SMTP Basics

Core Idea

SMTP is the protocol mail servers use to move email between systems. You do not administer mail in this course, but SMTP is a useful comparison point because it shows that the internet is many text-based protocols, not just HTTP.

Mental Model

HTTP usually means a client asks a web server for a resource. SMTP usually means a mail sender talks to a mail server to hand off a message. Both involve clients, servers, commands, replies, hostnames, ports, and logs.

Common Ports

• 25: server-to-server SMTP.
• 587: message submission with authentication and encryption negotiation.
• 465: SMTP over TLS in many deployments.

What To Notice

Mail delivery depends on DNS records, especially MX records. A domain can have a website and no mail service, or mail service and no website.

dig MX example.org

Why We Do Not Practice Sending Mail Here

Mail systems have spam, abuse, reputation, authentication, privacy, and operational risks. The course uses SMTP as protocol literacy, not as a shared-server exercise.

Common Confusions

• SMTP sends mail. IMAP and POP3 retrieve mail.
• A domain’s web server and mail server are often different machines.
• Port reachability does not mean you are allowed to relay mail.
• Modern mail requires authentication and anti-spoofing records such as SPF, DKIM, and DMARC.

Proof Check

Run dig MX example.org and identify whether the domain publishes mail exchanger records.

Docs Pointers

• Run man dig.
• Read Cloudflare: DNS MX record.
• Read RFC 5321 only if you want the formal SMTP protocol specification.