private static void putEmailRecipients(final RequestContext requestContext, final Map<String, CandidateRecipientAddress> validAddresses) {
        requestContext.getFlowScope().put(FLOW_SCOPE_ATTR_EMAIL_RECIPIENTS, validAddresses);
    }
Event attribute to allow registration of email.
support/cas-server-support-simple-mfa-core/src/main/java/org/apereo/cas/mfa/simple/web/flow/CasSimpleMultifactorSendTokenAction.java