### 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
Authentication auth = new Authentication("user", new Secret("passwd".toCharArray()));
        Profile profile = randomFrom(Profile.values());
        when(account.send(email, auth, profile)).thenReturn(email);
        EmailService.EmailSent sent = service.send(email, auth, profile, "account1");
        verify(account).send(email, auth, profile);
        assertThat(sent, notNullValue());
        assertThat(sent.email(), sameInstance(email));
        assertThat(sent.account(), is("account1"));
    }
    public void testAccountSmtpPropertyConfiguration() {
        Settings settings = Settings.builder()
            .put("xpack.notification.email.account.account1.smtp.host", "localhost")
            .put("xpack.notification.email.account.account1.smtp.starttls.required", "true")
            .put("xpack.notification.email.account.account2.smtp.host", "localhost")
            .put("xpack.notification.email.account.account2.smtp.connection_timeout", "1m")
            .put("xpack.notification.email.account.account2.smtp.timeout", "1m")
            .put("xpack.notification.email.account.account2.smtp.write_timeout", "1m")
            .put("xpack.notification.email.account.account3.smtp.host", "localhost")
            .put("xpack.notification.email.account.account3.smtp.send_partial", true)
            .put("xpack.notification.email.account.account4.smtp.host", "localhost")
            .put("xpack.notification.email.account.account4.smtp.local_address", "localhost")
            .put("xpack.notification.email.account.account4.smtp.local_port", "1025")
            .put("xpack.notification.email.account.account5.smtp.host", "localhost")
            .put("xpack.notification.email.account.account5.smtp.wait_on_quit", true)
            .put("xpack.notification.email.account.account5.smtp.ssl.trust", "host1,host2,host3")
            .build();
        EmailService emailService = new EmailService(
            settings,
            null,
            mock(SSLService.class),