The protocol used to send and receive emails on the Internet is "POP3/SMTP". POP3 is a protocol used to read emails from a mail server; SMTP is a protocol that provides reliable and effective email transmission. It is mainly used to transfer email information between systems and provide notifications about incoming letters.
The operating environment of this tutorial: Windows 10 system, Dell G3 computer.
The protocol used to send and receive emails on the Internet is "POP3/SMTP".
POP3 Protocol
POP3, the full name is "Post Office Protocol - Version 3", that is, "Post Office Protocol Version 3". It is a member of the TCP/IP protocol suite and is defined by RFC1939. This protocol is primarily intended to support remote management of email on the server using a client. The POP3 protocol that provides SSL encryption is called POP3S.
POP3 is a protocol used to read mail from the mail server and transmit the mail from the POP server to the user agent.
The user agent responsible for receiving mail is called a POP client, and the server responsible for sending mail is called a POP server
The POP protocol supports "offline" mail processing. The specific process is: the email is sent to the server, the email client calls the email client program to connect to the server, and downloads all unread emails. This offline access mode is a store and forward service that sends emails from the email server to a personal terminal machine, usually a PC or MAC. Once the email is sent to the PC or MAC, the email on the mail server will be deleted. However, most POP3 mail servers can "only download mails and not delete them on the server side", which is an improved POP3 protocol.
SMTP Protocol
SMTP is a protocol that provides reliable and efficient email transmission. SMTP is an email service built on the FTP file transfer service. It is mainly used to transfer email information between systems and provide notifications about incoming letters. SMTP is independent of a specific transmission subsystem and only requires the support of a reliable and orderly data flow channel. One of the important features of SMTP is its ability to transmit emails across the network, that is, "SMTP mail relay". Using SMTP, mail transmission can be realized between processing processes on the same network, and mail transmission between a processing process and other networks can also be realized through a relay or gateway.
The working process of the SMTP protocol can be divided into the following three processes:
(1) Establishing a connection: At this stage, the SMTP client requests to establish a TCP connection with the server's port 25. Once the connection is established, the SMTP server and client begin to advertise their domain names to each other and confirm each other's domain names.
(2) Mail transmission: Using commands, the SMTP client passes 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.
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