In Shopify, you can customize the content displayed depending on whether the customer is logged in on the store, but you can also implement something similar in the "order confirmation" email sent after a purchase. Here is an example of a setting that determines whether the customer has a login account or not and displays different content in the email body.
Example of setting to prompt customers who do not have an account to register via "Order confirmation email"
[Settings] Settings > Notifications > Order confirmation > Edit code
