The basics of the mail server are explained in detail

An email address can be divided into two parts. The first part is the user name and the second part is the host name. Take [email protected] as an example, yahoo.com is the host name and john is the username.

Assuming that Alex uses the mail address [email protected], when Alex wants to send a letter to [email protected], he will go through the following steps:

First Alex writes from the personal computer A good letter is sent to the host of his gmail.com.

gmail.com will first store the letter in the staging area on its own machine.

gmail.com will check the destination of the letter and look for the IP address of yahoo.com.

Then send the letter to the host of yahoo.com via the Internet.

After the yahoo.com host received the letter, it found that it was given to the user in the machine, so the letter was stored in the user's new folder.

When John is free, turn on the computer from home and take the initiative to go to yahoo.com to check for new mail. When you find a new letter, download it to your computer.

The process is shown below:

If [email protected] is to be sent Letter to [email protected], because the sender and the recipient use the same mail server, so in the second step above, the gmail.com host finds that the recipient is the user of the machine and will store the letter in use. The new ones are in the middle.

From the process of sending a letter to receiving a message, each component on the network plays a different role

Mail User Agent (MUA): a mail user agent, which is used The procedure used to write and receive letters. For example, we use Outlook Exporess, Thunderbird, and more. Its role is to provide users with a good way to send and receive mail interfaces, and to pass the letter to their own mail server.

Mail Transfer Agent (MTA): A mail transfer agent, a server responsible for forwarding mail. Sendmail and Postfix used in UNIX are MTA software. Its role is to receive the letter from MTU and forward the letter to the destination based on the address of the letter. In the destination, another MTA will be responsible for receiving the letter. Sometimes letters are not sent from the user's computer to the destination host at a time, but are forwarded to the destination host via many MTAs. The action that the MTA receives from its own letter and forwards it to another MTA is called "Relay."

Mail Delivery Agent (MDA): A mail delivery agent that assigns mail to local users to the user's mailbox. In UNIX, MDA is usually the mail program.

When we sent a letter, we used the SMTP (Simple Mail Transfer Protocol) protocol. On a mail server, we must first install the mail server software to receive the SMTP protocol. Letters. The SMTP protocol is also used when transferring messages between the MTA and the MTA. In the case of receiving a message, a POP3 (Post Office Protocol) or an IMAP (Internet Message Access Protocol) protocol is used. As shown below:

The commonly used SMTP software is Sendmail, Qmail, or Postfix included with UNIX. The POP3 and IMAP software UNIX used in receiving the mail is not built in, so we have to install it ourselves. However, as a Mail Server, we have to set a minimum of POP3 and SMTP to allow users to send and receive. In addition, we can also use the mail interface software of the web interface. The web interface mail sending and receiving software allows users to easily send and receive mails through the browser of any operating system without using software such as Outlook.

Traditionally, SMTP does not require identity authentication when accepting user mail. Anyone can use your host to create spam. Therefore, UNIX built-in Sendmail does not accept SMTP mail. The general ISP industry mostly controls the online source to prohibit users who are not allowed to RELAY. However, if we control the online source, we cannot use SMTP at other non-permitted IP addresses. People who can send letters anywhere are very inconvenient. Therefore, we must let users use SMTP authentication to send mail through UNIX, so users who want to use SMTP to send mail such as Outlook must first authenticate with this machine.

Before starting to set up the mail server, you must first set the DNS. Since the mail must be sent, you must find the IP of the destination host. If your host does not have a valid DNS setting, the mail will not be sent correctly. Da. In addition, your server IP must also be set to reverse the solution, the IP name must be mapped against your host name. If the reverse solution is not normal, some servers may reject the letter you sent.