5.3 accredit: Sender Accreditation
accreditation = 'accredit' '=' domain-spec
The argument to the accreditation modifier is a domain-spec to be
macro-expanded and queried. The result of the query is interpreted
according to the definitions set forth by the accreditation service.
For example,
accredit=%{d}.accreditation-provider.example.com
accredit=%{ir}.accreditation-provider.example.com
This facility allows the publishing domain to make independently
verifiable assertions about itself in machine-readable form.
Multiple "accredit" modifiers may appear in one SPF record.
The "accredit" modifier is OPTIONAL. SPF clients MAY ignore it.
SPF publishers MAY omit it. If a receiver does not recognize the
domain-spec argument, it is free to ignore the modifier.
It is expected that SPF-enabled receivers will maintain a library of
recognized accreditation providers, keyed by the domain-spec. An
accreditation providers is responsible for describing the protocol
it uses to encode assertions. For example, suppose an accreditation
provider supports DNS "A" queries against the expanded domain-spec.
Suppose a result of NXDOMAIN means "domain is not known to the
accreditation service." Suppose a result of "127.0.0.10" means "the
accreditation service vouches for the integrity of the sender
domain." Accreditation providers can make up any protocol they like
as long as they can convince receivers to parse it.