Technically there doesn't have to be live email validity checks on the sign up page- an email with the continue your signup with this link could be made available. OR an email saying you already have an account, do you need to reset your password?
If you aren't validating the email by sending an email that has to be replied to as part of the sign-up process (not after), then you are simply assuming that the email address was entered correctly without an opportunity for the person attempting the account sign up to correct it.