I can't see how it's unethical, unless even looking at the data is unethical, and you'll have to convince me of that. I'd be more concerned about the ramifications than the ethics. The least bad thing that could happen here is winding up on some big ISP's blacklist because enough of their users marked this unsolicited mail as spam. Once that happens, good luck communicating with your customers.

I'm sure you can imagine the worst that could happen. Courtrooms are not happy fun places.