I'd check the mail headers and see where it came from for real first off. If it is real, and even if it's not, your best bet is to just ignore it honestly. They either have something on you or they don't, and nothing you say is going to change that in your favor any. The only thing responding can do is provide them with further insight into your operation so they can stop anyone else from replicating it, or worse, use it against you in some sort of lawsuit.