How Email Verification Works

Email verification isn't one test — it's several small checks stacked together. Here's what actually happens when you check an address.

1. Syntax check

First, we check that the address is even written correctly: one @ symbol, a valid domain part, no illegal characters. This catches typos instantly, without needing the internet at all.

2. Does the domain exist?

Next we look up the domain in DNS — the internet's address book. If the domain doesn't exist at all, the email can never be delivered, so this is one of the few checks that can produce a confident INVALID on its own.

3. Does it have a mail server? (MX record)

An MX record tells us which mail server handles email for a domain. A domain can exist (have a working website) without being able to receive email at all — MX tells us whether mail routing is actually set up.

Important: having an MX record proves the domain can receive mail. It does not prove that any specific mailbox, like yourname@ that domain, actually exists.

4. Asking the mail server directly (SMTP)

The most direct check is to connect to the domain's mail server and ask, in the same technical language a real email would use, whether a specific mailbox exists — without actually sending anything.

This is the strongest signal available, but it doesn't always work. Some large providers (Gmail, Outlook, Yahoo, and others) deliberately give an unreliable or blocked answer here, specifically to prevent this kind of probing from being used to harvest which addresses are real. When that happens, honest tools say so instead of guessing.

5. Catch-all domains

Some domains are configured to accept mail sent to any address at that domain, valid or not, and sort out real vs fake mailboxes later. On a catch-all domain, even a perfect SMTP check can't tell you whether one specific mailbox is real — so the honest result is ACCEPT-ALL, not VALID.

6. Extra risk signals

On top of the above, we flag a few other things that matter for real-world sending: whether the domain is a known disposable/temporary provider, and whether the address is a generic role address like info@ or support@ rather than a personal mailbox. Neither of these alone means an address is invalid — they're just extra context.

Why "UNKNOWN" is a real, useful answer

When SMTP can't give a reliable answer and no other strong signal fills the gap, the honest result is UNKNOWN — not a guessed VALID. A tool that never says UNKNOWN isn't more accurate; it's just hiding its uncertainty from you.

Ready to try it? Verify a list now, or read our full results guide for what each status means in more detail.