### 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
private EmailService service;
    private Account account;
    @Before
    public void init() throws Exception {
        account = mock(Account.class);
        service = new EmailService(
            Settings.builder().put("xpack.notification.email.account.account1.foo", "bar").build(),
            null,
            mock(SSLService.class),
            new ClusterSettings(Settings.EMPTY, new HashSet<>(EmailService.getSettings()))
        ) {
            @Override
            protected Account createAccount(String name, Settings accountSettings) {
                return account;
            }
        };
    }
    public void testSend() throws Exception {
        when(account.name()).thenReturn("account1");
        Email email = mock(Email.class);