Every time an order is placed an account in Exact Online is being looked up based on either one of these conditions:
- The Shopify Customer ID
So in case no emailaddress is used, a new account is created based on the unique Shopify Customer ID. When a customer uses his or her emailadress the system tries to lookup the Account (type needs to be 'Customer') by the emailaddress.
So this connects existing accounts with Shopify as well.
Note that when an emailaddress changes, you need to change it as well manually in Exact Online in order to maintain the connection.