Opt-in email is a term used when someone is not initially added to an emailing list and is instead given the option to join the emailing list. Wikipedia

Closed-loop authentication, as applied to computer network communication, refers to a mechanism whereby one party verifies the purported identity of another party by requiring them to supply a copy of a token transmitted to the canonical or trusted point of contact for that identity. Wikipedia

Mechanism whereby one party verifies the purported identity of another party Wikidata