One way to do this would be to replace your common thank you page with something else, so that buyers do not see the download link in their browser immediately after completing checkout.
They will still get a product specific email message from us that contains a link to where they can get the download, so this way only people with valid email addresses can get the product. If you do this, note that your Common Thank You Email which can be seen via Seller Admin > Edit Account Preferences will link back to your common thank you page so it won't help anyone get their download links, you may want to blank that out and use only the product emails. See our help page here for information on editing your email messages:
http://www.e-junkie.com/ej/help.custom.thankyou-email.htm
