What is a verifier waterfall?
A verifier waterfall runs a list through several email verification services in turn. The first checks every address; only the addresses it could not decide, its unknowns, go on to the second, and so on. Because each later verifier sees fewer addresses, the order changes what the whole chain costs and how many addresses end up confirmed valid.
Reading the result
Enter the list size and, for each verifier you use, its price per check and the shares it marks valid and invalid. The calculator runs every order of your stack and ranks them by cost per valid address. The step table shows what each verifier checks, resolves and passes on. Every figure comes from what you entered; none comes from a verifier.
What changes the answer
Unknowns are the hard addresses
What a first verifier cannot decide is often a catch-all domain or a server that does not answer, and a second verifier may struggle with the same addresses. Enter shares measured on unknowns where you have them; fresh-list shares can overstate the gain.
Free unknown results
Some services do not charge for an address they return as unknown. Tick or untick it per verifier: it can move an expensive but accurate verifier to the front.
Cheapest is not always most valid
The order with the lowest cost per valid address can leave more addresses unknown than another. The ranking table shows both, so you can trade one for the other.
A verifier that earns nothing
If removing a verifier barely changes the valid count, it may cost more than it adds. Remove it from the list and compare the totals.