One method we use with clients is to state on the reg page: "Please complete the form below. The asset will be sent to the email address you provide." Then, on form submit, the lead is taken to a thank you page that reiterates that: "Thank you for your interest. The asset you requested has been sent to the email address you provided." And then, of course, you must have a Smart Campaign that delivers the fulfillment email with a link to the asset. This approach helps cut down on the number of bogus email addresses.