Hi Maarten,
Welcome to the SEBLOD forums.
To truly test the email, you need to use the regular Joomla validation process which forces the user to register and verify their email address. If you want to test that a field is indeed in an email format you can add an email validation rule to a field with SEBLOD, but they could use a fake email address if there is no registration process.
Once you have a valid user and email you could design it so that you must log in to activate the download page.
Hope that helps,
Randy