What is the basic principle of email?

The traditional method of sending letters is: the sender is the sender of the post office at the post office where the sender is located. The sending process of the e-mail is similar to this. The e-mail is first sent to the sending server to which the sender's mailbox belongs, that is, the SMTP server, which queues all the outgoing mails and sends them to the receiving server to which the recipient's mailbox belongs. The POP3 server, the last recipient downloads the mail from the POP3 server to the local hard disk, and completes the mail sending and receiving process.