I am trying to set the email address when have a new order. And I stored the new email in wp_postmeta. 
How to get the $order_id when using woocommerce_email_headers? 
I need to get the order_id to use it with get_post_meta() function.
Here is my code:
function techie_custom_wooemail_headers( $headers, $object) {
    $email = get_post_meta( $order_id, '_approver_email', true );
    // Replace the emails below to your desire email
    $emails = array('eee@hotmail.com', $email);
    switch($object) {
        case 'new_order':
            $headers .= 'Bcc: ' . implode(',', $emails) . "\r\n";
            break;
        case 'customer_processing_order':
            $headers .= 'Bcc: ' . implode(',', $emails) . "\r\n";
            break;
        case 'customer_completed_order':
        case 'customer_invoice':
            $headers .= 'Bcc: ' . implode(',', $emails) . "\r\n";
            break;
        default:
    }
    return $headers;
}
add_filter( 'woocommerce_email_headers', 'techie_custom_wooemail_headers', 10, 2);
How do I get back the data?
Thanks.