用c#写的smtp邮件发送类

//**********************Created by Chen**************************
using System;
using System.IO;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.Collections;
using System.Collections.Specialized;
using KSN.Exceptions;
using KSN.Validate;
namespace KSN.Web.Mail
{
 /// <summary
 /// 邮件内容
 /// </summary
 public class MailMessage
 {
  private string sender=null;
  private StringCollection receivers=new StringCollection();
  private string subject="";
  private string xMailer="";
  private StringCollection attachments=new StringCollection();
  private MailEncodings mailEncoding=MailEncodings.GB2312;
  private MailTypes mailType=MailTypes.Html;
  private byte[] mailBody=null;
  /// <summary
  /// 获取或设置发件人
  /// </summary
  public string Sender
  {
   get{return this.sender;}
   set{this.sender=value;}
  }
  /// <summary
  /// 获取收件人地址集合
  /// </summary
  public StringCollection Receivers
  {
   get{return this.receivers;}
  }
  /// <summary
  /// 获取或设置邮件主题
  /// </summary
  public string Subject
  {
   get{return this.subject;}
   set{this.subject=value;}
  }
  /// <summary
  /// 获取或设置邮件传送者
  /// </summary
  public string XMailer
  {
   get{return this.xMailer;}
   set{this.xMailer=value;}
  }
  /// <summary
  /// 获取附件列表
  /// </summary
  public StringCollection Attachments
  {
   get{return this.attachments;}
  }
  /// <summary
  /// 获取或设置邮件的编码方式
  /// </summary
  public MailEncodings MailEncoding
  {
   get{return this.mailEncoding;}
   set{this.mailEncoding=value;}
  }
  /// <summary
  /// 获取或设置邮件格式
  /// </summary
  public MailTypes MailType
  {
   get{return this.mailType;}
   set{this.mailType=value;}
  }
  /// <summary
  /// 获取或设置邮件正文
  /// </summary
  public byte[] MailBody
  {
   get{return this.mailBody;}
   set{this.mailBody=value;}
  }
 }
 /// <summary
 /// 邮件编码
 /// </summary
 public enum MailEncodings
 {
  GB2312,
  ASCII,
  Unicode,
  UTF8
 }
 /// <summary
 /// 邮件格式
 /// </summary
 public enum MailTypes
 {
  Html,
  Text
 }
 /// <summary
 /// smtp服务器的验证方式
 /// </summary
 public enum SmtpValidateTypes
 {
  /// <summary
  /// 不需要验证
  /// </summary
  None,
  /// <summary
  /// 通用的auth login验证
  /// </summary
  Login,
  /// <summary
  /// 通用的auth plain验证
  /// </summary
  Plain,
  /// <summary
  /// CRAM-MD5验证
  /// </summary
  CRAMMD5
 }
 /// <summary
 /// 邮件发送类
 /// </summary
 public class KSN_Smtp
 {
  #region "member fields"
  /// <summary
  /// 连接对象
  /// </summary
  private TcpClient tc;
  /// <summary
  /// 网络流
  /// </summary
  private NetworkStream ns;
  /// <summary