An unknown result means the receiving server gave no usable answer. It timed out, it greylisted the probe, or it refused the connection. No verdict was reached, so a vendor that charges for it is charging you for a check that failed. That's why the refund became the market norm.
Here is how the vendors word it, each read on their own pricing or docs page on 24 September 2026. Kickbox says "Unknown addresses don't count against your verification balance." Emailable says "Duplicates and Unknown results are refunded." Bouncer never charges unknowns or duplicates, Clearout returns the credit automatically, and Hunter counts no credit. ZeroBounce and Reoon don't bill them either.
Catch-all is where the rules split. ZeroBounce, Kickbox, Emailable and Clearout bill a catch-all result like any other. MillionVerifier charges a credit only when it resolves the address. DeBounce sells catch-all validation as a separate check at 10 credits each, Scrubby charges 1 or 3 credits depending on depth, and BounceBan charges 1 credit for every result it returns.
A catch-all result is a finding, not a failure. Your verifier had to send a second probe to a made-up address at the same domain, and the server accepted that too. You learn something real: a valid from that domain proves nothing. Around 28% of a typical B2B list sits on domains like this, so your catch-all share decides a big slice of the bill.
Before you buy, find two lines on the pricing page: whether unknowns and duplicates are billed, and what a catch-all result costs. If a rule isn't printed, ask for it in writing before you upload. Then set it against ZapBounce pricing, and read what an unknown result means to decide what to do with the rows that come back that way.
Related questions
Why do verifiers charge for catch-all results?
A catch-all result takes a second probe to prove, and it tells you the domain accepts every address. Every vendor that published a catch-all rule on 24 September 2026 billed it in some form.
Are duplicate addresses billed by email verifiers?
Not at every vendor. Bouncer and Emailable both state that duplicates are free. ZapBounce removes duplicates before billing, so each distinct address costs one credit at most.
Is a refunded unknown the same as a free check?
Yes, in cost terms: the credit goes back or is never taken. The row still comes back marked unknown, so you can retry it later or mail it in a separate batch.
How does ZapBounce bill a catch-all address?
ZapBounce bills a catch_all result once, at one credit, the same as valid or invalid. Unknown results, duplicates and errors cost nothing, and credits never expire.