Posted by Sean Cohen of Aweber
Confirmed opt-in as defined by SpamHaus, who is one of the most respected anti spam organizations in the world:
Known as “COI” in the legitimate bulk email industry, also known as “Confirmed Opt-in”, “Verified Opt-in” or sometimes “Double Opt-in”.
With Closed-Loop Opt-in the Recipient has verifiably confirmed permission for the address to be included on the specific mailing list, by confirming (responding to) the list subscription request verification. This is the standard practice for all responsible Internet mailing lists, it ensures users are properly subscribed, from a working address, and with the address owner’s consent.