function sendNotificationEmail(email) {
  const mailOptions = {
    from: '"Firebase Database Quickstart" <noreply@firebase.com>',
    to: email,
    subject: 'New star!',
    text: 'One of your posts has received a new star!'
  };
  return mailTransport.sendMail(mailOptions).then(function() {
    console.log('New star email notification sent to: ' + email);
  });
}
Send the new star notification email to the given email.