, VBR works in tandem with sender authentication schemes such as DomainKeys Identified Mail (DKIM), Sender Policy Framework (SPF) or Sender ID.
