SMTP, POP3, IMAP for email in Computer Networks - Full Explanation

Introduction

Imagine you want to send a letter to a friend and also receive letters from others. Email systems face similar challenges: how to send messages out, and how to receive and manage incoming messages. SMTP, POP3, and IMAP are the main tools that help computers handle these tasks smoothly.

Explanation

SMTP: Sending Email

SMTP stands for Simple Mail Transfer Protocol. It is the method computers use to send emails from your device to the recipient's email server. SMTP works like a postal service that takes your letter and delivers it to the correct post office for the recipient. It only handles sending, not receiving or storing emails.

SMTP is the protocol responsible for sending emails from one server to another.

POP3: Receiving and Downloading Email

POP3 means Post Office Protocol version 3. It lets your email program download messages from the email server to your device. After downloading, the emails are usually deleted from the server, so you can read them offline but they are stored only on your device. POP3 is simple and works well if you use one device to check email.

POP3 downloads emails to your device and usually removes them from the server.

IMAP: Managing Email on the Server

IMAP stands for Internet Message Access Protocol. Unlike POP3, IMAP keeps your emails on the server and lets you view and organize them from multiple devices. When you read or delete an email, the change happens on the server, so all your devices see the same mailbox. IMAP is useful if you check email from phones, tablets, and computers.

IMAP keeps emails on the server and syncs changes across all your devices.

Real World Analogy

Think of sending a letter through a postal service (SMTP), then picking up your mail from a mailbox and taking it home (POP3), or reading and organizing your mail while it stays in the mailbox so you can access it from anywhere (IMAP).

SMTP: Sending Email → The postal service that takes your letter and delivers it to the recipient's post office

POP3: Receiving and Downloading Email → Taking all your mail from the mailbox home to read and store

IMAP: Managing Email on the Server → Reading and organizing mail while it stays in the mailbox, accessible from multiple places

Diagram

Diagram

┌─────────────┐       ┌───────────────┐       ┌───────────────┐
│ Your Device │──────▶│ SMTP Server   │──────▶│ Recipient's   │
│ (Email App) │       │ (Send Email)  │       │ SMTP Server   │
└─────────────┘       └───────────────┘       └───────────────┘
       │
       │
       ▼
┌─────────────┐       ┌───────────────┐
│ POP3 Server │◀──────│ Your Device   │
│ (Download) │       │ (Email App)   │
└─────────────┘       └───────────────┘
       │
       │
       ▼
┌─────────────┐       ┌───────────────┐
│ IMAP Server │◀──────│ Your Device   │
│ (Sync Mail) │       │ (Email App)   │
└─────────────┘       └───────────────┘

Diagram showing how SMTP sends email out, while POP3 and IMAP retrieve emails from servers to your device.

Key Facts

SMTP → Protocol used to send emails from a client to a server or between servers.

POP3 → Protocol that downloads emails from the server to a single device and usually deletes them from the server.

IMAP → Protocol that keeps emails on the server and synchronizes mailbox changes across multiple devices.

Email Server → A computer system that stores, sends, and receives emails.

Email Client → Software or app used by a user to read, send, and manage emails.

Common Confusions

Believing SMTP is used to receive emails.

Believing SMTP is used to receive emails. SMTP only sends emails; receiving is handled by POP3 or IMAP.

Thinking POP3 keeps emails on the server after download.

Thinking POP3 keeps emails on the server after download. POP3 usually deletes emails from the server after downloading them to your device.

Assuming IMAP downloads emails permanently to your device.

Assuming IMAP downloads emails permanently to your device. IMAP keeps emails on the server and only downloads copies temporarily for viewing.

Summary

SMTP is the protocol that sends emails from your device to others.

POP3 downloads emails to one device and removes them from the server.

IMAP keeps emails on the server and syncs your mailbox across multiple devices.