Walk down memory lane...
Version 1: Circa 2013
The first version never launched. The model used challenge response to identify first time senders and request proof. The User would set their identity assurance threshold a sender would need to meet for message delivery. Senders would provide self asserted proof, 3rd party proof (SSO) or 3rd party verified (KYC) to meet the threshold requirements.
Version 2: Circa 2018
The second version implemented as an Add-in for Outlook, reported lookups to public sources (Full Contact + Haveibeenpwned) in addition to email authentication protocol validation to yield an opinion on authenticity. The Request Proof function requested Senders to establish proof through SSO, MFA and Message Confirmation.