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

function listing_expired_send_email( $new_status, $old_status, $post ) {
    if ( 'job_listing' !== $post->post_type || 'expired' !== $new_status || $old_status === $new_status ) {
        return;
    }
    $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( 'transition_post_status', 'listing_expired_send_email', 10, 3 );