### Description

When using the Watcher email action, it would be beneficial to be able to specify a list of allowed domains to which emails are allowed to be sent. This would probably be a non-dynamic setting, and allow specifying multiple domains.
x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailService.java
clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_CONNECTION_TIMEOUT, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_WRITE_TIMEOUT, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_SSL_TRUST_ADDRESS, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_LOCAL_ADDRESS, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_LOCAL_PORT, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_SEND_PARTIAL, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_WAIT_ON_QUIT, (s, o) -> {}, (s, o) -> {});
        
        reload(settings);
    }
    @Override
    protected Account createAccount(String name, Settings accountSettings) {
        Account.Config config = new Account.Config(name, accountSettings, getSmtpSslSocketFactory(), logger);
        return new Account(config, cryptoService, logger);
    }
    @Nullable
    private SSLSocketFactory getSmtpSslSocketFactory() {
        final SslConfiguration sslConfiguration = sslService.getSSLConfiguration(EMAIL_NOTIFICATION_SSL_PREFIX);
        if (sslConfiguration == null || sslConfiguration.explicitlyConfigured() == false) {
            return null;
        }
        return sslService.sslSocketFactory(sslConfiguration);
    }
    public EmailSent send(Email email, Authentication auth, Profile profile, String accountName) throws MessagingException {
        Account account = getAccount(accountName);
        if (account == null) {
            throw new IllegalArgumentException(
                "failed to send email with subject [" + email.subject() + "] via account [" + accountName + "]. account does not exist"
            );
        }