How Email Protocols Work: A Deep Dive into the Journey of an Email

Sandeep Chaudhary

Fri, 25 Apr 2025

In today’s digital communication landscape, email remains one of the most critical tools for personal, academic, and business communication. But have you ever wondered what happens behind the scenes when you hit the "Send" button on an email? This blog post unpacks the behind-the-scenes journey of an email, as beautifully illustrated in the visual guide above, and explains how various email protocols coordinate to deliver your message to the right inbox.

Step-by-Step Journey of an Email

1. Email Composition and Sending (Client Side)

The journey starts with a sender using an email client (like Gmail, Outlook, or Thunderbird), known technically as a Mail User Agent (MUA). The sender composes a message and hits send.

• • The email client uses SMTP or SMTPS (Simple Mail Transfer Protocol Secure) to connect with the Sender’s Email Server, also known as the Mail Transfer Agent (MTA).

2. SMTP Transfer

• • SMTP (Simple Mail Transfer Protocol) is responsible for sending emails from the client to the mail server and between mail servers.
• • SMTP uses TCP Port 25 by default.
• • Secure variants use SMTPS, generally via TCP Port 465 or with STARTTLS over port 587.

3. DNS Lookup and Routing

Once the sender’s server receives the email:

• • It queries the DNS (Domain Name System) for the recipient’s Mail Exchange (MX) Record.
• • The MX record returns the IP address of the receiver's mail server.

4. Message Processing & Mail Queue

Before delivery:

• • The message is processed to apply MIME encoding (for attachments, formatting), and security verification is applied.
• • The message is then placed in a Mail Queue, which acts as a holding area if the receiver's server is temporarily unavailable.

5. Delivery to Receiver’s Email Server

Once ready:

• • The email is transferred to the receiver’s email server (Mail Transfer Agent).
• • The email is stored until accessed by the recipient's email client.

Receiving the Email: POP3 vs IMAP

POP3 (Post Office Protocol v3)

• • Downloads the email from the server to the user’s device and usually deletes it from the server.
• • Works best for users who check emails from a single device.
• • Uses TCP Port 110.

IMAP (Internet Message Access Protocol)

• • Allows emails to remain on the server, syncing across multiple devices.
• • Supports folder structures, flags, and server-side searching.
• • Ideal for modern email usage where users access mail from multiple devices.
• • Uses TCP Port 143.

Security Protocols Ensuring Safe Delivery

• • SPF (Sender Policy Framework) – Verifies the sender's domain to prevent spoofing.
• • DKIM (DomainKeys Identified Mail) – Adds a cryptographic signature to prove authenticity.
• • DMARC (Domain-based Message Authentication, Reporting & Conformance) – Enforces domain-level email policies and reports email delivery statistics.

These protocols work together to protect users from phishing, spoofing, and spam attacks, ensuring secure and authenticated email transmission.