Unacceptable email address Alert during portal account setup

Need an alert to inform users at setup that an email address is not acceptable for use with setting up their portal account. Currently when users enter their demographic information and email address at setup, after accepting the terms and conditions, the 'Next' button is not highlighted if the email address is not an accepted form of email address. Users do not get any indication why they cannot click 'Next' to continue on with portal setup. This creates confusion for users.