The practice of matching anonymous website visitors against a third-party identity graph so you can email or text them without them ever having given you their address. Vendors like Wunderkind, Instant and Retention.com sit on your site, recognise a share of your traffic from data they hold elsewhere, and push those contacts into your email and SMS flows as abandoned-browse or abandoned-cart audiences. It is the opposite approach to zero-party data: instead of the customer telling you who they are on purpose, a broker tells you on their behalf. That distinction is the whole argument, because it decides what you actually own and what you can explain to a regulator or an acquirer.
Benchmark. Match rates are typically quoted between 15% and 40% of US traffic, but those are vendor-published figures rather than independently verified benchmarks. The only number that counts is what survives a holdout: suppress a random slice of recovered contacts for 30 days and compare. If the lift does not hold up, you are paying to email customers who were already coming back.