If you want to make your parents happy, take a piece of paper and a pen and ask them what their concerns are about your boyfriend. Don't answer any of them on the spot, just write them down and say you'll think about it and see how you feel.
It may be really hard to avoid telling them your side, but it will give you some time to see what things they're saying are valid and what isn't. If you argue on the spot, you're much more likely to feel like you need to defend your boyfriend.
As for the email, I do understand both your dad's concern and your need for privacy. In the meantime, open an account somewhere else (e.g., yahoo, google, hotmail) and password protect it. Stay out of trouble so that your parents don't need to worry about you :-)