[mothered]

Only particular proxy servers that are commonly used for sending many authentication requests in short period of time

Can you use public proxies that are not In the IP range/list they're blocking?

Yeah but I can't really know which of them are blocked, as far as I've researched

I understand.

I'd say It's based on both commonly used proxies and an IP range. The only way to get around this (that I can think of), Is to Identify through trial and error- which can be a very lengthy process. Of course, private proxies would most likely suffice but they come at a cost.