The 99% accuracy problem

Line up the claims: ZeroBounce 99.6%, NeverBounce 99.9%, Clearout 99.73%, Reoon over 99%, DeBounce a 97.5% deliverability guarantee. Bouncer and Emailable publish nothing, which is more honest than the rest of the market and worth saying out loud.

Now consider what a verifier does with an address it cannot resolve. A catch-all domain returns 250 for every recipient. Yahoo returns a positive response by design to defeat address harvesting. Microsoft throttles and returns temporary failures. Gmail and Microsoft 365 block probes from many cloud IP ranges outright.

None of those produce a mailbox verdict. The verifier either reports them as catch-all or unknown, or it guesses. A guess that lands becomes a correct verdict in the accuracy calculation. A guess that misses becomes your bounce.

Hunter's 40,000-verification benchmark measured the field at 63% to 70% because it counted the whole dataset. That is not a contradiction of the vendor figures. It is the same market measured with the denominator left in.

Suppose 10,000 addresses go in. Around 2,800 sit on catch-all domains, based on a B2B benchmark of that size. Several hundred more are Yahoo, Microsoft-throttled, greylisted or on servers that refuse the connection.

Exclude those and roughly 6,500 addresses remain, most of them straightforward: a working mailbox at a domain that answers honestly, or an address that plainly does not exist. Getting 99% of those right is not a remarkable feat. It is the easy part of the problem.

The claim is then printed next to a picture of your whole list. Nothing false was said, and the impression is wrong by about a third.