As a postscript to the above, it is with sadness and regret that I announce that Mailblocks will cease operations on 16th November. It seems the miserable gits at AOL (who bought Mailblocks) have dumped it. They are offering their AIM service as a replacement. This is a truly inferior product that lacks Mailblocks' challenge/response system and customisable trackers. I shall be asking for a refund, which is an option to paid users.
I have done some research into replacing this valuable service, and have run across the following.
Bluebottle --
www.bluebottle.com -- From Australia. Free, touts a challenge/response system but in my testing it didn't seem to work.
Mail-Block --
www.mail-block.com -- A paid challenge/response service, with a clunky interface. Not happy with it, will be requesting a refund.
Spam Arrest --
www.spamarrest.com -- the one I finally settled on. Includes not only challenge/response but trackers. They offer a 30-day free trial, but you must upgrade to a paid account before you can upload your address book into their approved senders list.