特点：
1. 支持SMTP验证
2. 支持群发功能
3. 支持各种免费邮箱，如sohu,163等
4. 支持Win2k Server自带的smtp服务器，即只要你联网，配置好smtp服务器，即可本机发邮件
5. 完整的错误代码，拥有Logs和HTMLlogs两个日志功能，可以输出详细的服务器--客户端交互日志
代码如下:
----------------------------------------------
using System;
using System.Text;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Collections;

namespace Sanxing.Mail.Data
{
    public class ESmtpMail
    {

//内部变量
/// <summary>
/// 设置换行变量
/// </summary>
private string enter="\r\n";

/// <summary>
/// 设定语言代码，默认设定为GB2312，如不需要可设置为""
/// </summary>
private string Charset="GB2312";

/// <summary>
/// 发件人地址
/// </summary>
private string From="";

/// <summary>
/// 发件人姓名
/// </summary>
private string FromName="";

/// <summary>
/// 回复邮件地址
/// </summary>
//public string ReplyTo="";

/// <summary>
/// 收件人姓名
/// </summary>    
private string RecipientName="";

/// <summary>
/// 收件人列表
/// </summary>
private Hashtable Recipient=new Hashtable();

/// <summary>
/// 邮件服务器域名
/// </summary>    
private string mailserver="";

/// <summary>
/// 邮件服务器端口号
/// </summary>    
private int mailserverport=25;

/// <summary>
/// SMTP认证时使用的用户名
/// </summary>
private string username="";

/// <summary>
/// SMTP认证时使用的密码
/// </summary>
private string password="";

/// <summary>
/// 是否需要SMTP验证
/// </summary>       
private bool ESmtp=false;

/// <summary>
/// 是否HTML邮件
/// </summary>       
private bool HTML=false;

/// <summary>
/// 邮件附件列表
/// </summary>
private System.Collections.ArrayList Attachments;

/// <summary>
/// 邮件发送优先级，可设置为"High","Normal","Low"或"1","3","5"
/// </summary>
private string priority="Normal";

/// <summary>
/// 邮件主题
/// </summary>       
private string Subject="";

/// <summary>
/// 邮件正文
/// </summary>       
private string Body="";

/// <summary>
/// 收件人数量
/// </summary>
private int RecipientNum=0;

/// <summary>
/// 最多收件人数量
/// </summary>
private int recipientmaxnum=1;

/// <summary>
/// 密件收件人数量
/// </summary>
//private int RecipientBCCNum=0;

/// <summary>
/// 错误消息反馈
/// </summary>
private string errmsg;

/// <summary>
/// TcpClient对象，用于连接服务器
/// </summary>    
private TcpClient tc;

/// <summary>
/// NetworkStream对象
/// </summary>    
private NetworkStream ns;

/// <summary>
/// SMTP错误代码哈希表
/// </summary>
private Hashtable ErrCodeHT = new Hashtable();

/// <summary>
/// SMTP