What is SMTP?

SMTP (Simple Mail Transfer Protocol)is the standard protocol for sending and relaying email between mail servers. When your app or mail client sends a message, it speaks SMTP to a mail server, which relays it — server to server — until it reaches the recipient’s mailbox.

How SMTP works

SMTP is a text-based conversation, usually over port 587 or 465. A simplified exchange:

HELO mail.example.com
MAIL FROM:<you@example.com>
RCPT TO:<friend@example.org>
DATA
Subject: Hi
...message body...
.

SMTP only handles sending and delivery. Readingmail from a server is a different job — that’s what IMAP and POP3 are for.

Why it matters

Almost every app that emails users does so over SMTP, directly or through a provider’s API. In testing, you usually want to avoid sending real SMTP mail to real people — so you either point SMTP at a sandbox that traps it, or send to a disposable address you can read over an API. See receiving email with an API.