SMTP - Simple Mail Transfer Protocol

SMTP - Simple Mail Transfer Protocol

What is SMTP?

Protocol for sending email from client to server, or between email servers. When you click "Send" in your email app, SMTP delivers your message.

How SMTP Works:

• Sending only: SMTP is for SENDING emails, not receiving
• Port 25: Default SMTP port (also 587 for submission)
• Server-to-server: Your mail server forwards message to recipient's mail server
• Store-and-forward: If recipient server is unavailable, SMTP retries later
• Text-based: Uses simple text commands (HELO, MAIL FROM, RCPT TO, DATA)

Email Sending Process:

1. You write email and click Send
2. Your email client uses SMTP to send to your mail server
3. Your mail server uses SMTP to forward to recipient's mail server
4. Recipient's mail server stores the email
5. Recipient uses POP3/IMAP to download email

Receiving Email (POP3/IMAP):

• POP3 (Post Office Protocol): Downloads emails from server to device, then deletes from server
• IMAP (Internet Message Access Protocol): Keeps emails on server, syncs across multiple devices
• SMTP vs POP3/IMAP: SMTP = sending, POP3/IMAP = receiving

SMTP Ports:

• Port 25: Traditional SMTP (server-to-server)
• Port 587: SMTP submission (client-to-server, preferred for modern email)
• Port 465: SMTPS (SMTP over SSL, legacy encrypted version)