Do you ever send out mass marketing through the mail? Or have you ever received spam? They tell you not to click on any links in a spam email because it could expose you to harm. In the same way, I expect that replying to this threat would do more harm than good. CLEARLY, they don't have 10,000 lawyers working for them, and yet they've probably sent out that many notifications. So, clearly they cannot actually make good on all of those threats. I could be wrong, I suppose.
I would be careful sending any of these posters emails. Suppose they are working for the law firm and trying to get evidence against you. Do not underestimate the vileness of lawyers and their tactics.