GTM ClubNo provider finds every email. Provider A might verify 55% of a list, B another 15% of what A missed, C another 8%. Chained in a waterfall, the list goes from half-usable to mostly usable, and every address is verified on the way.
The order matters: cheapest and most accurate first, expensive and broad last. The output is a list where bounce rate stays under 2%, which is the line deliverability depends on.