In order to check whether the customer opened the message with the code or file after the purchase, go to the transaction registry and find the item with information about the code sent:

Information "message open on [...] at [...]" will be displayed only when the message has been read by the Customer.

Note: sometimes it happens that older e-mail programs do not confirm message open, therefore the lack of this information in the registry does not mean that the message has not been read.