SMTP is a protocol that provides reliable and efficient e-mail transmission. It is a mail service based on FTP file transfer service, which is mainly used to transfer mail information between systems and provide notifications related to letters. ?

An important feature of SMTP is that it can transmit mail across the network, namely SMTP mail relay. Using SMTP, mail transmission between processors on the same network can be realized, and mail transmission between a processor and other networks can also be realized through repeaters or gateways. A mail exchange server with Domain Name Service System (DNS) function can also be used to identify the next-hop IP address of the mail. ?

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The working process of SMTP protocol can be divided into the following three processes:?

(1) Establish a connection: At this stage, the SMTP client requests to establish a TCP connection with port 25 of the server. Once the connection is established, SMTP server and client begin to announce their own domain names and confirm each other's domain names.

(2) Mail transmission: By command, the SMTP client transmits the source address, destination address and specific content of the mail to the SMTP server, and the SMTP server responds accordingly and receives the mail.

(3) Connection release: the SMTP client issues an exit command, the server responds after processing the command, and then closes the TCP connection.