I plan on using a write-back technique where the robot sends mail back to the sender, asking to validate the address by simply replying or clicking a URL.

So if someone who happens to have an ISP that's blacklisted by the ORB writes me a message entitled "URGENT FIX !!!" about using my shareware library to calculate low home mortgages, then it still won't be a Black Hole, but will delay to validate the sender has a proper return address and is a real person not a bulk mailer.