邮件发送类，支持HTML格式，支持优先级设置。通过SOCKET类实现的。问题终于解决了。

using System;
using System.Text;
using System.IO;
using System.Net;
using System.Net.Sockets;
namespace Blood.Com.ClassLib
{    
    /// <summary>
    /// TcpClient派生类，用来进行SMTP服务器的连接工作
    /// </summary>
    public class SMTPClient : TcpClient
    {
        public bool isConnected()
        {
       return Active;
        }
        public void SendCommandToServer(string Command)
        {
       NetworkStream ns = this.GetStream() ;
       byte[]    WriteBuffer ;
       WriteBuffer = new byte[1024] ;
       WriteBuffer = Encoding.Default.GetBytes(Command) ;
       ns.Write(WriteBuffer,0,WriteBuffer.Length);
       return    ;
        }    
        public string GetServerResponse()
        {