You paid to verify the list. You sent to it. Some of it bounced anyway.
That is not necessarily a bad verifier. It is more often a verifier being asked a question it cannot answer, on a share of your list that nobody warned you about. What follows is what the tool actually checks, in order, and where the checking stops.
The three checks
A verifier works through an address in stages, and each stage can end the process.
It reads the syntax first. An address that breaks the format rules is invalid without anything leaving your machine.
Then it looks up the domain's mail exchange records in DNS. No MX record means no mail server is advertised for that domain, so nothing sent there arrives, and again the answer comes back without contacting anyone.
Then it opens an SMTP conversation with the mail server that the MX record points to and asks, without sending a message, whether that server will accept mail for that particular mailbox. This is the step that does the real work, and it is the step with the hole in it.
The domain that accepts everything
A catch-all domain, sometimes called accept-all, is configured to accept mail addressed to anything at that domain, whether or not the mailbox exists. Plenty of companies run one deliberately so that a message to a misspelled name still reaches someone.
For a verifier, that setting removes the signal entirely. The server says yes to every address it is asked about, including ones nobody has ever used. ZeroBounce puts it in one sentence on its own blog: "SMTP probing cannot distinguish individual addresses on a catch-all domain, no matter why the setting was enabled." Its status-code documentation is blunter still, defining the catch-all result as addresses that "are impossible to validate without sending a real email and waiting for a bounce."
That last clause is the honest description of your options. You can find out. The way you find out is by sending.
How much of a list this affects depends on who the list is. The number ZeroBounce publishes about its own traffic is that more than 9% of all addresses it processed in 2025 came back catch-all, which it puts at more than a billion addresses out of more than eleven billion verified. That is a share of everything its customers submitted rather than a share of B2B domains, so read it as an order of magnitude rather than a rate for your list.
ZoomInfo states the limit more aggressively, and it is worth knowing who is speaking: on its own blog it writes that because security services like Mimecast obscure whether an address is valid, "SMTP checks can only ever accurately validate roughly 50 percent of business domains, regardless of provider." ZoomInfo sells contact data, and that sentence sits in a section arguing for ZoomInfo's own multi-stage verification over ordinary SMTP tools. The mechanism it describes is real. The figure is the vendor's.
We wrote separately about what to do with the catch-all portion of a list once you have identified it, in catch-all email addresses, and whether to keep them.
The same word, four meanings
The result you get back is a status, and the statuses are not standardised. Four vendors, in their own documentation, read 6 September 2026.
| Status | ZeroBounce | NeverBounce | Emailable | DeBounce |
|---|---|---|---|---|
| Valid | "valid and safe to email. They will have a very low bounce rate of under 2%" | "Verified as real address" | "we can determine with high confidence that the email address exists and can receive mail" | "Verified as real address" |
| Invalid | "we determined to be invalid. Please delete them from your email list" | "Verified as not valid" | "we can determine with high confidence that the email address is invalid or should not be mailed" | "Verified as invalid (Bounce)" |
| Catch-all | "impossible to validate without sending a real email and waiting for a bounce" | "A domain wide-setting, also known as Accept-all (Unverifiable)" | Documentation page did not resolve when checked | "all emails on this domain will be reported as a catch-all" |
| Unknown | "couldn't validate for one reason or another. Typical cases include an offline mail server or an anti-spam email system" | "The server cannot be reached" | "could not determine with confidence whether the email address is valid or invalid at the time of verification" | "The server cannot be reached" |
Look at the unknown row. At NeverBounce it means one specific thing, a server that did not answer. At ZeroBounce it also covers an anti-spam system that interfered. Those are different populations, and if you drop unknowns from one list and keep them on another you are applying two different policies while believing you applied one.
Notice also what ZeroBounce writes into its definition of valid: a bounce rate of under 2%. The best bucket the tool has is not a promise of zero.
"Deliverable" is not "will not bounce"
Emailable is the vendor that states this most precisely, because it has to. Its guarantee covers addresses that were "classified as Deliverable" and then "bounced due to an invalid (non-existent) email address," provided they were emailed within 24 hours of verification. A guarantee written that carefully is a concession: the classification and the outcome are two different things, and the vendor bridges them with a contract rather than with a claim.
The technical version of the same point comes from MailTester, which sells an email finder and still writes it plainly: SMTP "can detect that all addresses are accepted, even if they don't actually receive mail." A server accepting a message is not a person reading it.
Two other things a verifier is not measuring. It is not measuring whether the person still works there, which is a different decay curve entirely and the subject of what list decay actually costs you. And it is not measuring whether your mail arrives, which depends on your domain and your sending behaviour rather than on their mailbox.
What the accuracy percentage is standing on
Every vendor in this category publishes a number. ZeroBounce says 99.6%. MillionVerifier says 99% and up. Verifalia says 99%. NeverBounce frames it as delivering up to 99.9% of your emails with confidence.
Eight of the nine vendors named here could be read at source on 6 September 2026, Kickbox being unreachable, and not one of the eight publishes a method, a sample size or a date behind that figure on the page where it appears. Emailable is the closest to an exception, and what it publishes is a conditioned guarantee rather than a percentage.
Prices, where a vendor publishes a packaged tier rather than a slider: Bouncer lists 10,000 credits at $60, and NeverBounce lists pay-as-you-go at $8 per 1,000, which is $80 for the same volume. Most of the rest quote a per-credit rate that only resolves into a price once you pick a quantity.
The comparison posts are their own case. Six published accuracy tests were examined. Three were run by companies that sell a verifier, and in all three the publisher's own tool came first: Sparkle's test ranks Sparkle first, Bouncer's ranks Bouncer first, LeadMagic's 10,000-address test ranks LeadMagic first at 99.5%. The other three were published by companies that do not sell a verifier. Two of those three name a winner and link the reader to nothing, every link in the piece pointing back at the publisher. Searching for a test run by someone with no product and no marketplace stake turned up nothing.
One ownership fact is worth carrying into any comparison you read: NeverBounce has been a ZoomInfo product since ZoomInfo's predecessor acquired it in 2019, and ZoomInfo is one of the largest sellers of B2B contact data. ZeroBounce, DeBounce and Hunter each sell contact data or email finding alongside verification too, so for four of the nine the company checking the data also sells data. We looked at how those accuracy claims are constructed across the wider data market in what vendor accuracy claims actually measure.
The only number that is about your list
None of the above is a reason to skip verification. Syntax and MX checks are free of ambiguity, the SMTP probe resolves most consumer and small-business domains cleanly, and removing known-invalid addresses before a send is the cheapest protection your sending domain has.
It is a reason to stop treating the dashboard percentage as the result. The number that describes your list is the bounce rate you measure after you send it, on your own domain, on your own send. Run a verification, send to the clean portion, count what came back, and you have a figure with a known denominator that belongs to you. That is the measurement we walked through in bounce rate as a data-quality metric.
Then decide the catch-all portion separately, as a deliberate choice about risk rather than a rounding error inside somebody else's accuracy claim.
Sources in this article were read on 6 September 2026 from the vendors' own pages: ZeroBounce's catch-all article and its email validation API status codes, NeverBounce's status documentation, Emailable's deliverability guarantee and result definitions, DeBounce's status codes, Hunter's verifier documentation, MailTester's SMTP article, and ZoomInfo's page on email accuracy verification. Prices are quoted only where a vendor published a packaged tier. Emailable's accept-all definition is marked as unresolved because the documentation page did not load, rather than paraphrased from elsewhere.
Frequently asked
- How does an email verifier work?
- In three stages. It checks the syntax of the address, then queries DNS for the domain's mail exchange records to confirm a mail server exists, then opens an SMTP connection to that server and asks whether it will accept mail for that specific mailbox, without sending a message. Each stage can end the process with an answer, and the third stage is the one that determines whether an individual mailbox exists.
- What is a catch-all email address?
- A catch-all, also called accept-all, is a domain configured to accept mail addressed to any name at that domain, whether or not the mailbox exists. Companies often set this up so that mail to a misspelled address still reaches someone. For verification it removes the signal, because the server answers yes to every address it is asked about, including invented ones.
- Can email verification detect catch-all addresses?
- It can detect that a domain is catch-all, and it cannot resolve individual addresses on one. ZeroBounce's own documentation describes catch-all results as impossible to validate without sending a real message and waiting for a bounce. NeverBounce returns the same case as accept-all and labels it unverifiable. Every vendor examined returns catch-all as its own status rather than folding it into valid or invalid.
- Does verified mean the email will not bounce?
- No. The best status a verifier returns is a confidence judgment about whether a mailbox exists and will accept mail, and vendors write it that way. ZeroBounce's definition of a valid address includes an expected bounce rate under 2%. Emailable guarantees deliverability on addresses it classified as deliverable, on the condition that they were emailed within 24 hours of verification and that the bounce was caused by a non-existent address, which is a contract rather than a claim of identity between the two.
- How accurate are email verification tools?
- Published accuracy claims cluster between 99% and 99.9%, and they are claims about the vendor's whole processing volume rather than about any particular list. The figure that describes a specific list is the bounce rate measured after sending to it, on the sending domain that will be used, because that has a denominator the sender controls. Catch-all domains sit outside both numbers, since no verifier resolves individual addresses on them.
- What do the statuses mean?
- They vary by vendor, which matters most for the ambiguous ones. NeverBounce defines unknown strictly as a server that cannot be reached. ZeroBounce also includes cases where an anti-spam system interfered. Role-based, disposable, spam trap and risky are similarly defined per vendor, so a rule written for one tool's export does not transfer to another's without rereading the definitions.