public function send($template, $email, $vars = [ ], $options = [ ], $schedule_time = null) {
        $post = [ ];
        $post['template'] = $template;
        $post['email'] = $email;
        $post['vars'] = $vars;
        $post['options'] = $options;
        if ($schedule_time) {
            $post['schedule_time'] = $schedule_time;
        }
        $result = $this->apiPost('send', $post);
        return $result;
    }