Discover email service

Email Service is one of the most frequently used and used applications on the Internet. Ordinary users can use their usernames and passwords to enable the mailbox, and read, store, reply, forward, and delete emails. Like traditional mails delivered by a postman, e-mails are asynchronous. That is to say, people send and read mails at convenience without having to cooperate with others in advance. Unlike traditional emails, emails are fast, easy to distribute, and cost-effective, in addition, modern email messages include hyperlinks, HTML text, images, sounds, video data, and other rich forms, which are increasingly popular.

Email System Structure

The Internet email system uses the "Storage and forwarding" Store and Forward) method. The sender sends the edited email from the sender's computer through the email client program, in the process of network transmission, it passes through multiple computers and finally arrives at the destination computer and sends it to the recipient's email box. Figure 1 shows the composition of the Internet mail system. The system consists of three main components: User proxy, email server, and transmission protocol.

Figure 1 Composition of the email system

User Agent

The Mail User proxy MUA is part of the interface between the user and the mail system. You can use it to create, read, and manage your own mail. MUA can be implemented in different ways. For example, OutLook and Foxmail are MUA running on Windows; Pine and Kmail are MUA running on Unix; free Hotmail and Yahoo emails provide Web-based MUA, and even some short messages and notifications can be seen as auxiliary functions of MUA.
When MUA delivers emails to the mail system, the mail system sends them to the correct receiving host. This task is completed by the mail Transmission proxy MTA. The design of the email system has nothing to do with the specific network structure. Whether it is the Internet or UUCP network, in principle, as long as the message system has an MTA that understands the corresponding network protocol, the mail system can use the MTA, send emails from one host to another.
When the mail reaches the target host, the mail system needs to send the mail to the specific user through the mail distribution proxy MDA. A complete mail system must support the Mail User proxy MUA.

Email server　

The email server is the core of the email system. Each recipient has a Mailbox on a mail server ). A typical journey of mail message is from the user proxy of the sender, to the email server of the sender, and then to the email address of the recipient. When you want to view the mail messages in your mailbox, the email server that stores the email will authenticate the user name and password. If the sender's email server cannot immediately deliver the email Message to the recipient's email server, the sender's server will store the Message in the Message Queue and try again later. This kind of attempt is usually executed every 30 minutes or so. If the attempt fails after several days, the server removes the message from the message queue and notifies the sender of another email.

Transmission Protocol

The Simple Mail Transfer Protocol (SMTP) is the primary application layer protocol of the Internet email system. It is used to send mail messages from the sender's email server to the recipient's email server. Like most application-layer protocols, SMTP also has two ends: the client executed on the sender's email server and the server executed on the recipient's email server. The SMTP client and server run on each mail server at the same time. When a mail server sends a mail message to another mail server, it is running as an SMTP client. When a mail server receives a mail message from another mail server, it is running as an SMTP server.

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