WP Job Manager: Send an email to employer when their job listing expires

functions.php

function listing_expired_send_email($post_id) {
   $post = get_post($post_id);
   $author = get_userdata($post->post_author);

   $message = "
      Hi ".$author->display_name.",
      Your listing, ".$post->post_title." has now expired: ".get_permalink( $post_id );
   wp_mail($author->user_email, "Your job listing has expired", $message);
}
add_action('expired_job_listing', 'listing_expired_send_email');