Also known as: SMTP Probe, Mailbox Probe
Checking whether a specific mailbox exists by starting an SMTP conversation with the receiving server without sending a real email.
SMTP verification confirms that an individual mailbox exists by beginning the SMTP delivery conversation and watching how the server responds to the RCPT TO command. If the server accepts the recipient, the mailbox is very likely real; if it returns a 550 "no such user," it does not exist. The probe is aborted before any message is actually sent.
It is the most precise signal a verifier has, but it depends on the receiving server cooperating. Catch-all domains accept every recipient, so the probe is inconclusive there. Some servers greylist or rate-limit unknown senders, returning temporary errors that have to be retried.
Because results can be ambiguous, an honest engine reports inconclusive probes as unknown or catch-all rather than guessing — and uses reputation data to fill the gaps.
The standard protocol mail servers use to send and receive email across the internet.
A mail server configured to accept email for every possible address at a domain, even ones that do not exist.
An anti-spam technique where a server temporarily rejects mail from unknown senders, expecting legitimate ones to retry.
The process of checking whether an email address is real, correctly formatted, and able to receive mail before you send to it.
See it in action
Check any address against BounceShift's multi-layer engine — syntax, MX, disposable, and role checks free, with full SMTP verification on signup.