Principles of the email system (1)

Preface:
In the past few days, the company's email server has always encountered some problems. As there were not many contacts, and the email server had to be maintained by myself, I think it is necessary to learn the relevant knowledge!

Two Pillars of email
Here, we will detail the principles of the Internet email system. First, let's start with the basic concepts and concepts: providing Internet postal servicesProgramWhat is it? What technical terms and terminologies are there?

SMTP (Simple Mail Transfer Protocol, Simple Mail Transfer Protocol)

The main protocol used to send emails on the Internet. SMTP contains a set of rules on how applications sending and receiving emails interact.

POP3 (Post Office Protocol 3, Post Office Protocol Version 3)

The main protocol that provides the email receiving service. POP3 is a service that supports common online machines to receive emails from the mail server.

They are!

The Internet connects thousands of different networks. Each network has its own computer network and software. How can we make these different systems work together and exchange emails?

The answer is that the SMTP system standardizes mail delivery.The SMTP protocol is part of the TCP/IP protocol series.It explains the mail format and instructions on how to process the delivered mail. Each Internet computer automatically ensures that emails are selected and transmitted in the standard format when running the postal program. This program is calledTransport agent ),It works according to the SMTP protocol and sends your postal contact to the outside world.

In most systems, the transfer receiving program runs in the "Background" and responds to any request that may be received at any time. In UNIX system terms, this program is called the "patron saint", that isIntelligent Program (Daemon )(Yes, it can be called like this ).

Every UNIX system has a variety of smart programs hidden in the "background" to serve you silently. Theoretically, No matter what transfer handling program your system uses, you only need to use SMTP to send and receive postal files. Most Unix systems use a smart program called "send mail.

Do you think it is necessary to understand all the technical details of the work system? Of course not required. What we want you to appreciate is the work of the postal system, because each person on the network has at least one computer, which runs the transfer receiving program based on SMTP standards to send and receive emails.In many systems, the transfer acceptance program is called the "send mail" intelligent program.At least you need to know its name and what work it can do.

Although SMTP can transfer an email from your computer to the recipient's mailbox, it does not support the recipient to download the email to his own computer. The POP protocol is used to help us do this. Currently, the third version of POP3 is widely used.OnlyThe mail server where the recipient's mailbox is located supports the POP3 protocol (usually known as the POP3 server), so that the recipient can retrieve your email from the mailbox to his computer.

If there is no pop but only SMTP, the recipient will have to use Telnet to log on to the mail server where the mailbox is located to read your mail, and then use ftp to download the mail.

we can also use IMAP (Internet Message Access Protocol, Internet Packet Access Protocol ). When you use an email application (such as Outlook Express or Foxmail) to access the IMAP server, you can decide whether to copy the email to your computer, and whether to retain the email copy on the IMAP server. When accessing the POP3 server, emails in the mailbox are copied to the user's computer and no copies of emails are retained. Currently, there are not many servers that support the IMAP Protocol. A large number of mail servers are POP3 servers.

after the POP3 server downloads the email, it no longer retains the copy of the email. After IMAP downloads the email, it can retain the copy of the email on the mail server.