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**POP3 (Post Office Protocol version 3)** is an **application-layer protocol** used to **retrieve (download) email from a mail server to a client**.

In short:
**POP3 lets you pull messages from the server down to a computer or phone**.

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## Why POP3 is needed

POP3 is used when you need to:

- retrieve messages from a mail server
- save them locally on a device
- work with mail **offline**

Historically POP3 was built for this scenario:

> "connect → download mail → disconnect."

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## How POP3 works (simplified)

1. The mail client connects to the mail server
2. It authenticates (login/password)
3. The client requests a list of messages
4. The client **downloads the messages**
5. The messages are usually **deleted from the server**

This is POP3's key trait.

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## POP3's main trait

**Messages are stored on the client, not on the server**

Consequences:

- if mail was downloaded to one computer,
it will no longer be on another device
- the state of messages (read / unread)
does not sync

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## POP3 vs IMAP (in short)

- **POP3**
  - downloads messages
  - often deletes them from the server
  - convenient for a single device
- **IMAP**
  - messages stay on the server
  - suits multiple devices

In practice, today **IMAP is used more often** than POP3.

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## POP3 security

Classic POP3:

- does not encrypt data
- transfers the login and password as plain text

So people use:

- **POP3S (POP3 + SSL/TLS)**

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## A real-life example

POP3 is like:

- taking paper letters out of a mailbox
- carrying them home
- there are no more letters left in the box

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