### 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
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import javax.mail.MessagingException;
import javax.net.ssl.SSLSocketFactory;
import static org.elasticsearch.xpack.core.watcher.WatcherField.EMAIL_NOTIFICATION_SSL_PREFIX;
/**
 * A component to store email credentials and handle sending email notifications.
 */
public class EmailService extends NotificationService<Account> {
    private static final Setting<String> SETTING_DEFAULT_ACCOUNT = Setting.simpleString(
        "xpack.notification.email.default_account",
        Property.Dynamic,
        Property.NodeScope
    );
    private static final Setting.AffixSetting<String> SETTING_PROFILE = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "profile",
        (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<Settings> SETTING_EMAIL_DEFAULTS = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "email_defaults",
        (key) -> Setting.groupSetting(key + ".", Property.Dynamic, Property.NodeScope)
    );
    
    private static final Setting.AffixSetting<Boolean> SETTING_SMTP_AUTH = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.auth",
        (key) -> Setting.boolSetting(key, false, Property.Dynamic, Property.NodeScope)