- Verification coverage, defined
- Verification coverage is the share of a list that a verifier gives a definitive verdict, valid or invalid, rather than leaving it catch-all or unknown.
Coverage is plain arithmetic. Say you upload 10,000 addresses and get back 6,200 valid, 1,300 invalid, 1,900 catch-all and 600 unknown. The definitive rows add up to 7,500, so coverage on that file is 75%.
Your list sets most of that number, not the tool. A catch-all domain answers 250 for every address, so no SMTP check can reach a verdict there. In one 10,000-address B2B benchmark about 28% of addresses sat on catch-all domains, which caps coverage on a list like that well below 100% for every verifier.
Other rows drop out for reasons that change day to day. Greylisting servers stall a first attempt, Microsoft rate-limits probe traffic, and Gmail and Microsoft 365 block probes from cloud IPs. Those rows come back unknown, and many of them resolve on a later retry.
That's why coverage belongs beside accuracy, never instead of it. Coverage tells you how much of the file you can act on with confidence. Accuracy tells you how far to trust those verdicts, and its false-positive rate tells you how many dead addresses slipped through as valid.
Most vendors publish only the second number. ZeroBounce claims 99.6% accuracy, read on its own site in September 2026, with no coverage figure beside it. Hunter's benchmark of 15 verifiers on 3,000 real business addresses, dated 3 November 2025, measured overall accuracy between 31.2% and 70.0% once every row counted. What accuracy is actually achievable explains the gap.
You can use coverage to compare tools on your own data. Run the same sample through each one and count the definitive rows. Then send a small batch of each tool's valid rows and see which bounce.
How ZapBounce reports it
ZapBounce reports two numbers together: coverage, the share of your list that reached a definitive verdict, and the accuracy of those verdicts. Each batch summary counts valid, invalid, catch_all and unknown, so your coverage for that file is on screen the moment it finishes. Catch-all rows are labeled as catch_all, never counted as valid, and unknown rows are never billed. The accuracy page sets out the method, and the pricing page shows what each check costs.
Verification coverage: common questions
What is a good verification coverage rate?
It depends on the list more than the tool. A consumer list on Gmail and Outlook addresses can resolve almost entirely, while a B2B list with a quarter of its addresses on catch-all domains can't pass about 75%. Compare tools on the same sample rather than against a single benchmark.
Do catch-all addresses count toward coverage?
No. A catch-all result means the server accepted a made-up address too, so the mailbox itself was never confirmed. Coverage counts only valid and invalid, the rows with a definitive answer.
How is coverage different from accuracy?
Coverage is how much of the list got a verdict. Accuracy is how many of those verdicts were right. A tool can score high on one and low on the other, which is why a single headline percentage tells you little.
Can I raise coverage on my own list?
Partly. Rechecking unknown rows a few hours later resolves many greylisted and throttled addresses. Catch-all domains stay catch-all however often you check them, so plan a separate, slower send for those.