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
PCFET0NUWVBFIEhUTUwgUFVCTElDICItLy9XM0MvL0RURCBIVE1MIDQuMCBUcmFuc2l0aW9uYWwv
L0VOIj4NCjxIVE1MPjxIRUFEPg0KPE1FVEEgY29udGVudD0idGV4dC9odG1sOyBjaGFyc2V0PWdi
MjMxMiIgaHR0cC1lcXVpdj1Db250ZW50LVR5cGU+DQo8TUVUQSBuYW1lPUdFTkVSQVRPUiBjb250
ZW50PSJNU0hUTUwgOC4wMC42MDAxLjE4NzAyIj48TElOSyByZWw9c3R5bGVzaGVldCANCmhyZWY9
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PC9GT05UPiZuYnNwOzwvRElWPg0KPERJViBhbGlnbj1sZWZ0PjxGT05UIGNvbG9yPSNjMGMwYzAg
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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_=====--