gistfile1.txt

add_filter('wp_mail', 'handle_wp_mail');
function handle_wp_mail($args) {
  if ($args['subject'] == 'Welcome to YOUR SITE') {
    $newMessage = <<<EOT
<h2>Welcome to YOUR SITE</h2>
<p>Dear User,</p>
<p>Thanks for registering!</p>
EOT;

    $newMail = [
      'to' => $args['to'],
      'subject' => $args['subject'],
      'headers' => $args['headers'],
      'attachments' => $args['attachments'],
      'message' => $newMessage,
    ];
    
    return $newMail;
  }
}