Simple HTML

<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>

****************************************************************

Servlet

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");
    }
}
************************************************************************
Java Class

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();

     /* Specifies the IP address of your default mail server
     	   for e.g if you are using gmail server as an email sever
           you will pass smtp.gmail.com as value of mail.smtp host.
           As shown here in the code.
           Change accordingly, if your email id is not a gmail id
        */
     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
   {

 	/* Create an instance of MimeMessage,
 	      it accept MIME types and headers
 	   */

    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 class is used to deliver the message to the recipients */

       Transport.send(message);


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

web.xml file

<?xml version="1.0" encoding="UTF-8"?>

<web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">

    <servlet>
        <servlet-name>mail</servlet-name>
        <servlet-class>MailApp</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>mail</servlet-name>
        <url-pattern>/mail</url-pattern>
    </servlet-mapping>

    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>

</web-app>