I might have misunderstood you, but brute force what exactly? Sure, you could use your GPU to generate hashes for 500m email addresses / phones (if they weren'

I might have misunderstood you, but brute force what exactly? Sure, you could use your GPU to generate hashes for 500m email addresses / phones (if they weren't already pre-generated) but then what? You'd need to then run a DNS check against a target domain – this would introduce 15-300ms DNS check overhead, per hash and you'd get blocked in seconds by your ISP / DNS resolver. Even if you managed it, it would only reveal an association between a domain and email address, it wouldn't brute force anything. You'd still need to prove control over that email/phone. You can even remove the chance of an association being revealed through salting but that introduces a dependency, more info on the website.