### 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
);
    private static final Setting.AffixSetting<Boolean> SETTING_SMTP_STARTTLS_ENABLE = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.starttls.enable",
        (key) -> Setting.boolSetting(key, false, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<Boolean> SETTING_SMTP_STARTTLS_REQUIRED = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.starttls.required",
        (key) -> Setting.boolSetting(key, false, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<String> SETTING_SMTP_HOST = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.host",
        (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<Integer> SETTING_SMTP_PORT = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.port",
        (key) -> Setting.intSetting(key, 587, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<String> SETTING_SMTP_USER = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.user",
        (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<SecureString> SETTING_SECURE_PASSWORD = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.secure_password",
        (key) -> SecureSetting.secureString(key, null)