add_action('woocommerce_email_customer_details', 'send_customer_ip_adress', 10, 4);
function send_customer_ip_adress($order, $sent_to_admin, $plain_text, $email){


    // Just for admin new order notification
    if( 'new_order' == $email->id ){
        // WC3+ compatibility
        $order_id = method_exists( $order, 'get_id' ) ? $order->get_id() : $order->id;



        echo '<br><p><strong>Customer IP address:</strong> '. get_post_meta( $order_id, '_customer_ip_address', true ).'</p>';
    }
}