### 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
SETTING_DEFAULT_ACCOUNT,
            SETTING_PROFILE,
            SETTING_EMAIL_DEFAULTS,
            SETTING_SMTP_AUTH,
            SETTING_SMTP_HOST,
            SETTING_SMTP_PORT,
            SETTING_SMTP_STARTTLS_ENABLE,
            SETTING_SMTP_USER,
            SETTING_SMTP_STARTTLS_REQUIRED,
            SETTING_SMTP_TIMEOUT,
            SETTING_SMTP_CONNECTION_TIMEOUT,
            SETTING_SMTP_WRITE_TIMEOUT,
            SETTING_SMTP_LOCAL_ADDRESS,
            SETTING_SMTP_LOCAL_PORT,
            SETTING_SMTP_SEND_PARTIAL,
            SETTING_SMTP_WAIT_ON_QUIT,
            SETTING_SMTP_SSL_TRUST_ADDRESS
        );
    }
    private static List<Setting<?>> getSecureSettings() {
        return Arrays.asList(SETTING_SECURE_PASSWORD);
    }
    public static List<Setting<?>> getSettings() {
        List<Setting<?>> allSettings = new ArrayList<Setting<?>>(EmailService.getDynamicSettings());
        allSettings.addAll(EmailService.getSecureSettings());
        allSettings.addAll(SSL_SETTINGS.getEnabledSettings());
        return allSettings;
    }
}