Prediction: it will fail on many email addresses that are out there in the wild and in daily use. The problem with 99% cases is that if you handle enough accounts that 1% becomes a serious tech support issue. In the end; the best test whether an email address is valid or not is if the mail arrives with the intended recipient. So what you do if an email address does not pass verification is prompt the user to that effect but do not force them to do anything at all.