Unfortunately, there's no surefire way to determine how many leads are unqualified without further investigation. However, emailing a lead list and asking for responses is a great way to start weeding out the unqualified ones. You can also use email deliverability software to help you gauge whether or not your email is getting through to potential customers. Finally, consider using a lead scoring system to help you prioritize and track leads so that you can focus your attention on the most promising prospects.