SendMailServlet example & server setup ?

I tried this example :

// import the JavaMail packages
import javax.mail.*;
import javax.mail.internet.*;

// import the servlet packages
import javax.servlet.*;
import javax.servlet.http.*;

// import misc classes that we need
import java.util.*;
import java.io.*;

public class SendMailServlet extends HttpServlet {
    String smtpServer;

    public void init(ServletConfig config) throws ServletException
    {
        super.init(config);

        // get the SMTP server from the servlet properties
        smtpServer = config.getInitParameter("smtpServer");
    }

    public void doPost(HttpServletRequest req, HttpServletResponse res)
                       throws ServletException, IOException
    {
        // get the message parameters from the HTML page
        String from = req.getParameter("from");
        String to = req.getParameter("to");
        String subject = req.getParameter("subject");
        String text = req.getParameter("text");

        PrintWriter out = res.getWriter();
        res.setContentType("text/html");

        try {
            // set the SMTP host property value
            Properties properties = System.getProperties();
            properties.put("smtp.mail.yahoo.com", smtpServer);

            // create a JavaMail session
            Session session = Session.getInstance(properties, null);

            // create a new MIME message
            MimeMessage message = new MimeMessage(session);

            // set the from address
            Address fromAddress = new InternetAddress(from);
            message.setFrom(fromAddress);

            // set the to address
            if (to != null) {
                Address[] toAddress = InternetAddress.parse(to);
                message.setRecipients(Message.RecipientType.TO, toAddress);
            }
            else
                throw new MessagingException("No \"To\" address specified");

            // set the subject
            message.setSubject(subject);

            // set the message body
            message.setText(text);

            // send the message
            Transport.send(message);

            out.println("Message sent successfully.");
        }
        catch (AddressException e) {
            out.println("Invalid e-mail address.<br>" + e.getMessage());
        }
        catch (SendFailedException e) {
            out.println("Send failed.<br>" + e.getMessage());
        }
        catch (MessagingException e) {
            out.println("Unexpected error.<br>" + e.getMessage());
        }
    }
}

and got this:

type Exception report

message

description The server encountered an internal error () that prevented it
from fulfilling this request.

exception

java.lang.NullPointerException
    java.util.Hashtable.put(Hashtable.java:396)
    SendMailServlet.doPost(SendMailServlet.java:40)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:362)
with : properties.put("smtp.mail.yahoo.com", smtpServer);in line
40.That is my first servlet so if you be so kind to give me a detailed
answer.