What is SMTP?
Also called: Simple Mail Transfer Protocol
SMTP, the Simple Mail Transfer Protocol, is how mail servers hand messages to each other. It is a short text conversation: the sender connects, identifies itself, names the sender and recipient, sends 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 for each recipient, DATA to send headers and body, then QUIT. Every command gets a numeric reply, which is where codes like 250, 421 and 550 come from.
Verification relies on the recipient being named before anything is sent. A probe issues RCPT TO for the address, reads whether the server accepts it, and disconnects. No message body is transmitted and nothing appears in anyone's inbox.
Server-to-server SMTP runs on port 25. Ports 587 and 465 are for a mail client submitting to its own provider with a login. Many cloud hosts block outbound port 25 to limit spam, which is why a verifier has to run somewhere it is open.
Example
220 mx.example.com ESMTP → EHLO verify.example.net → MAIL FROM:<[email protected]> → RCPT TO:<[email protected]> → 250 OK → QUITGo deeper
Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms