https://datatracker.ietf.org/doc/draft-ietf-dkim-dkim2-spec/
draft-ietf-dkim-dkim2-spec-01 - DomainKeys Identified Mail Signatures v2 (DKIM2)
DomainKeys Identified Mail v2 (DKIM2) permits a person, role, or organization that owns a signing domain to document that it has handled an email message by...
domainkeys identified maildraftietfdkimspec