If a customer's invoice or paid receipt doesn't contain an email address, it means there won't be an email associated with that particular order.
You can conveniently check your orders desktop dashboard to identify orders without a customer email address.
If an email address was not imported, you will see the words "Add Email" displayed in red text.
To add an email address, simply click on the "Add Email" button.
This will prompt a notification box where you can either paste or manually type in the customer's email address.
Once you have entered the email address, click "OK," and it will be linked to both the order and the customer's information.