Unfortunately, it can happen that servers block us, especially when we send a lot of requests to them.
If you have an external link on your domain, we do follow it in order to check the status code of this external site and we make sure that you do not link to broken pages.
If this external site is denying the access, our crawler show header status codes 4xx or even 5xx depending on the response. In this case please check the external site manually. Often the link is technically fine but our crawler is not allowed to analyze it.