What is a smtp?
Also called: Simple Mail Transfer Protocol
SMTP is the protocol mail servers use to hand messages to each other. It is a short text conversation: the sender opens a connection, identifies itself, names the sender and recipient, transmits the message, and disconnects. Verification uses the same conversation but stops before the message is sent.
A normal exchange runs EHLO to introduce the connecting server, MAIL FROM to name the envelope sender, RCPT TO to name each recipient, then DATA to transmit headers and body, and QUIT. Every command gets a numeric reply, which is where the 250, 421 and 550 codes come from.
Verification exploits the fact that the recipient is named before anything is transmitted. A probe connects, issues RCPT TO for the address in question, reads whether the server accepts it, and disconnects at that point. No message body is ever sent, and nothing appears in the recipient's inbox.
That is also why verification needs outbound port 25 specifically. Ports 587 and 465 are for authenticated submission to your own provider; port 25 is for server-to-server transfer, and most cloud hosts block it outbound by default to limit spam.
Last reviewed 2026-09-11