I'm not sure if you explicitly want an RFC-style description (i.e. follows https://www.rfc-editor.org/rfc/rfc2119 for MUST vs SHOULD vs etc) or if you are using RFC as a colloquial term for the technical details of the protocol.
In case of the latter, the "protocol" link at the top resolves to this GitHub repo: https://github.com/Open-Email/MailHTTPS-Protocol