Hi John M., as Arturo mentioned, we check to see if the website returns certain error codes or statuses. However, there are a couple of reasons why a website that you can access and reach from your browser may be throwing an error to our domain checker, but the most likely reason in your case (checked out your validation requests and almost all of the invalid websites are returning 403 - Forbidden errors to us) is that some websites have have bot-protection mechanisms like captchas which block automated scripts such as ours.