Can you please tell me where the email preview data comes from and if it is possible to change it? The preview data I currently see does not contain all the registration situations I want to validate. Thanks.
The preview data cannot be changed. What you can do instead is copy an event (or use an existing event) and do a test registration. Then you can re-send an email for that registration via the Event Espresso > Registrations page so you can validate the contents of the email for that test registration.