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.
Comments
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.