So far I have worked 8 years on eMail Verifier. The software has evolved a lot because Internet, mail servers and spam filtering has evolved a lot as well. The idea with eMail Verifier is to identify
addresses that are bad for sure, reason why I use that 'Looks Valid' and 'Looks Bad' results. My principal objective is not marking as bad addresses that are actually good. That would be terrible.

In the case of <squibble@sillydomain.com> i get the following response from the server:

554 Your access to this mail system has been rejected due to spam or virus content. If you believe that this failure is in error, please submit an unblock request at http://unblock.secureserver.net

eMail Verifier only knows that the access has been rejected but nothing says the address is bad, perhaps it is, maybe not. As a result we mark it as 'Valid' or 'Looks Valid'. Let's says it is a security. eMail Verifier will NEVER mark an address as bad until it is 100% sure of it!