Great question and perfect time to plug my start up.
1%-4% is a common response rate. Anything higher is, well... I never heard of anything higher and especially, in the last few years, the percentage has gone down. Email is simply not effective. HBR, IDC, Forrester, and (I hforget the other) has a great white paper on this to support the numbers associated with email not being effective. And from my personal experience, I avoided email. Decision makers get +200 plus emails per day. And not to mention,