Exception in thread "main" java.lang.RuntimeException: javax.mail.MessagingException: Could not convert socket to TLS;
  nested exception is:
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
at SendEmail.main(SendEmail.java:64)

import java.util.Properties;
 import javax.mail.Message;
 import javax.mail.MessagingException;
 import javax.mail.PasswordAuthentication;
 import javax.mail.Session;
 import javax.mail.Transport;
 import javax.mail.internet.InternetAddress;
 import javax.mail.internet.MimeMessage;

 public class SendEmail {
 public static void main(String[] args) {
  // Recipient's email ID needs to be mentioned.
  String to = "myemail@gmail.com";

  // Sender's email ID needs to be mentioned
  String from = "javamailcoursework@gmail.com";
  final String username = "javamailcoursework@gmail.com";//change accordingly
  final String password = "password";//change accordingly

  // Assuming you are sending email through relay.jangosmtp.net
  String host = "smtp.gmail.com";

  Properties props = new Properties();
  props.put("mail.smtp.auth", "true");
  props.put("mail.smtp.starttls.enable", "true");
  props.put("mail.smtp.host", "smtp.gmail.com");
  props.put("mail.smtp.port", "587");

  // Get the Session object.
  Session session = Session.getInstance(props,
     new javax.mail.Authenticator() {
        protected PasswordAuthentication getPasswordAuthentication() {
           return new PasswordAuthentication(username, password);
   }
     });

  try {
   // Create a default MimeMessage object.
   Message message = new MimeMessage(session);

   // Set From: header field of the header.
   message.setFrom(new InternetAddress(from));

   // Set To: header field of the header.
   message.setRecipients(Message.RecipientType.TO,
           InternetAddress.parse(to));

   // Set Subject: header field
   message.setSubject("Testing Subject");

   // Now set the actual message
   message.setText("Hello, this is sample for to check send " +
    "email using JavaMailAPI ");

   // Send message
   Transport.send(message);

   System.out.println("Sent message successfully....");

  } catch (MessagingException e) {
     throw new RuntimeException(e);
  }