SMTP

Also called: Simple Mail Transfer Protocol.

An SMTP session is a short text conversation. The client greets the server (EHLO), names the sender (MAIL FROM), names each recipient (RCPT TO), sends the message after DATA, and quits. The server answers each step with a three-digit reply code.

Reply codes are grouped by first digit: 2xx success (for example 250), 4xx temporary failure (try again later), 5xx permanent failure (for example 550 for no such user). Enhanced status codes from RFC 3463, such as 5.1.1, add detail.

Email verification uses the first half of this conversation. A verifier sends RCPT TO for the address, reads the reply, and quits before DATA, so no message is delivered. The VRFY command looks like it was made for this, but RFC 5321 lets servers decline to answer it and most do.

Knowing the codes helps you read bounce reports. 5.1.x codes mean bad addresses (a list problem); 5.7.x codes usually mean policy or authentication (a setup or reputation problem); 4xx codes mean wait and retry, not delete.

Example

RCPT TO:<office@smithplumbing.com> gets 250 OK; RCPT TO:<nobody@smithplumbing.com> gets 550 5.1.1 User unknown. The first mailbox exists; the second doesn't, and the domain isn't catch-all.