### 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
);
    private static final Setting.AffixSetting<Boolean> SETTING_SMTP_SEND_PARTIAL = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.send_partial",
        (key) -> Setting.boolSetting(key, false, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<Boolean> SETTING_SMTP_WAIT_ON_QUIT = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.wait_on_quit",
        (key) -> Setting.boolSetting(key, true, Property.Dynamic, Property.NodeScope)
    );
    private static final SSLConfigurationSettings SSL_SETTINGS = SSLConfigurationSettings.withPrefix(EMAIL_NOTIFICATION_SSL_PREFIX, true);
    private static final Logger logger = LogManager.getLogger(EmailService.class);
    private final CryptoService cryptoService;
    private final SSLService sslService;
    public EmailService(Settings settings, @Nullable CryptoService cryptoService, SSLService sslService, ClusterSettings clusterSettings) {
        super("email", settings, clusterSettings, EmailService.getDynamicSettings(), EmailService.getSecureSettings());
        this.cryptoService = cryptoService;
        this.sslService = sslService;
        
        clusterSettings.addSettingsUpdateConsumer(SETTING_DEFAULT_ACCOUNT, (s) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_PROFILE, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_EMAIL_DEFAULTS, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_AUTH, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_STARTTLS_ENABLE, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_STARTTLS_REQUIRED, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_HOST, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_PORT, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_USER, (s, o) -> {}, (s, o) -> {});
        clusterSettings.addAffixUpdateConsumer(SETTING_SMTP_TIMEOUT, (s, o) -> {}, (s, o) -> {});