What happens if...

If I am using CAS and one of my friends is also using it, the first time he sends me an email, CAS sends a challenge (say he has a new email address). If I am not on his friends list yet, his CAS will send a challenge to me, and my CAS is still waiting for his response to my first challenge, so it will not pass through to me, and so on. I imagine that I might look at the pending list at some point and resolve the daisy chain, but it might take a while. I currently get only about 1% of the spam that is sent to me, and that seems to be diminishing. I don't want to begin creating hassles for my friends in order to improve on that result. Even without the daisy chain, I would think that the challenge email would be a bit rude to receive and for a business, it is a waste of time and resources to process. There has to be a better way.