E-mail overview : E-mail sends the mail to the mail server used by the recipient and puts it in the recipient's mailbox. The recipient can go online to the mail server they use to read it at their convenience.
Two of the most important draft standards for e-mail : Simple Mail Transfer Protocol (SMTP) and Internet Text Message Format.
Common Internet Mail Extensions MIME : MIME describes the data type of the mail (such as text, sound, image, etc.) in its mail header. Multiple types of data can be transmitted simultaneously in MIME mail.
The three main components of the email system : user agent, mail server, mail sending protocol (such as SMTP) and mail reading protocol (such as POP3).
-
User Agent (UA) : The interface between the user and the email system. In most cases, it is a program running on the user's computer, so it is also called email client software. A user agent should have at least the following four functions:
- Compose : Provide users with an environment to edit letters.
- Display : It can conveniently display the message on the computer screen.
- Processing : Processing includes sending mail and receiving mail.
- Communication : After composing the email, the sender should use the email sending protocol to send the email to the mail server used by the user.
-
mail server :
- Basic concept : the mail server works 24 hours a day and has a large-capacity mailbox.
- The function of the mail server : The function of the mail server is to send and receive mail, and report the result of mail delivery to the sender at the same time.
- Mail server protocol : The mail server needs to use two different protocols: one protocol is used for the user agent to send mail to the mail server or send mail between mail servers, such as the SMTP protocol; and the other protocol is used for the user agent to send mail from the mail server Read mail in, such as Post Office Protocol POP3. Both the SMTP protocol and the POP3 protocol use TCP connections to transmit mail, which is for reliable transmission.
- Note : The mail server should be able to act as client and server at the same time.
Steps to send and receive email between computers :
- The sender invokes the user agent in the computer to compose and compose the message to be sent;
- The sender clicks the "Send Mail" button, and the user agent does all the work of sending the mail. The user agent sends the mail to the sender's mail server using the SMTP protocol, the user agent acts as the SMTP client, and the sender's mail server acts as the SMTP server. As for the user, after the user delivers the written letter to the user agent, there is nothing to worry about.
- After the SMTP server receives the mail from the user agent, it stores the mail in the mail cache queue and waits to be sent to the receiver's mail server (the waiting time depends on the processing capacity of the mail server and the number of letters waiting to be sent in the queue. quantity). But this waiting is generally far greater than the queuing time of packets waiting to be forwarded in routers.
- The SMTP client of the sender's mail server establishes a TCP connection with the SMTP server of the receiver's mail server, and then sends out the mails in the mail cache queue in sequence. Mail doesn't land at some intermediate mail server on the Internet. If the SMTP client still has some mail to send to the same mail server, it can be sent repeatedly on the originally established TCP connection. If the SMTP client cannot establish a TCP connection with the SMTP server, the mail to be sent will continue to be stored in the sender's mail server, and a new attempt will be made in a later period of time. If the SMTP client cannot send the mail beyond the specified time, the sending mail server will notify the user agent of this situation.
- After the SMTP server process running in the recipient's mail server receives the mail, it puts the mail into the recipient's user mailbox and waits for the recipient to read it.
- When the recipient intends to receive the mail, he runs the system agent in the computer and uses the PO3 (or IMAP) protocol to read the mail sent to him.
The components of an email : envelope and content.
Email format :用户名@邮件服务器的域名