WTB: US backconnect/private proxies for mailing

Hello,

I'm looking for proxy providers who sell either backconnect USA rotating proxies that work with SMTP mailing or who can provide private bulk USA mailing proxies.

I already know about StormProxies, they work ok for me, but can't find any others. If they are just private IPs i need large quantities like 300 - 1000 IPs/month, but not priced astronomically, and if they are backconnect i need GEO option for USA.

Thank you