The classic problem associated with Sender-Challenge-Response systems such as you describe for your anti-spam product is that it may reject automated mails from websites such as Store Purchases, Air Travel Bookings, Hotel Bookings and even the Comodo website etc. These websites are unable to respond to your automated challenge and so cannot authenticate themselves.
Furthermore it is usually the case that the user does not know what email address the automatic website will use and so the user cannot pre-authorise that address.
How does your AntiSpam product deal with this situation? I can find no mention of it in the sales blurbs.
(:KWL)
Bill