[mmccaff]

Yes, this. Processing fraudulent cards can result in a costly sum of chargebacks for the seller, and a fake email address is a very strong indicator that the person is a fraudster. Other services like Maxmind minFraud consider the email address and many other attributes when ranking the likelihood that the order is coming from a fraudster. It doesn't mean that you have to disallow the order from being saved (or whatever), just that you might want to flag it for manual review before processing the CC to capture payment.