Also known as: Simple Mail Transfer Protocol
The standard protocol mail servers use to send and receive email across the internet.
SMTP (Simple Mail Transfer Protocol) is the language mail servers speak to hand messages to one another. A sending server connects to the recipient's MX server and works through a short conversation — HELO/EHLO, MAIL FROM, RCPT TO, DATA — to deliver the mail.
That conversation is also how deep verification works. By starting the handshake and issuing RCPT TO for a specific address, a verifier can often learn whether the mailbox exists — without ever sending an actual message. This is called an SMTP probe or SMTP verification.
SMTP probing is powerful but imperfect: catch-all servers accept everything, some providers rate-limit or greylist probes, and large mailbox providers can answer inconsistently under load.
A DNS record that tells other servers which mail server is responsible for receiving email for a domain.
Checking whether a specific mailbox exists by starting an SMTP conversation with the receiving server without sending a real email.
An anti-spam technique where a server temporarily rejects mail from unknown senders, expecting legitimate ones to retry.
A mail server configured to accept email for every possible address at a domain, even ones that do not exist.
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.