### 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
new ClusterSettings(Settings.EMPTY, new HashSet<>(EmailService.getSettings()))
        );
        Account account1 = emailService.getAccount("account1");
        Properties properties1 = account1.getConfig().smtp.properties;
        assertThat(properties1, hasEntry("mail.smtp.starttls.required", "true"));
        assertThat(properties1, hasEntry("mail.smtp.connectiontimeout", "120000"));
        assertThat(properties1, hasEntry("mail.smtp.writetimeout", "120000"));
        assertThat(properties1, hasEntry("mail.smtp.timeout", "120000"));
        assertThat(properties1, not(hasKey("mail.smtp.sendpartial")));
        assertThat(properties1, not(hasKey("mail.smtp.waitonquit")));
        assertThat(properties1, not(hasKey("mail.smtp.localport")));
        Account account2 = emailService.getAccount("account2");
        Properties properties2 = account2.getConfig().smtp.properties;
        assertThat(properties2, hasEntry("mail.smtp.connectiontimeout", "60000"));
        assertThat(properties2, hasEntry("mail.smtp.writetimeout", "60000"));
        assertThat(properties2, hasEntry("mail.smtp.timeout", "60000"));
        Account account3 = emailService.getAccount("account3");
        Properties properties3 = account3.getConfig().smtp.properties;
        assertThat(properties3, hasEntry("mail.smtp.sendpartial", "true"));
        Account account4 = emailService.getAccount("account4");
        Properties properties4 = account4.getConfig().smtp.properties;
        assertThat(properties4, hasEntry("mail.smtp.localaddress", "localhost"));
        assertThat(properties4, hasEntry("mail.smtp.localport", "1025"));
        Account account5 = emailService.getAccount("account5");
        Properties properties5 = account5.getConfig().smtp.properties;
        assertThat(properties5, hasEntry("mail.smtp.quitwait", "true"));
        assertThat(properties5, hasEntry("mail.smtp.ssl.trust", "host1,host2,host3"));
    }
}