I'm sorry for the trouble this has is causing @ashleyleebeauty. I checked with the Marketing team and at this time there isn't a way to authenticate the emails collected on your embedded sign up form. But the team is definitely aware of this need and we really appreciate your feedback. 

Keep in mind that when you send a Marketing Campaign that system does keep any eye on emails that bounce back. If an email bounces back twice outside of a 14 day-window the email is removed. I hope this helps in the meantime, but if it's costing you too much you could consider removing the form and manually adding emails or switching to collecting emails from the Point of Sale app only.