If you’re an email provider, try to reject unwanted mail during the SMTP exchange where possible. Once you accept a message for delivery, then decide that it shouldn’t be delivered, you are faced with the unenviable task of where to send the non-delivery report. You can’t rely on the validity of the “From:” and “Reply-to:” headers, and sometimes even the “Envelope Sender” or “Retrun path” details, because sender credentials are commonly forged in order to divert attention away from the real sender. A misdirected bounce only adds to the spam problem. Even the “Received:” headers can be tampered with.
Don’t be fooled by claims that you are obliged to pay for removal from one of the UCEPROTECT and Backscaterer lists. If you fix the problem that resulted in the listing, the removal will happen automatically. The payment is only required if you can’t wait a week (or more) after fixing the problem.