Shulou(Shulou.com)05/31 Report--

In this issue, the editor will bring you about the principle of Email. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

E-mail and Internet

In a professional E-mail solution, the components of the entire system are usually software from the same company, and you can't replace individual components at will, because there is usually no "equivalent program" to replace. On the other hand, Internet Email system adopts open design, and anyone can use different software combinations to build a mail system and exchange information with other interner Email systems. The only premise is that all software does not hesitate to abide by a set of public standards and protocols. This set of protocols defines the standard format of the message, the location and task of each role in the mail system, and the detailed process of message delivery from the sender to the receiver. Exchange itself is not a complete Interner Email mail system, it is only an important component of the whole system. Most Email users are only familiar with the software used to read and write letters, called "mail user agent" (Mail User Agent,MUA). Common MUA include: mutt, elm, pine, netscape and outlook express. Its task is to enable users to read, write and send letters. However, instead of delivering the Email directly to the recipient, MUA delivers the Email on its behalf by the Mail transfer Agent (Mail Transfer Agent,MTA). The role played by exchange is MTA.

The composition of the mail system

When you post a letter, the letter is handed over to the post office for processing. Similarly, when you ask MUA to send an email, it can only send it to a server running MTA software (such as exchange, postfix). Figure 1-1 shows the process of Email from sender to recipient. The task of MTA is to accept the appointment of MUA to deliver Email from one system to another. And accept the mail from the distant MTA. Whenever MTA receives a letter request from MUA, it will first determine whether it should be accepted. In general, if the message is from a user on the local system, or a system on the local network, or any network through which the message can be forwarded (relay) to another destination, MTA will accept the request to send the letter. On the other hand, MTA will also decide whether to receive the email or not based on the "recipient". If the recipient is a user of the local system, or if the recipient is on another system that it knows how to forward (forword), MTA receives the message.

After MTA receives the email, it must decide what to do next. It may deliver the message to users on its own system, or it may send the message to another MTA to continue delivery. Messages to be delivered to other networks may be delivered through multiple MTA relays. If the MTA cannot deliver the information and cannot hand it over to another MTA for processing, return the message to the original sender or send a notification letter to the system administrator. Generally speaking, the MTA Server of an individual is usually controlled by ISP, while that of a company's employees may be controlled by the information systems department of the enterprise.

The MTA of the mail terminal must deliver the message to the Mail delivery Agent (message delivery agent,MDA) after discovering that the recipient is a user of the local system. MDA may store information in a regular folder or in a special database dedicated to e-mail. No matter which form it is, any mechanism used to keep mail for a long time is collectively called a mailbox (message storage), or commonly known as a "mailbox".

After the mail is deposited in the mailbox, it stays there, waiting for the recipient to take it away. Recipients use MUA to pick up and read letters. The server software that provides mailbox access service is not the MTA that received the information at the beginning, and the roles of the two are separate. MUA must successfully authenticate the user before it can take the mail from the mailbox and present it to the user to read.

Because Internet Email adopts open standards, different roles such as MTA, MDA, MUA and so on can be played by many different software. Different packages that implement the same protocol can communicate with each other, no matter what system they are running on. If you put a complete Email mail system together, you may find that it is one set of software that deals with SMTP and another that deals with POP/IMAP. But for each role in the mail system, there are many different software to choose from.

These are the Email principles that the editor shares with you. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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