The honest answer: highly accurate for addresses that can be checked, and honest about the ones that cannot. Mailbox-level SMTP verification reliably separates real from dead addresses on domains that respond - but catch-all domains and probe-blocking ISPs put a hard ceiling on certainty. What matters is whether a verifier admits that or hides it.
For domains that answer SMTP, mailbox-level verification is highly accurate - valid and invalid are reliable. Accuracy is capped on catch-all domains and probe-blocking ISPs, which an honest verifier returns as catch-all or unknown rather than guessing valid.
A domain configured to accept mail for every address answers "yes" even for mailboxes that do not exist. SMTP alone cannot confirm a single address, so it must be scored, not guessed. Learn more
Some servers defer or refuse verification probes (greylisting, or large ISPs blocking checks). The check cannot complete, so the honest result is unknown. Learn more
These are usually valid but high-risk. Accuracy is not just valid/invalid - a good verifier flags disposable and role addresses so you can act on risk. Learn more
A correct result goes stale. Around 19% of valid addresses die within 90 days, so accuracy depends on how recently the list was checked. Learn more
Almost every verifier advertises a high accuracy figure, and for clean, responsive domains most are genuinely accurate. The number that matters is what a tool does with the hard cases. A verifier that never returns unknown and folds catch-all into valid is not more accurate - it is guessing, and those guesses become your bounces.
The measured breakdown across 10.2M verifications - including how much of what looks catch-all is really unprobeable - is in our deliverability benchmarks and catch-all study.
BounceZero runs a five-stage pipeline - syntax, MX, SMTP probe, catch-all and disposable detection, then ML scoring - and returns valid, invalid, catch-all, or unknown. It never converts an unconfirmable address into a valid one to pad the numbers. Greylisted and probe-blocked domains are re-probed from a retry queue rather than marked invalid. See how email verification works for the full pipeline.
For domains that answer SMTP probes, mailbox-level verification is highly accurate - "valid" and "invalid" results are reliable. Accuracy is inherently limited on catch-all domains (which accept everything) and on large ISPs that block probes; a good verifier returns those as catch-all or unknown rather than guessing.
Headline accuracy numbers usually describe the subset of addresses that can be checked cleanly. The honesty test is what a tool does with catch-all and probe-blocked addresses: an honest verifier labels them separately, while a tool inflating its "valid" rate may quietly count catch-all as valid, which is a guess, not a check.
Four things: catch-all domains that accept mail for any address; greylisting and probe blocks that stop the SMTP check from completing; disposable and role addresses that are valid but risky; and list decay, since a correct result goes stale as people abandon inboxes. Each is handled by labelling, not guessing.
Test it on addresses you know (a live mailbox, a known-dead one, a catch-all domain, and a disposable address) and check that it separates catch-all and unknown instead of forcing everything into valid/invalid. A verifier that never returns unknown is guessing on the hard cases.
Valid, invalid, catch-all, unknown - labelled honestly, never guessed. 100 free verifications.
Start FreeDeep-dive guides on how email verification and inbox placement work
Check if an email is real, valid or fake - free, bulk, or API, and how to read results
info@, sales@, admin@ - why role addresses bounce more and when to suppress them
Permanent 5xx failures vs soft bounces, safe thresholds, and how to prevent them
The RCPT TO handshake explained - codes, catch-alls, limits
Why domains accept everything and how ML classifies deliverability
Temp addresses, how to detect them, and why they matter
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