I don't think it's even an empty threat. He's worried that you'll mess with him. He's filed for a patent application on something similar to what you do, but w

I don't think it's even an empty threat. He's worried that you'll mess with him. He's filed for a patent application on something similar to what you do, but which does not in his opinion cover what you do (because he couldn't patent it if you did). He's sent you an email to specifically clarify that his patent application doesn't overlap with what you do, just to make sure you won't cause trouble for his application. Now, if he knew what he was doing he probably wouldn't have sent this email at all, because if you know what you're doing then you'll let your patent attorney handle all communication to do with your patent application. Anyway, I wouldn't worry about this guy. Unless you really want to go and look at the patent application, decide whether it does in fact clash with your prior art, and object to it on those grounds. But unless you're dying of free time or swimming in cash it's not worth the effort and/or expense.