### 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
return send(email, auth, profile, account);
    }
    private EmailSent send(Email email, Authentication auth, Profile profile, Account account) throws MessagingException {
        assert account != null;
        try {
            email = account.send(email, auth, profile);
        } catch (MessagingException me) {
            throw new MessagingException(
                "failed to send email with subject [" + email.subject() + "] via account [" + account.name() + "]",
                me
            );
        }
        return new EmailSent(account.name(), email);
    }
    public static class EmailSent {
        private final String account;
        private final Email email;
        public EmailSent(String account, Email email) {
            this.account = account;
            this.email = email;
        }
        public String account() {
            return account;
        }
        public Email email() {
            return email;
        }
    }
    private static List<Setting<?>> getDynamicSettings() {
        return Arrays.asList(