What is smtp?
SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending email between servers. Email verifiers use a controlled SMTP conversation to probe whether a mailbox will accept mail, without actually delivering a message.
SMTP is the language mail servers speak to each other. When you send an email, your server opens an SMTP conversation with the recipient's server, announces who the mail is from and to, and hands over the message. It's the plumbing underneath every email ever sent.
Verifiers borrow that same conversation for a different purpose. To check whether a mailbox exists, a verifier opens an SMTP connection and walks through the opening steps — identifying itself, naming the sender, naming the recipient — and reads how the server responds to the recipient address. A clean rejection means the mailbox doesn't exist; an acceptance means it does.
Crucially, a proper probe stops before delivery. It learns whether the server would accept the address and then ends the conversation without ever sending a message, so the recipient receives nothing. That's what makes verification safe to run at scale.
The protocol is also where catch-all behaviour shows up. A catch-all server accepts the recipient address in the SMTP conversation no matter what you ask, which is exactly why those addresses can't be confirmed and get their own verdict. SMTP gives the verifier an honest answer; on a catch-all domain, that honest answer is simply "I accept everything."
Frequently asked questions
Does an SMTP probe send an email?
No. A proper probe stops short of delivery — it only checks whether the server would accept the address, so the recipient never receives anything.
Why can't SMTP confirm catch-all addresses?
Because a catch-all server accepts every recipient address in the SMTP conversation, real or not. The probe gets the same acceptance for a genuine mailbox and a fake one, so the address can only be reported as catch-all.