C# – Error when trying to send mails using smtp

I have got the following error message when i tried to sent message using smtp.

A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 67.69.240.69:25
System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 67.69.240.69:25
   at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress)
   at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Int32 timeout, Exception& exception)
   --- End of inner exception stack trace ---
   at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6, Int32 timeout)
   at System.Net.PooledStream.Activate(Object owningObject, Boolean async, Int32 timeout, GeneralAsyncDelegate asyncCallback)
   at System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate asyncCallback)
   at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout)
   at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at Handler.BLL.cSendMail.SendMail(String p_strFrom, String p_strDisplayName, String p_strTo, String p_strSubject, String p_strMessage, String strFileName)
[ staffID:,21/08/2013 9:49:34 AM ]: status code : GeneralFailure

code

   public bool SendMail(string p_strFrom, string p_strDisplayName, string p_strTo, string p_strSubject, string p_strMessage , string strFileName)
 {
     try
     {
         p_strDisplayName = _DisplayName;
         string smtpserver = _SmtpServer;
         SmtpClient smtpClient = new SmtpClient();
         MailMessage message = new MailMessage();
         MailAddress fromAddress = new MailAddress(_From,_DisplayName);
         smtpClient.Host = _SmtpServer;
         smtpClient.Port = Convert.ToInt32(_Port);
         string strAuth_UserName = _UserName;
         string strAuth_Password = _Password;
         if (strAuth_UserName != null)
         {
             System.Net.NetworkCredential SMTPUserInfo = new System.Net.NetworkCredential(strAuth_UserName, strAuth_Password);
             smtpClient.UseDefaultCredentials = false;
             if (_SSL)
             {
                 smtpClient.EnableSsl = true;
             }
             smtpClient.Credentials = SMTPUserInfo;
         }
         message.From = fromAddress;

         message.Subject = p_strSubject;
         message.IsBodyHtml = true;
         message.Body = p_strMessage;
         message.To.Add(p_strTo);
         try
         {
             smtpClient.Send(message);
             Log.WriteSpecialLog("smtpClient mail sending first try success", "");
         }
          catch (Exception ee)
         {
             Log.WriteSpecialLog("smtpClient mail sending first try Failed : " + ee.ToString(), "");
             return false;
         }
         return true;
     }
     catch (Exception ex)
     {
         Log.WriteLog("smtpClient mail sending overall failed : " + ex.ToString());  
         return false;
     }
 }

when 3 or more mail sending only one or two fails