# SMTP Protocol
Confidence: high
Last verified: 2026-05-22
Generation: human_only


## TL;DR

SMTP (Simple Mail Transfer Protocol) is the standard for sending email across the Internet. It uses TCP port 25 (or 587 for submission). Email clients submit mail; MTAs relay it between servers. SMTP is text-based and uses `MAIL FROM`, `RCPT TO`, `DATA` commands.

## Core Explanation

SMTP extensions (ESMTP): STARTTLS (encryption), 8BITMIME (non-ASCII), SIZE (message size declaration). SPF, DKIM, DMARC authenticate email origin and content. IMAP/POP3 are for receiving (reading) email — SMTP is send-only. Gmail, Outlook, etc. use SMTP for outgoing, IMAP for incoming.

## Further Reading

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