本篇文章为smtp协议解析源码注释的第一篇，简单介绍一下smtp协议格式，大部分格式内容是从网上复制过来的。

一。smtp的客户端与服务器交互过程

常用客户端命令：

HELO/EHLO              发出请求

AUTH LOGIN             身份认证

MAIL FROM:              发件人email地址

RCPT TO:                   收件人email地址，可以写多个地址，发送给多人，这是一个列表

DATA                          邮件内容，如：hello，你好。

QUIT                          会话结束

服务器响应码：

220 <domain> Service ready

221 <domain> Service closing transmission channel

250 Requested mail action okay, completed

354 Start mail input; end with <CRLF>.<CRLF>    

二。信件头数据实例（从网上抄的）

以下内容来自：http://blog.csdn.net/kerry0071/article/details/28604267

C: telent SMTP.163.com 25  //以telenet方式连接163邮件服务器  
S: 220 163.com Anti-spam GT for Coremail System //220为响应数字，其后的为欢迎信息  
C: HELO SMTP.163.com //除了HELO所具有的功能外，EHLO主要用来查询服务器支持的扩充功能   
S: 250-mail  
S: 250-AUTH LOGIN PLAIN  
S: 250-AUTH=LOGIN PLAIN  
S: 250 8BITMIME //最后一个响应数字应答码之后跟的是一个空格，而不是'-'   
C: AUTH LOGIN   //请求认证  
S: 334 dxNlcm5hbWU6  //服务器的响应——经过base64编码了的“Username”=  
C: Y29zdGFAYW1heGl0Lm5ldA==  //发送经过BASE64编码了的用户名  
S: 334 UGFzc3dvcmQ6  //经过BASE64编码了的"Password:"=  
C: MTk4MjIxNA==  //客户端发送的经过BASE64编码了的密码  
S: 235 auth successfully  //认证成功   
C: MAIL FROM: bripengandre@163.com  //发送者邮箱  
S: 250 … .  //“…”代表省略了一些可读信息  
C: RCPT TO: bripengandre@smail.hust.edu.cn //接收者邮箱  
S: 250 … .    // “…”代表省略了一些可读信息  
C: DATA //请求发送数据  
S: 354 Enter mail, end with "." on a line by itself  
C: Enjoy Protocol Studing  
C: .  
S: 250 Message sent  
C: QUIT //退出连接   
S: 221 Bye  

三。信件体MIME实例（从网上抄的）

以下内容来自：https://blog.csdn.net/u014608280/article/details/89380417

Date: Mon, 29 Jun 2009 18:39:03 +0800

From: "=?gb2312?B?26zQocHB?=" <gaoxl@legendsec.com>

To: "moreorless" <moreorless@live.cn>

Cc: "gxl0620" <gxl0620@163.com>

BCC: "=?gb2312?B?26zQocHB?=" <venus.oso@gmail.com>

Subject: attach

Message-ID: <200906291839032504254@legendsec.com>

X-mailer: Foxmail 6, 15, 201, 21 [cn]

Mime-Version: 1.0

Content-Type: multipart/mixed;

      boundary="=====001_Dragon777814155473_====="

This is a multi-part message in MIME format.

--=====001_Dragon777814155473_=====

Content-Type: multipart/alternative;

      boundary="=====003_Dragon777814155473_====="

--=====003_Dragon777814155473_=====

Content-Type: text/plain;

      charset="gb2312"

Content-Transfer-Encoding: base64

DQoNCjIwMDktMDYtMjkgDQoNCg0KDQrbrNChwcEgDQo=

--=====003_Dragon777814155473_=====

Content-Type: text/html;

      charset="gb2312"

Content-Transfer-Encoding: base64

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MjMxMiIgaHR0cC1lcXVpdj1Db250ZW50LVR5cGU+DQo8TUVUQSBuYW1lPUdFTkVSQVRPUiBjb250

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PC9IVE1MPg0K

--=====003_Dragon777814155473_=====--

--=====001_Dragon777814155473_=====

Content-Type: application/octet-stream;

      name="readme.txt"

Content-Transfer-Encoding: base64

Content-Disposition: attachment;

      filename="readme.txt"

YWJjZGVkZg==

--=====001_Dragon777814155473_=====--