Certified Server Validation (CSV) is a technical method of email authentication intended to fight spam.
As of January 3, 2007, all Internet Drafts have expired and the mailing list has been closed down since there had been no traffic for 6 months.
If the level of trust is in between, document the result in a header in each email delivered or forwarded, and/or perform additional checks.
Mail sources are motivated to make the answers yes, and it's easy for them to do so (unless their email flow is so toxic that no reputable independent accreditation service will vouch for them).
CSV is designed to be efficient and elegant, and in this respect it certainly beats SPF's coverage of HELO identities.