Unless it's some super obscure one, the email provider in itself won't cause suspensions, otherwise legit Russian users would lose their accounts too, because mail.ru is pretty popular there. It's more than likely the ips, which were used to create the accounts and the email provider is mere coincidence. It's just that those email addresses are the cheapest or easiest to create, so cheaper account providers use those mostly.
I also think that it's perfectly fine to use US proxies with accounts, which were created from non-US ips, if those ips are clean.
On buyaccs.com, the ips which are used to create the accounts are pretty mixed, they are from all over the globe and came with mail.ru email addresses, maybe you're talking about this providet.
But for instance i know for a fact, that Kastez sells PVAs, which are created from US ips, no email address is bundled with those though.
Btw you can easily check the ip, which was used to create an account:
https://twitter.com/settings/your_twitter_data and the login history too, all the ips are listed, which accessed the account. in the last 2 months.
