Anti-spam systems based on challenge-response are fundamentally broken in several ways. You should not be using them.
I get many challenges each week. The only ones that I respond to are the ones that are the result of spam with forged headers-- if you spam me with your challenge for something that I never sent, then I'm going to make damned certain that you get the original spam. It only seems fair.
But that's not why I'm writing today.
I'm writing because you, Mr. Moron, sent me email inquiring about a particular function of FIBS (my backgammon server). When I sent you a polite answer to your inquiry, the hounds that guard your inbox responded by sending me a challenge. I declined to jump through those additional hoops, and you will therefore not get an answer to your question.
If you send mail to someone and expect them to respond, you should ensure that they will be able to respond easily via normal channels. It's up to you to whitelist people when you send them mail; don't expect them to do it for you.