How Email Works

YOU GOT A MAIL!

1. Overview

• Format
• Header
• Process of Sending
• 3 protocols: SMTP, POP3, IMAP

2. Decription

2.1 Email Format

Email generally follow a format called IMF (Internet Message Format). It is a set of rule of how an email meesage must follow (like From:, To:, Subject:)

2.2 Email Header

• Received - the last hop before it reaches your email server (the is the ONLY header that can be trusted, other fields can be masqueraded)
• Return Path - the return email addr if the email fails to delivered
• Replay-To - The addr when users hit reply
• X-Originating-ip - sender’s ip
• The Bottom-Up Rule: Read them from bottom to top. The top-most is the last server receiving it.

2.3 Process of Sending

1. Client to Server: Your email client (like Gmail or Outlook) connects to your email provider’s outgoing mail server (the SMTP server) and hands off the message.
2. Routing: Your SMTP server looks up the recipient’s domain to find their mail server’s address using the Domain Name System (DNS) and its MX Record (Mail Exchanger).
3. Server to Server: Your SMTP server connects to the recipient’s mail server (also using SMTP) and transfers the email.
4. Delivery: The recipient’s mail server accepts the message, runs checks (spam, virus), and places it in the recipient’s digital mailbox.
5. Retrieval: The recipient’s email client later connects to their mail server (using POP or IMAP) to retrieve and display the message.

2.4 Protocols

• For Sending:
  • SMTP is the only choice (port 25, and 587 for Secure Transport)
• For Reseiving:
  • IMAP - the mail stay on server. Users can view mail from different devices.
  • POP3 - the users download the mails, and they will get deleted from servers.

3. Extended Readings

• POP3 vs IMAP
• Email Configuration

last modified: 2025-12-12