Your Intercom will work on any domain unless you specifically stated where you want it to be allowed.

One setting is within Messanger > Keep your Messenger secure where you can list trusted domains and Intercom will work just on them.

The other setting to use a custom domain when sending messages (and thus needs to be authenticated) is under Settings > Outbound > Addresses & domains, where you can add the custom address on your domain to be used for communication and there's domain authentication when you do so.

Is there anyway of removing addresses from this area in this area Settings > Email > Domains ?