Beijing website construction Company Shangpin China: E-mail, whose English name is E-mail, is a fast, simple and inexpensive modern means of communication to contact other users through the Internet. It is also the most frequently used service function for Internet users. Compared with traditional communication methods, e-mail has the following obvious advantages:
① E-mail is faster than traditional mail delivery, has a wide range of reach, and is more reliable.
② E-mail can realize one to many e-mail transmission, which can make it easy for one user to send notifications to multiple people.
③ E-mail can integrate various types of information such as text, image and voice into one e-mail, so it will become an important means of multimedia information transmission.
To fully understand how email works. This section will be elaborated from the following aspects.
1. Email delivery process
The e-mail system adopts the working mode of "Store and Forward". An e-mail is sent from the sending computer. In the process of network transmission, it passes through multiple computers and finally arrives at the destination computer and is sent to the recipient's e-mail, as shown in Figure 4-4. This delivery process of e-mail is a bit like the delivery process of conventional letters in the traditional postal system. When a user writes a letter to a distant friend and puts it into a post office box, the letter will be received by the local post office, and transported by sorting and mail trucks. It may need to be forwarded through one post office after another, and finally arrive at the post office where the recipient is located. Then the postman will deliver the letter to the recipient or put it into his mailbox.
In TCP/IP and e-mail systems, a "Delayed Delivery" mechanism is also provided, which is one of the outstanding advantages of e-mail systems. With this mechanism, when the mail is forwarded between Internet hosts (mail servers), if the remote destination host is temporarily inaccessible (possibly due to shutdown), the sending host will store the mail in the buffer store, and then continue to try to send until the destination host can access it.
2. E-mail related agreements
(1) Simple Mail Transfer Protocol
Simple Mail Transfer Protocol (SMTP) is an application layer protocol based on TCP/IP on the Internet, which is suitable for e-mail exchange between hosts. SMTP includes two standard subsets, one is to define the format of e-mail information and the other is to transport e-mail.
In the Internet, the delivery of e-mail depends on SMTP. In other words, SMTP is mainly responsible for the delivery of e-mail between servers. Its biggest feature is simplicity, because it only stipulates how e-mail is transmitted through the TCP connection between the sender and receiver in the Internet. Other operations, such as interaction with users, e-mail storage, and the time interval of sending e-mail by the mail system, are not involved.
In an e-mail system, SMTP works as a client server. The sender's host is the client side, and the recipient's mail server is the server side. The SMTP on both machines cooperate with each other to send the e-mail from the sender's host to the recipient's mailbox. In the process of sending mail, TCP is required to connect (the default port number is 25). SMTP specifies the interaction between the sender and receiver, as shown in Figure 4-5. The sending host first sends the mail to the local SMTP server, which establishes a reliable TCP connection with the recipient's mail server, establishing a direct channel from the sender's host to the recipient's mail server, thus ensuring the reliability of mail transmission.
(2) Post Office Agreement
At present, the Post Office Protocol (POP) mainly uses POP version 3, namely POP3. The main task of POP3 is to directly transmit the mail in the e-mail box of the mail server to the user's local computer when the user's computer is connected to the mail server, as shown in Figure 4-6. This function is similar to the temporary saving of mail in the post office. Users can pick up mail at any time. If POP3 is not used, users can only connect to the local mail server through remote login to view mail. It is troublesome to deliver mail to the local computer. This article was published on SEO website optimization Company Shangpin China //ihucc.com/
