While we don't outright ban free email addresses during sign up, we do generally trust our Maxmind plugin. As someone already mentioned, Maxmind scores proxies very high and free email addresses relatively high so pretty much any signup with a combination of those will be outright rejected before we even get to see the order.
I suppose it's possible that someone using an open proxy AND a free email address is a legitimate customer, but the odds are so far against it that we'd rather not even take the chance.
There is no shortage of customers on WHT and HD screaming that their host is not honest with them. Well, in my view, starting a relationship with me through a proxy and a free email address puts up some red flags for me that customer isn't being honest with me.
I'm sure we lose a customer or two because of this, but when I compare how well our servers are running now as to how well they were running the beginning when we weren't screening carefully, the trade off is well worth it.