Thank you for your reply. I applied the Server Validation behavior to the wrong page. Applying it to the confirm page makes it work. I have set the redirect to the checkout page. Also I applied the Validation Show If behavior to the required fields on the checkout page.
There is a problem though. When I fill out the entire form, and on purpose put in the wrong email format, the entire information is wiped out, and all fields with the Validation Show If behavior show the error message.
Is there a way to retain the information, so that the customer does not have to fill out the form all over again, just because one field was left out?