Internet e-mail is how to send?
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Since the birth of the Internet, e-mail became popular.
In this section, we will discuss the core application layer protocol in the Internet e-mail application.
Before we delve into these agreements, we stand at a very high angle look at the Internet mail system and its key components.
Internet mail system
We can see from this chart, the Internet e-mail application has three main parts
- user agents
For example, Microsoft's outlook client - mail servers
- Simple Mail Transfer Protocol(SMTP)
These three components in the end what it was, no concept.
Here we give an example, you will understand.
Suppose Alice is sitting in front of the computer to bob through outlook email.
user agent
user agents allow users to read, reply, forward, save, create messages.
For example: Microsoft's outlook and Apple Mail
Apple Mail is below this stuff
When Alice written message:
then click Send Alice, Alice took the apple mail message is sent to the Server Alice's mail, the WHERE IS at The Placed in the Message at The outgoing mail the Message Queue Server's the When Bob Wants A to the Read the Message, His-. The User Agent Retrieves at The the Message from Bob's mail Box in Bob's mail Server .
Mail Server
Mail servers form the core of the e-mail infrastructure. 每一个收信者has a mailbox located in one of the mail servers.
我想问，所以有bob’s mail server这一说吗？bob’s mail server可以存放其他人的mailbox吗？
Bob’s mailbox manages and maintains the messages that have been sent to him.
A typical message starts its journey in the sender’s user agent, travels to the sender’s mail server, and travels to the receipt’s mail server, where it is deposited in the recipient’s mailbox.
所以我想问Bob’s mail server可以放其他人的mail box吗？
当Bob想要access the messages in Bob’s mailbox, the mail server containing his mailbox authenticates Bob ( with usernames and passwords).
Bob: 不是随便一个人都可以access我的邮箱的。 首先你要报我的名号，然后你还要报上我的密码。呵呵，你不是我，你怎么可能知道我的密码是多少。
但是Alice的服务器必须要处理failures in Bob’s mail server. 如果Alice’s server cannot deliver mail to Bob’s server, Alice’s server holds the message in a message queue and attempts to transfer the message later.
Alice’s server: 这个时候Bob’s server的女朋友和他分手了，他油盐不进，我还是过一会儿再给他喂油吧。
Reattempts are often done every 30 minutes or so.
至少过个半个小时吧，没有半个小时，这家伙消不了气的。
如果几天之后还是没有成功，Alice的服务器就会removes the message and notifies the sender(Alice) with an e-mail message.
我想知道这个notify消息是如何发送给Alice的？
SMTP
SMTP是因特网电子邮件中主要的应用层协议。SMTP uses the reliable data transfer service of TCP to transfer mail from the sender’s mail server to the receipt’s mail server.
像大多数应用一样，SMTP有两端, a client side and a server side.
我突然有个疑问，电子邮件使用的是什么网络应用体系结构？
- client side, which executes on the sender’s mail server
- server side, which executes on the recipient’s mail server
Both the client and server sides run on every mail server.
This saying is very abstract
When a mail server sends mail to other mail servers, it acts as an SMTP client.
When a mail server receives mail from other mail servers, it acts as an SMTP server.