How many private proxies needed to hide IP in email headers?

I use EAR (and it leaks your IP in the email headers) as an autoresponder and will be sending like 400-500 replies a day back to my responses that I got from posting on CL. I just don't want my home ip to be shown in the gmail headers. So if I just get 1 private SOCK5 proxy (I think they go for $10/mo) would that work for me as long as the proxy is up? I will sending my replies through many gmail accounts.

Also is proxybonanza a good place to buy?