protected boolean isAcceptableEmailAddress(final String emailAddress) {
        return EmailValidator.getInstance().isValid(emailAddress)
            && properties.getMail().isRegistrationEnabled()
            && RegexUtils.matches(properties.getMail().getAcceptedEmailPattern(), emailAddress);
    }
Token property to hold the email address that one wants to register.
support/cas-server-support-simple-mfa-core/src/main/java/org/apereo/cas/mfa/simple/web/flow/CasSimpleMultifactorVerifyEmailAction.java