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# POP3 Protocol POP3 (Post Office Protocol version 3) is a protocol used to download emails from a mail server. POP3 allows users to retrieve emails from the server through clients (such as Outlook, Thunderbird) and store them on local devices. ## How POP3 Works POP3 uses a client-server model, transmitting emails through plaintext or encrypted communication channels. Its core function is to download emails from the server. ### 1. **POP3 Connection Establishment** !(#) * The client connects to port 110 on the server (the default POP3 port). * The server returns `+OK`, indicating the service is ready. * The client sends the username (USER command), and the server returns `+OK`, indicating a password is required. * The client sends the password (PASS command), and the server returns `+OK`, indicating successful login. * * * ### 2. **Email Download** After the connection is established, the client can download emails. Here is a typical email download process: !(#) * **STAT**: The client retrieves the number of emails and total size. * **LIST**: The client retrieves the email list. * **RETR**: The client downloads the specified email. * **DELE**: The client marks the email for deletion (the email is deleted after the QUIT command). * * * ### 3. **Connection Closure** After the email download is complete, the client can close the connection: !(#) * The client sends the QUIT command to request