C#实现SMTP服务器，使用TCP命令实现，功能比较完善

using System;
using System.Text;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Collections;

namespace SkyDev.Web.Mail
{
 public enum MailFormat{Text,HTML};
 public enum MailPriority{Low=1,Normal=3,High=5};

 #region Class mailAttachments
 public class MailAttachments
 {
  private const int MaxAttachmentNum=10;
  private IList _Attachments;

  public MailAttachments()
  {
   _Attachments=new ArrayList();
  }

  public string this[int index]
  {
   get { return (string)_Attachments[index];}
  }
  /// <summary>
  /// 添加邮件附件
  /// </summary>
  /// <param name="FilePath">附件的绝对路径</param>
  public void Add(params string[] filePath)
  {
   if(filePath==null)
   {
    throw(new ArgumentNullException("非法的附件"));
   }
   else
   {   
    for(int i=0;i<filePath.Length;i++)
    {
     Add(filePath[i]);
    }
   }
  }

  /// <summary>
  /// 添加一个附件,当指定的附件不存在时，忽略该附件，不产生异常。
  /// </summary>
  /// <param name="filePath">附件的绝对路径</param>
  public void Add(string  filePath)
  {
   //当附件存在时才加入,否则忽略
   if (System.IO.File.Exists(filePath))
   {
    if (_Attachments.Count<MaxAttachmentNum)
    {
     _Attachments.Add(filePath);
    }
   }
  }

  public void Clear()//清除所有附件
  {
   _Attachments.Clear();
  }

  public int Count//获取附件个数
  {
   get { return _Attachments.Count;}
  }

 }
 #endregion//end Class mailAttachments

 #region Class MailMessage
 /// <summary>
 /// MailMessage 表示SMTP要发送的一封邮件的消息。
 /// </summary>
 public class MailMessage
 {
  private const int MaxRecipientNum=10;
  public MailMessage()
  {  
   _Recipients=new ArrayList();//收件人列表
   _Attachments=new MailAttachments();//附件
   _BodyFormat=MailFormat.Text;//缺省的邮件格式为Text
   _Priority=MailPriority.Normal;
   _Charset="GB2312";
  }

  /// <summary>
  /// 设定语言代码，默认设定为GB2312，如不需要可设置为""
  /// </summary>
  public string Charset
  {
   get { return _Charset;}
   set { _Charset=value;}
  }

  public string From
  {
   get{ return _From;}
   set { _From=value;}
  }

  public string FromName
  {
   get { return _FromName;}
   set { _FromName=value;}
  }
  public string Body
  {
   get { return _Body;}
   set { _Body=value;}
  }

  public string Subject
  {
   get { return _Subject;}
   set { _Subject=value;}
  }

  public MailAttachments Attachments
  {
   get {return _Attachments;}
   set { _Attachments=value;}
  }

  public MailPriority Priority
  {
   get { return _Priority;}
   set { _Priority=value;}
  }

  public IList Recipients
  {
   get { return _Recipients;}
  }
  /// <summary>
  /// 增加一个收件人地址
  /// </summary>
  /// <param name="recipient">收件人的Email地址</param>
  public void AddRecipients(string recipient)
  {
   //先检查邮件地址是否符合规范
   if (_Recipients.Count<MaxRecipientNum)
   {
    _Recipients.Add(recipient);//增加到收件人列表
   }
  }

  public void AddRecipients(params string[] recipient)
  {
   if (recipient==null)
   {
    throw (new ArgumentException("收件人不能为空."));
   }
   else
   {
    for (int i=0;i<recipient.Length;i++)
    {
     AddRecipients(recipient[i]);
    }
   }
  }

  public MailFormat BodyFormat
  {
   set { _BodyFormat=value;}
   get { return _BodyFormat;}
  }

  private string _From;//发件人地址
  private string _FromName;//发件人姓名
  private IList _Recipients;//收件人
  private MailAttachments _Attachments;//附件
  private string _Body;//内容
  private string _Subject;//主题
  private MailFormat _BodyFormat;//邮件格式
  private string _Charset="GB2312";//字符编码格式
  private MailPriority _Priority;//邮件优先级
 }
 #endregion

 #region Class SmtpMail
 public class SmtpServerHelper
 {
  private string CRLF="\r\n";//回车换行

  /// <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 tcpClient;

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

  /// <summary>
  /// 服务器交互记录
  /// </summary>
  private string logs="";

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

  /// <summary>
  /// SMTP正确代码哈希表
  /// </summary>
  private Hashtable RightCodeHT = new Hashtable();

  public SmtpServerHelper()
  {
   SMTPCodeAdd();//初始化SMTPCode
  }

  ~SmtpServerHelper()
  {
   networkStream.Close();
   tcpClient.Close();
  }

  /// <summary>
  /// 将字符串编码为Base64字符串
  /// </summary>
  /// <param name="str">要编码的字符串</param>
  private string Base64Encode(string str)
  {
   byte[] barray;
   barray=Encoding.Default.GetBytes(str);
   return Convert.ToBase64String(barray);
  }

  /// <summary>
  /// 将Base64字符串解码为普通字符串
  /// </summary>
  /// <param name="str">要解码的字符串</param>
  private string Base64Decode(string str)
  {
   byte[] barray;
   barray=Convert.FromBase64String(str);
   return Encoding.Default.GetString(barray);
  }

  /// <summary>
  /// 得到上传附件的文件流
  /// </summary>
  /// <param name="FilePath">附件的绝对路径</param>
  private string GetStream(string FilePath)
  {
   //建立文件流对象
   System.IO.FileStream FileStr=new System.IO.FileStream(FilePath,System.IO.FileMode.Open);
   byte[] by=new byte[System.Convert.ToInt32(FileStr.Length)];
   FileStr.Read(by,0,by.Length);
   FileStr.Close();
   return(System.Convert.ToBase64String(by));
  }

  /// <summary>
  /// SMTP回应代码哈希表
  /// </summary>
  private void SMTPCodeAdd()
  {
   //[RFC 821 4.2.1.]
   /*
     4.2.2.  NUMERIC ORDER LIST OF REPLY CODES

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