- Unknown verification result, defined
- An unknown result means the verifier could not reach a defensible verdict, usually because the receiving server refused to answer clearly.
Servers produce this state deliberately. Microsoft throttles repeated probes. Gmail blocks verification traffic from cloud addresses outright. Yahoo sometimes answers 252, which says in so many words that it won't verify the recipient.
Vendors handle the same situation very differently. Some return unknown honestly, some fold it into a risky bucket, and some resolve it to valid, which flatters the accuracy figure while making the number meaningless.
The useful question about any verifier is not its accuracy percentage but what share of your list it declined to judge. That number is the denominator the accuracy claim was calculated on.
ZapBounce reports unknown as its own verdict and does not bill for it.
How ZapBounce reports it
Unknown is a first-class result in our API, not a footnote, and it is never billed. The reason field names what happened: greylisted, throttled, probe_blocked, timeout, temporary_failure, or cannot_verify for a 252. A domain that accepts every recipient is a different result, catch_all, and that one is billed.
Where 1,100 unknowns come from on one list
Take a hypothetical 10,000-address list that comes back as 6,100 valid, 900 invalid, 1,900 catch-all and 1,100 unknown. Open the unknowns and sort by the reason given. You might find 520 at Yahoo and AOL hosts that answered 252, the reply that says outright the server won't verify anyone. Another 380 are at Microsoft-hosted domains that throttled the probe, and 200 are at servers that greylisted twice or never answered.
Those three groups need different handling, which is why the reason matters as much as the label. The Yahoo group won't resolve next week or next year, because the provider designed it that way. Throttled addresses often will resolve if you run them again on another day. For the last group it's a coin toss.
On billing, the 1,100 credits for those unknowns stay in your balance, and ZapBounce credits don't expire. The 1,900 catch-all rows did cost a credit each, because a catch-all is a finding about the domain and an unknown is the lack of one.
What a vendor's unknown share tells you
Ask two verifiers to process the same file and compare the unknown counts before you compare anything else. If one reports 11% unknown and the other reports 0.5%, the second one didn't get better answers from Yahoo. Yahoo gave both of them the same non-answer, and one of them chose to print it as valid or invalid.
That choice is where the accuracy claim gets made. Accuracy is a fraction, and the unknowns are what a vendor removes from the bottom of it. A figure of 99% measured on the easy 70% of a list and a figure of 99% measured on all of it are very different products, even though the brochure number is identical.
So read the pair together. Coverage says how much of your list got a verdict, and accuracy says how often those verdicts were right. If you're handed either number without the other, the claim can't be checked.
Unknown verification result: common questions
What share of a list usually comes back unknown?
It depends almost entirely on the mix of domains. Lists heavy in Microsoft 365 tenants and throttling corporate hosts produce far more than a consumer Gmail list.
Should I send to unknown addresses?
Send to them separately from your verified mail so any bounces do not sit inside your main send, and let the result tell you.
Why do vendors report unknowns so differently?
Because the unknown bucket is where the accuracy percentage is made. Resolving unknowns to valid raises the headline figure without improving the check.