SMTP is a protocol that provides reliable and effective email transmission. SMTP is a mail service based on the FTP file transfer service, which is mainly used to transfer mail information between systems and provide notifications about incoming letters. SMTP is independent of specific transmission subsystems, and only requires reliable and orderly data flow channel support. One of the important features of SMTP is that it can transmit mail across the network, that is, "SMTP mail relay." Using SMTP, mail transmission between the same network processing process can be realized, and mail transmission between a certain processing process and other networks can also be realized through a relay or gateway.
SMTP is a relatively simple text-based protocol. One or more recipients of a message are specified on it, and the message text is transmitted. SMTP began to be widely used in the early 1980s. At the time, it was only a supplement to UUCP, which was more suitable for handling mail sent between intermittently connected machines. On the contrary, SMTP works best when the sending and receiving machines are continuously connected to the network.
The working process of the SMTP protocol can be divided into the following three processes:
1. Connection establishment: At this stage, the SMTP client requests to establish a TCP connection with port 25 of the server. Once the connection is established, the SMTP server and the client begin to advertise their domain names to each other, and at the same time confirm each other's domain names. [4]
2. Mail transmission: Using commands, 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.
SMTP usually has two working modes. Send SMTP and receive SMTP. The specific working method is:
Send SMTP After receiving the user’s mail request, determine whether the mail is a local mail, if it is directly delivered to the user’s mailbox, otherwise query the MX of the remote mail server to DNS Record and establish a two-way transmission channel with the remote receiving SMTP. After that, the SMTP command is issued by the sending SMTP, received by the receiving SMTP, and the response is sent in the opposite direction. Once the transmission channel is established, the SMTP sender sends a MAIL command to indicate the sender of the mail. If the SMTP receiver can receive the mail, an OK response is returned. The SMTP sender then issues the RCPT command to confirm whether the mail has been received. If the SMTP receiver receives it, it will return an OK response; if it cannot receive it, it will send a rejection response (but not suspend the entire mail operation), and both parties will repeat this many times. When the recipient receives all the mails, it will receive a special sequence. If the recipient successfully processes the mail, it will return an OK response.
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 mainly used to support the use of the client to remotely manage emails on the server. The POP3 protocol that provides SSL encryption is called POP3S.
The POP protocol supports "offline" mail processing. The specific process is:
The email is sent to the server, and the email client calls the email client program to connect to the server and download all unread emails. This offline access mode is a store and forward service that sends mail from the mail server to the personal terminal machine. Once the mail is sent to the terminal, the mail on the mail server will be deleted. But most POP3 mail servers can "only download mails without deleting them on the server side", which is the improved POP3 protocol.
The difference and connection between POP3 and SMTP protocol:
POP3 specifies how to connect a personal computer to the Internet mail server and the electronic protocol for downloading e-mails. It is the first offline protocol standard for Internet e-mail. POP3 allows users to store mail from the server to the local host (ie their own computer) and delete the mail stored on the mail server. The POP3 server follows POP3. The receiving mail server of the protocol, used to receive e-mails.
The POP3 protocol allows email clients to download emails on the server, but operations on the client (such as moving emails, marking as read, etc.) will not be fed back to the server, such as receiving emails through the client 3 emails in and moved to other folders, these emails on the mailbox server were not moved at the same time.
SMTP is a set of specifications used to transfer mail from source address to destination address, and it controls the way in which mail is transferred. The SMTP protocol belongs to the TCP/IP protocol suite, which helps each computer find the next destination when sending or transferring letters. The SMTP server is the outgoing mail server that follows the SMTP protocol.
SMTP authentication simply means that you must provide your account name and password before you can log in to the SMTP server, which makes spammers inaccessible. The purpose of adding SMTP authentication is to prevent users from being harassed by spam.
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