A verifier asks the mail server a direct question. It looks up the domain's MX record, opens a connection on port 25 and names the address at the RCPT TO step. A 550 reply means no such user, so that row is invalid and you delete it before it bounces. The connection closes before DATA, so nobody on your list receives a thing.
So why the gap between 99% and 70%? Two different numbers get called accuracy. ZeroBounce claims 99.6% and Clearout 99.73% (fact sheet, September 2026), and each figure counts only the rows where the tool gave a definite answer. Hunter's benchmark scored every address, including the hard ones. Both can be true at once, and only one tells you what happens to your whole list.
The hard ones are predictable. Catch-all domains accept every address, so the server says yes to real and fake mailboxes alike. About 28% of a 10,000-email B2B benchmark sat on catch-all domains. Yahoo and AOL answer yes to everything by design, and Gmail and Microsoft 365 often block probes from cloud servers. No verifier can confirm a mailbox on those servers by SMTP alone.
That's why you should judge a verifier on two numbers. Coverage is the share of your list that got a definite verdict. Accuracy is how often those verdicts were right. ZapBounce reports both, returns catch_all and unknown as results of their own, and never bills an unknown. The accuracy page sets out the method, and the ceiling on domains that respond honestly is 95% to 98%.
For your list, the practical test is simple. Verify, send to the valid rows, and watch your bounce rate. A verifier earns its fee when the bounce rate on those rows stays low. A list of 10,000 costs $25 at ZapBounce, and credits never expire, so re-checking before each campaign costs little.
Related questions
Are email verifiers accurate?
Email verifiers are accurate on the addresses a server answers for, and weaker on catch-all and blocked domains. Hunter's November 2025 benchmark of 15 tools measured overall accuracy from 31.2% to 70.0% on 3,000 business addresses. Ask any vendor for coverage and accuracy together.
Why did a verified email still bounce?
A verified address can bounce because mailboxes close after the check, or because the domain was catch-all and accepted the probe. Hunter found 5.5% of verified addresses had degraded within eight weeks. Verify within days of a send, not months.
Is a 99% accuracy claim a lie?
A 99% claim is usually true of the rows the tool answered, and silent about the rows it skipped. Vendors count unknown and catch-all results outside the figure. Ask what share of your list got a verdict, and the claim becomes checkable.
Is email verification worth paying for?
Yes, when the list is older than a few months or came from more than one source. A verifier removes hard bounces before they count against your domain. ZapBounce charges $5 for 1,000 checks and gives 100 free credits a month with no card.