The default Shopify order CSV will export the email address entered by the customer when the order was placed.

However, if the customer registered for an account and changed the email address, this won't update the original email address associated with the order.

EZ Exporter can export both the original email address that was entered when the order was placed and also the customer's current email address as part of an order CSV export.

In the app, simply select the field customer.email to include this in your export.