4-1-6 SMTP (Simple Mail Transfer Protocol) Explained

Key Concepts

• SMTP Overview
• SMTP Operation
• SMTP Commands
• SMTP Ports
• SMTP Security

SMTP Overview

SMTP (Simple Mail Transfer Protocol) is a communication protocol for electronic mail transmission. It is used to send and receive email messages between mail servers. SMTP is a text-based protocol that operates on the application layer of the TCP/IP model.

SMTP Operation

SMTP operates in a client-server model. The client, typically an email client or a mail user agent (MUA), sends an email to the SMTP server. The SMTP server then forwards the email to the recipient's mail server. The recipient's mail server stores the email until the recipient retrieves it using a mail retrieval protocol like IMAP or POP3.

SMTP Commands

SMTP uses a set of commands to manage the email transmission process. Key commands include:

• HELO/EHLO: Initiates the session with the SMTP server.
• MAIL FROM: Specifies the sender's email address.
• RCPT TO: Specifies the recipient's email address.
• DATA: Initiates the transfer of the email message content.
• QUIT: Ends the SMTP session.

SMTP Ports

SMTP uses specific ports for communication:

• Port 25: The default port for SMTP communication between mail servers.
• Port 587: Used for SMTP submission, where email clients send messages to an SMTP server.
• Port 465: Historically used for SMTP over SSL (SMTPS), though it is now deprecated in favor of STARTTLS on port 587.

SMTP Security

SMTP security is enhanced through:

• STARTTLS: An extension that allows SMTP connections to be encrypted using Transport Layer Security (TLS).
• SMTP-AUTH: Authentication mechanisms that require clients to authenticate themselves to the SMTP server before sending emails.
• DMARC, SPF, DKIM: Email authentication protocols that help prevent email spoofing and ensure the authenticity of email messages.

Examples and Analogies

Think of SMTP as a postal service for emails. The email client (sender) writes a letter (email) and hands it to the post office (SMTP server). The post office then sorts and forwards the letter to the recipient's post office (recipient's mail server). The recipient's post office holds the letter until the recipient picks it up (retrieves it using IMAP or POP3).

Another analogy is a courier service. The sender packages the item (email) and hands it to the courier (SMTP server). The courier delivers the package to the recipient's address (recipient's mail server). The recipient's address holds the package until the recipient collects it.