I am using the below code to receive the message via serial port which is working fine. Now i need to send back a acknowledgment message to the sender machine. How can send the message ?

       private void MonitorSP_DataReceived(object sender, SerialDataReceivedEventArgs e)
    {
        try
        {
            System.IO.Ports.SerialPort SP = (System.IO.Ports.SerialPort)sender;

            //Get the ports available in system
            string[] theSerialPortNames = System.IO.Ports.SerialPort.GetPortNames();
            string strAvlPortNames = "";
            foreach (string s in theSerialPortNames)
            {
                strAvlPortNames += s.ToString() + ", ";
            }

            //Read an contruct the message
            Thread.Sleep(1000);
            string msg = SP.ReadExisting();
            string ConstructedMsg = "Port's Found : " + strAvlPortNames + "\n" + "Port Used : " + SP.PortName + "\n" + "Message Received : " + msg;

            if (InvokeRequired)
            {
                richTextBox1.Invoke(new MethodInvoker(delegate { richTextBox1.Text = ConstructedMsg; }));
                //Send acknowlegement to sender port
                SP.Write(SP.PortName);
                return;
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.StackTrace.ToString());
        }
    }