When we return an email, we verify it to ensure the email is recognized. It works 95% of the time but in some cases, the mail server tries to trick us and returns a false positive. This tells us it exists when it doesn't. In that specific case, we believe the mail is 100% valid but in reality, it's not, and that's why you get a bounce back. When that occurs, feel free to send us a list of invalid emails and we'll re-credit your account, as we believe we failed at giving you a correct email, so you shouldn't have to pay for it!