Contrary to popular belief, not even activation messages are a 100% correct way to validate an email address. Due to the many hilarities involved in delivering

Contrary to popular belief, not even activation messages are a 100% correct way to validate an email address. Due to the many hilarities involved in delivering an email, they can result in both false positives and false negatives. Sadly there is no perfect way to validate an email address. You can either optimise for false positives or false negatives, but perfect precision is beyond hope. Also note, the more validation you require, the more users you deter.