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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesVouch By Reference (VBR) is an IETF email protocol that lets a third party vouch for a domain’s mail of a stated type. The sender identifies the domain, labels the message category, and names potential certifiers; the receiving system validates the domain and checks a certifier it already trusts. A positive result is one input to email handling—not proof that a message is safe or a command to deliver it.
What Vouch By Reference does
VBR adds third-party certification to email. It separates three questions that are easy to confuse: whether a message is associated with a legitimate domain, whether a trusted third party vouches for that domain’s specified kind of mail, and what the receiving system should do with the result. VBR addresses the second question; it does not replace identity validation or the receiver’s own filtering policy. The protocol is specified in IETF RFC 5518, a Standards Track document published in April 2009.
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How a VBR check works
- The sender adds a VBR-Info header. It names an accountable domain, describes the message content, and lists certifier domains that may vouch for that category of mail.
- The receiver validates the domain identity. RFC 5518 recommends an available identity mechanism such as DKIM. A valid DKIM signature can establish the domain identity used with VBR; the RFC also recommends protecting the VBR-Info header with DKIM.
- The receiver chooses a certifier it trusts. The sender’s list is a set of candidates, not a trust decision. A receiver should not accept a certifier merely because the sender named it.
- The receiver checks DNS for the trusted certifier’s statement. It constructs a lookup using the accountable domain, the literal
_vouchlabel, and the vouching-service host name, then queries a TXT record. The record indicates which message types that service certifies for the domain. - The receiver applies its own policy. It can use the result as an additional certification or reputation signal, but local policy determines whether that changes filtering or delivery.
What the VBR-Info fields mean
RFC 5518 defines three required parameters in the VBR-Info header:
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mdidentifies the accountable domain.mcstates the message-content category.mvgives a colon-separated list of certifier domains.
The RFC illustrates the format as:
VBR-Info: md=somebank.example; mc=transaction; mv=certifier-a.example:certifier-b.example;The content labels
all: all mail from the sender.list: identical or very similar messages sent to multiple recipients.transaction: a response to a user action or a notice about an event in the user’s account.
The sender asserts the mc label. It helps describe the mail for certification and auditing, but the label itself is not independent proof that a message fits the category.
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What a positive VBR result does—and does not—tell you
A positive result means a third party the receiver has chosen to trust is willing to vouch for the specified domain and type of mail. It does not establish that the particular message is harmless, that the recipient consented to receive it, or that it must reach the inbox. VBR is a certification signal, not a content-safety verdict.
RFC 6212 registers vbr as an Authentication-Results method. The result can be recorded alongside other authentication information, but what a receiver does with Authentication-Results is a matter of local policy. See RFC 6212, published in April 2011.
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Why certifier choice matters
If a receiver uses VBR to influence spam scores, the chosen certifier can have meaningful operational influence over filtering. RFC 5518 therefore advises recipients to select vouching services carefully. Using more than one certifier may reduce reliance on a single service, but multiple endorsements do not guarantee that a message is correct or safe.
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| Function | Question it answers | Role in email handling |
|---|---|---|
| Domain identity validation | Is the message associated with the domain it claims to represent? | Establishes the identity used for a VBR check; RFC 5518 recommends DKIM as an identity-validation method. |
| VBR certification | Does a trusted third party vouch for this domain’s stated category of mail? | Adds a certification or reputation signal after identity validation. |
| Receiver policy | How should this message be handled? | Determines how authentication and certification results affect filtering or delivery. |
Standard and current deployment evidence
RFC 5518 is the protocol specification; RFC 6212 defines how VBR results are registered for use in Authentication-Results. The existence of these standards does not establish how widely mail systems currently deploy VBR. Neither document supplies adoption statistics, so an adoption percentage or claim of widespread use cannot be inferred from standards status alone.
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