Consider the economics from the vendor's side. You upload 10,000 addresses. On some share of them the receiving server never gives a straight answer: it greylists, throttles, blocks the probe or times out. If the vendor bills for all 10,000, then those non-answers are the most profitable rows in the file, because they cost the least work.
That is a bad incentive to leave lying around. It does not mean vendors act on it deliberately. It means there is no pressure in the system to improve on the hard cases, because the hard cases pay the same as the easy ones.
Not billing them inverts it. An address we could not get an answer on is a row we did work on and earned nothing from, which is the correct pressure to put on ourselves.
Duplicates follow the same logic. Deduplicating a file costs almost nothing and charging for both copies is charging twice for one answer. Bouncer, Kickbox, Reoon and Emailable all decline to bill unknowns, so this is table stakes rather than a differentiator.
What this means in practice
Upload 10,000 addresses and you are billed for the ones that came back with a result: valid, invalid or catch-all. Unknowns and duplicate rows come back labeled and cost nothing. Catch-all sits on the billed side on purpose. It took a full handshake plus a second probe with an invented address, and it tells you something true about the domain: it accepts everything, so nobody's valid there is a measurement.
You will see the catch-all and unknown shares as headline numbers rather than buried in an export. Together they are the coverage gap on your particular list, and it varies enormously: a list of Gmail consumer addresses resolves far better than a list of business domains behind security gateways.
Credits do not expire, so an unused balance is still there next quarter. Kickbox and Hunter both expire credits after twelve months, which is worth checking against whatever you are comparing us to.
Where this argument costs us something
The short version
- Check whether a verifier bills for unknowns before you compare per-address prices, and ask what it does with catch-all.
- Ask whether duplicates are charged, because on a merged list that can be a meaningful share.
- Check credit expiry, since a cheaper rate with a twelve-month clock can cost more.
Questions people ask
What counts as an unknown?
Any address where the receiving server would not give us a usable answer: a throttle, a refused connection, a temporary failure that persisted, or a probe that was blocked. It is a statement about the server rather than the address.
Are catch-all results billed?
Yes. Catch-all is a finding, and it took a full handshake plus a second probe to reach it. It tells you the whole domain accepts everything, which is the reason not to trust a valid anyone else sells you for that domain. Unknown is the absence of a finding, and that is the one we never bill.