The full name of POP is post office protocol, which is used to receive e-mail. It uses TCP port 1 10. The third edition is commonly used now, so it is called POP3 for short. POP3 still works in client/server mode. A client is called a client. Generally speaking, our daily computer serves as a client, while the server is managed by a network administrator. For example, the server is a collection of many small mailboxes, just like the mailbox structure of the building where we live, and the client is like a person holding a key to open the mailbox and get a letter.

Let's take a look at the process of receiving email with email software. Generally, we set the URL of a POP server (such as pop. 163.com), and the account number and password of the email address are in the account attribute of the email software. This is used in the process of receiving letters. When we press the receive button in the email software, the email software will first call the DNS protocol to analyze the IP address of the POP server. When the IP address is resolved, the e-mail program will start to connect to the port 1 10 of the e-mail server using TCP protocol. Because the POP server is busy, we have to wait a long time in this process. When the mail program successfully connects to the POP server, it will first use the USER command to transfer the account of the mailbox to the POP server, and then use the PASS command to transfer the account of the mailbox to the server. When this authentication process is completed, the mail program uses the STAT command to request the server to return mailbox statistics, such as the total number of messages and the size of messages, and then the list will list the number of messages in the server. Then the mail program will use the RETR command to receive the mail, and after receiving a mail, it will use the DELE command to set the mail in the mail server to the deleted state. When QUIT is used, the mail server will delete messages marked for deletion. Generally speaking, receiving mail from a server by a mail program is actually a dialogue process, and POP protocol is a language for e-mail.