What is SMTP? (Email glossary definition)

SMTP (Simple Mail Transfer Protocol) is the standard protocol mail servers use to send and relay email between each other. Originally defined in 1982 and formalized in RFC 5321, it remains the foundation of every modern email system.

An SMTP conversation between two mail servers looks like this:

> 220 mail.example.com ESMTP ready
< EHLO sender.example.com
> 250-mail.example.com Hello
< MAIL FROM:<sender@example.com>
> 250 OK
< RCPT TO:<recipient@example.com>
> 250 OK
< DATA
> 354 Start mail input; end with <CRLF>.<CRLF>
< (message content)
< .
> 250 OK: queued as 12345
< QUIT
> 221 Bye

Each line is a command (left) and a numeric response code (right). 2xx codes mean success. 4xx codes are temporary failures (retry later). 5xx codes are permanent failures.

For email verification: our engine uses SMTP without DATA. We open the connection, send EHLO, MAIL FROM, and RCPT TO, then capture the server's response to RCPT TO. A 250 means the mailbox exists. A 550 means it does not. We then QUIT without ever sending the actual message. The recipient never sees anything. See how verification works.