You can ask for confirmation if it’s an unusual construction, but denying it outright is user hostile. Also, how many users are actually manually typing their

You can ask for confirmation if it’s an unusual construction, but denying it outright is user hostile. Also, how many users are actually manually typing their e-mail address into a form, instead of using autofill? As the referenced page says, a valid email address is one that can receive email. If you really care about the e-mail address being correct, then you need to validate it by sending a verification e-mail.