Email verifier
Four outcomes, not two. Including an honest "we could not find out" — which you are never charged for.
Unknown is not undeliverable
When a server blocks the probe or DNS fails, the mailbox was never tested. Reporting that as invalid is how verifiers quietly delete real customers from real lists.
Catch-all is called out
On a domain that accepts everything, a positive result proves the domain accepts mail — not that the person exists. That distinction is surfaced rather than folded into the score.
Warmed sending IPs
Probes go out through addresses under a managed warmup and pacing schedule with per-provider limits, which is what keeps accuracy from collapsing the moment a list gets large.
How email verification works
Verification opens an SMTP conversation with the mail server responsible for a domain and asks whether it would accept mail for an address — then stops before any message is sent. Nothing reaches the recipient.
Before that, the address is normalised and classified: syntax, internationalised domain names, disposable providers, role accounts and free providers. MX records are resolved with proper handling of the failure cases, so a resolver incident produces "we could not check" rather than "this domain is dead".
The four outcomes, and why there are four
Deliverable: the mail server accepted the address. Undeliverable: it said the mailbox does not exist. Risky: real mail, but not a safe send — a catch-all domain, a role account, a disposable provider. Unknown: we could not find out.
That fourth outcome is the one most tools collapse into the second, and it is the most expensive mistake in this category. A false undeliverable removes a real customer from a list permanently, and nobody ever finds out it happened.
VerifyInbox also never charges for an unknown. If the mailbox was not tested, there is nothing to bill for.
Why sending reputation decides accuracy
A verifier is only as accurate as the IP addresses it probes from. A brand-new address opening thousands of SMTP sessions looks exactly like a dictionary attack, and it gets blocked — after which every result it produces is "unknown" or, worse, a false negative.
Every probe here is metered against a specific sending IP with its own warmup day, pacing curve, per-provider sub-caps, reputation score and cooldown. It is the least visible part of the product and the one that decides whether the answers are any good.
Questions, answered.
How do I check if an email address is valid?
Ask the mail server. A verifier opens an SMTP conversation and asks whether the address would be accepted, stopping before any message is sent — so the recipient never receives anything.
Does verifying an email address send them an email?
No. The conversation stops before the message body. Nothing appears in their inbox and nothing is logged against them.
What is a catch-all email address?
An address at a domain configured to accept mail for everything, whether the mailbox exists or not. On such a domain a positive verification is much weaker evidence than it appears, which is why we report it explicitly.
Why do some addresses come back as unknown?
Because the mailbox was genuinely not tested — the receiving server blocked the probe, greylisted it, or DNS failed. It is not a soft invalid, it is retryable, and it is not charged.
What is a good bounce rate?
Below 2%. Above 5% providers filter aggressively and most email services begin suspending accounts. Verifying before sending is the single largest lever on that number.
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