HTML FILE
*****************************************************
<form action="mail" method="post">
  To:<input type="text" name="to"  /><br/>
  Subject:<input type="text" name="subject"  /><br/>
  Message:<input type="text" name="message"  /><br/>
  Your Email id:<input type="text" name="user" ><br/>

  <input type="submit" value="send" />
</form>
*******************************************************
JAVA CLASS

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.mail.*;
import javax.mail.internet.*;

public class MailApp extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        String to = request.getParameter("to");
        String subject = request.getParameter("subject");
        String message =  request.getParameter("message");
        String user = request.getParameter("user");
        String pass = request.getParameter("pass");
        SendMail.send(to,subject, message, user, pass);
        out.println("Mail send successfully");
    }
}
********************************************************************
SERVLET

import java.io.*;
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;

public class SendMail
{
    public static void send(String to, String sub,
                         String msg, final String user,final String pass)
    {
     //create an instance of Properties Class
     Properties props = new Properties();

     props.put("mail.smtp.host", "smtp.gmail.com");
     //below mentioned mail.smtp.port is optional
    // props.put("mail.smtp.port", "587");
     props.put("mail.smtp.auth", "true");
     props.put("mail.smtp.starttls.enable", "true");

     /* Pass Properties object(props) and Authenticator object
           for authentication to Session instance
        */

    Session session = Session.getInstance(props,new javax.mail.Authenticator()
    {
  	  protected PasswordAuthentication getPasswordAuthentication()
  	  {
  	 	 return new PasswordAuthentication(user,pass);
  	  }
   });

   try
   {

    MimeMessage message = new MimeMessage(session);
       message.setFrom(new InternetAddress(user));
       message.addRecipient(Message.RecipientType.TO,new InternetAddress(to));
       message.setSubject(sub);
       message.setText(msg);

       Transport.send(message);


    }
    catch(Exception e)
    {
    	 e.printStackTrace();
    }
  }
}