### 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/test/java/org/elasticsearch/xpack/watcher/notification/email/EmailServiceTests.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.watcher.notification.email;
import org.elasticsearch.common.settings.ClusterSettings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.xpack.core.ssl.SSLService;
import org.elasticsearch.xpack.core.watcher.common.secret.Secret;
import org.junit.Before;
import java.util.HashSet;
import java.util.Properties;
import static org.hamcrest.Matchers.hasEntry;
import static org.hamcrest.Matchers.hasKey;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.not;
import static org.hamcrest.Matchers.notNullValue;
import static org.hamcrest.Matchers.sameInstance;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;
public class EmailServiceTests extends ESTestCase {