SMTP protocol communication model

The SMTP protocol is a member of the TCP/IP protocol family, which is mainly about how to transfer e-mail from the sender address to the receiving address, which is the rule of transmission. The communication model for the SMTP protocol is not complex, and the primary focus is on sending SMTP and receiving SMTP: First, a duplex communication link to receive SMTP is established by sending SMTP for the mail request sent by the user, where the receiving SMTP is relative to the sending SMTP. In fact, it can be either the final receiver or intermediate transporter. Send SMTP is responsible for sending SMTP commands to receive SMTP, while receiving SMTP is responsible for receiving and feedback responses. Can be roughly represented by the following communication model diagram: