What is SMTP verification?

SMTP verification checks whether an email address can receive mail by asking the domain's mail server about the mailbox, without sending a message.

Also called: email verification, mailbox check.

A verifier looks up the domain's MX records to find its mail server, opens an SMTP connection, and begins the steps of delivering a message up to the point where the server says whether it will accept mail for that recipient. It then disconnects. Nothing is delivered and the recipient sees nothing.

The server's answer gives three useful outcomes. Accepted means the mailbox exists (valid). Refused means it does not (invalid), which is the address that would hard-bounce. No clear answer, or an answer of yes to every address on a catch-all domain, means the mailbox cannot be confirmed either way (risky or unknown).

Verification is a point-in-time check. A mailbox valid today can be closed next month when an employee leaves or a business shuts, so a check is only as good as its date; see data decay. Syntax checks and domain checks alone are not verification: they catch typos, not closed mailboxes.

A good verifier never reports an unconfirmed mailbox as valid, because a false valid is exactly the address that bounces. 67Leads reads emails from each company's own website and then runs this check; an address the probe could not confirm is reported as unverified.