Networking - Simple Mail Transfer Protocol

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is a standard protocol used to send emails from one server to another over the internet. It’s the primary protocol responsible for email delivery, especially from an email client (like Outlook or Gmail) to an email server, or between two email servers.

 Features of SMTP:

Feature Description
Full Form Simple Mail Transfer Protocol
Port Numbers Default: 25, Secure: 465 (SSL) or 587 (TLS)
Protocol Type Application layer protocol (uses TCP for reliable transmission)
Direction Handles sending and forwarding of email (not receiving)

How SMTP Works:

  1. Email Composition:

    • You write an email using an email client (e.g., Gmail, Outlook).

  2. Email Submission:

    • The client sends the message to the SMTP server using SMTP (usually on port 587).

  3. Email Routing:

    • The SMTP server checks the recipient's domain (e.g., @example.com) and finds the destination mail server using DNS and MX (Mail Exchange) records.

  4. Forwarding:

    • The message is transferred to the recipient’s mail server using SMTP.

  5. Receiving:

    • The recipient retrieves the email using POP3 or IMAP, not SMTP.

Basic SMTP Commands:

Command Purpose
HELO / EHLO Greet the server
MAIL FROM: Specify sender’s email address
RCPT TO: Specify recipient’s email address
DATA Begins the message body
QUIT Ends the session

SMTP vs POP3 vs IMAP:

Protocol Purpose Used By
SMTP Send emails Email clients → servers
POP3 Retrieve & delete emails (downloads to one device) Email clients ← servers
IMAP Access and manage email on the server (syncs across devices) Email clients ← servers

Advantages of SMTP:

  • Simple and reliable

  • Widely supported across platforms

  • Works well with other email protocols (POP3, IMAP)

Limitations of SMTP:

  • No encryption by default (needs SSL/TLS for security)

  • Can be exploited for spam if not secured properly

  • Does not handle receiving or storing emails

 

SMTP (Simple Mail Transfer Protocol) is the backbone of email sending on the internet. It defines how messages are sent from clients to servers and from server to server. While SMTP handles the sending, protocols like IMAP and POP3 are used for retrieving email. Together, they power the global email system we use every day.