/// <summary>
 /// 发送邮件
 /// </summary>
 public void SendMail ()
 {
  // 创建TcpClient对象， 并建立连接
  TcpClient tcp = null;
  try
  {
  tcp = new TcpClient (server, port);
  }
  catch (Exception)
  {
  throw new Exception ("无法连接服务器");
  }

  ReadString (tcp.GetStream());//获取连接信息

  // 开始进行服务器认证
  // 如果状态码是250则表示操作成功
  if (!Command (tcp.GetStream(), "EHLO Localhost", "250"))
  throw new Exception ("登陆阶段失败");

  if (userName != "")
  {
  // 需要身份验证
  if (!Command (tcp.GetStream(), "AUTH LOGIN", "334"))
   throw new Exception ("身份验证阶段失败");
  string nameB64 = ToBase64 (userName); // 此处将username转换为Base64码
  if (!Command (tcp.GetStream(), nameB64, "334"))
   throw new Exception ("身份验证阶段失败");
  string passB64 = ToBase64 (password); // 此处将password转换为Base64码
  if (!Command (tcp.GetStream(), passB64, "235"))
   throw new Exception ("身份验证阶段失败");
  }

  // 准备发送
  WriteString (tcp.GetStream(), "mail From: " + from);
  WriteString (tcp.GetStream(), "rcpt to: " + to);
  WriteString (tcp.GetStream(), "data");

  // 发送邮件头
  WriteString (tcp.GetStream(), "Date: " + DateTime.Now); // 时间
  WriteString (tcp.GetStream(), "From: " + fromName + "<" + from + ">"); // 发件人
  WriteString (tcp.GetStream(), "Subject: " + subject); // 主题
  WriteString (tcp.GetStream(), "To:" + toName + "<" + to + ">"); // 收件人

  //邮件格式
  WriteString (tcp.GetStream(), "Content-Type: multipart/mixed; boundary="unique-boundary-1"");
  WriteString (tcp.GetStream(), "Reply-To:" + from); // 回复地址
  WriteString (tcp.GetStream(), "X-Priority:" + priority); // 优先级
  WriteString (tcp.GetStream(), "MIME-Version:1.0"); // MIME版本

  // 数据ID,随意
//  WriteString (tcp.GetStream(), "Message-Id: " + DateTime.Now.ToFileTime() + "security.com");
  WriteString (tcp.GetStream(), "Content-Transfer-Encoding:" + encoding); // 内容编码
  WriteString (tcp.GetStream(), "X-Mailer:JcPersonal.Utility.MailSender"); // 邮件发送者
  WriteString (tcp.GetStream(), "");

  WriteString (tcp.GetStream(), ToBase64 ("This is a multi-part message in MIME format."));
  WriteString (tcp.GetStream(), "");

  // 从此处开始进行分隔输入
  WriteString (tcp.GetStream(), "--unique-boundary-1");

  // 在此处定义第二个分隔符
  WriteString (tcp.GetStream(), "Content-Type: multipart/alternative;Boundary="unique-boundary-2"");
  WriteString (tcp.GetStream(), "");

  if(!isHtml)
  {
  // 文本信息
  WriteString (tcp.GetStream(), "--unique-boundary-2");
  WriteString (tcp.GetStream(), "Content-Type: text/plain;charset=" + languageEncoding);
  WriteString (tcp.GetStream(), "Content-Transfer-Encoding:" + encoding);
  WriteString (tcp.GetStream(), "");
  WriteString (tcp.GetStream(), body);
  WriteString (tcp.GetStream(), "");//一个部分写完之后就写如空信息，分段
  WriteString (tcp.GetStream(), "--unique-boundary-2--");//分隔符的结束符号，尾巴后面多了--
  WriteString (tcp.GetStream(), "");
  }
  else
  {
  //html信息
  WriteString (tcp.GetStream(), "--unique-boundary-2");
  WriteString (tcp.GetStream(), "Content-Type: text/html;charset=" + languageEncoding);
  WriteString (tcp.GetStream(), "Content-Transfer-Encoding:" + encoding);
  WriteString (tcp.GetStream(), "");
  WriteString (tcp.GetStream(), htmlBody);
  WriteString (tcp.GetStream(), "");
  WriteString (tcp.GetStream(), "--unique-boundary-2--");//分隔符的结束符号，尾巴后面多了--
  WriteString (tcp.GetStream(), "");
  }

  // 发送附件
  // 对文件列表做循环
  for (int i = 0; i < attachments.Count; i++)
  {
  WriteString (tcp.GetStream(), "--unique-boundary-1"); // 邮件内容分隔符
  WriteString (tcp.GetStream(), "Content-Type: application/octet-stream;name="" + ((AttachmentInfo)attachments).FileName + """); // 文件格式
  WriteString (tcp.GetStream(), "Content-Transfer-Encoding: base64"); // 内容的编码
  WriteString (tcp.GetStream(), "Content-Disposition:attachment;filename="" + ((AttachmentInfo)attachments).FileName + """); // 文件名
  WriteString (tcp.GetStream(), "");
  WriteString (tcp.GetStream(), ((AttachmentInfo)attachments).Bytes); // 写入文件的内容
  WriteString (tcp.GetStream(), "");
  }

  Command (tcp.GetStream(), ".", "250"); // 最后写完了，输入"."

  // 关闭连接
  tcp.Close ();
 }

 /// <summary>
 /// 向流中写入字符
 /// </summary>
 /// <param name="netStream">来自TcpClient的流</param>
 /// <param name="str">写入的字符</param>
 protected void WriteString (NetworkStream netStream, string str)
 {
  str = str + " "; // 加入换行符

  // 将命令行转化为byte[]
  byte[] bWrite = Encoding.GetEncoding(languageEncoding).GetBytes(str.ToCharArray());

  // 由于每次写入的数据大小是有限制的，那么我们将每次写入的数据长度定在７５个字节，一旦命令长度超过了７５，就分步写入。
  int start=0;
  int length=bWrite.Length;
  int page=0;
  int size=75;
  int count=size;
  try
  {
  if (length>75)
  {
   // 数据分页
   if ((length/size)*size<length)
   page=length/size+1;
   else
   page=length/size;
   for (int i=0;i<page;i++)
   {
   start=i*size;
   if (i==page-1)
    count=length-(i*size);
   netStream.Write(bWrite,start,count);// 将数据写入到服务器上
   }
  }
  else
   netStream.Write(bWrite,0,bWrite.Length);
  }
  catch(Exception)
  {
  // 忽略错误
  }
 }

 /// <summary>
 /// 从流中读取字符
 /// </summary>
 /// <param name="netStream">来自TcpClient的流</param>
 /// <returns>读取的字符</returns>
 protected string ReadString (NetworkStream netStream)
 {
  string sp = null;
  byte[] by = new byte[1024];
  int size = netStream.Read(by,0,by.Length);// 读取数据流
  if (size > 0)
  {
  sp = Encoding.Default.GetString(by);// 转化为String
  }
  return sp;
 }

 /// <summary>
 /// 发出命令并判断返回信息是否正确
 /// </summary>
 /// <param name="netStream">来自TcpClient的流</param>
 /// <param name="command">命令</param>
 /// <param name="state">正确的状态码</param>
 /// <returns>是否正确</returns>
 protected bool Command (NetworkStream netStream, string command, string state)
 {
  string sp=null;
  bool success=false;
  try
  {
  WriteString (netStream, command);// 写入命令
  sp = ReadString (netStream);// 接受返回信息
  if (sp.IndexOf(state) != -1)// 判断状态码是否正确
   success=true;
  }
  catch(Exception)
  {
  // 忽略错误
  }
  return success;
 }

 /// <summary>
 /// 字符串编码为Base64
 /// </summary>
 /// <param name="str">字符串</param>
 /// <returns>Base64编码的字符串</returns>
 protected string ToBase64 (string str)
 {
  try
  {
  byte[] by = Encoding.Default.GetBytes (str.ToCharArray());
  str = Convert.ToBase64String (by);
  }
  catch(Exception)
  {
  // 忽略错误
  }
  return str;
 }

 /// <summary>
 /// 附件信息
 /// </summary>
 public struct AttachmentInfo
 {
  /// <summary>
  /// 附件的文件名 [如果输入路径，则自动转换为文件名]
  /// </summary>
  public string FileName {
  get { return fileName; }
  set { fileName = Path.GetFileName(value); }
  } private string fileName;

  /// <summary>
  /// 附件的内容 [由经Base64编码的字节组成]
  /// </summary>
  public string Bytes {
  get { return bytes; }
  set { if (value != bytes) bytes = value; }
  } private string bytes;

  /// <summary>
  /// 从流中读取附件内容并构造
  /// </summary>
  /// <param name="ifileName">附件的文件名</param>
  /// <param name="stream">流</param>
  public AttachmentInfo (string ifileName, Stream stream)
  {
  fileName = Path.GetFileName (ifileName);
  byte[] by = new byte [stream.Length];
  stream.Read (by,0,(int)stream.Length); // 读取文件内容
  //格式转换
  bytes = Convert.ToBase64String (by); // 转化为base64编码
  }

  /// <summary>
  /// 按照给定的字节构造附件
  /// </summary>
  /// <param name="ifileName">附件的文件名</param>
  /// <param name="ibytes">附件的内容 [字节]</param>
  public AttachmentInfo (string ifileName, byte[] ibytes)
  {
  fileName = Path.GetFileName (ifileName);
  bytes = Convert.ToBase64String (ibytes); // 转化为base64编码
  }

  /// <summary>
  /// 从文件载入并构造
  /// </summary>
  /// <param name="path"></param>
  public AttachmentInfo (string path)
  {
  fileName = Path.GetFileName (path);
  FileStream file = new FileStream (path, FileMode.Open);
  byte[] by = new byte [file.Length];
  file.Read (by,0,(int)file.Length); // 读取文件内容
  //格式转换
  bytes = Convert.ToBase64String (by); // 转化为base64编码
  file.Close ();
  }
 }
}
}

--------------------------------------------------------------------------------

// 使用:

 MailSender ms = new MailSender ();
 ms.From = "jovencXrss.com";
 ms.To = "jovencpclala.net";
 ms.Subject = "Subject";
 ms.Body = "body text";
 ms.UserName = "########"; // 怎么能告诉你呢
 ms.Password = "********"; // 怎么能告诉你呢
 ms.Server = "smtp.tom.com";

 ms.Attachments.Add (new MailSender.AttachmentInfo ("D: est.txt"));

 Console.WriteLine ("mail sending...");
 try
 {
  ms.SendMail ();
  Console.WriteLine ("mail sended.");
 }
 catch (Exception e)
 {
  Console.WriteLine (e);
 }