I am hosting on a residential IP where I am not blacklisted on anything - I can sign in to google on my normal browser and search google, but I cannot do that with the proxy.
When searching on google I get a "Our systems have detected unusual traffic from your computer network. This page checks to see if it's really you sending the requests, and not a robot" message. Clicking "I'm not a robot" makes it keep loading forever.
Additionally, pressing the "Sign in" button on google pages tells me that the application is not trusted, even though it works on this IP without a proxy.