Tachyon said:
As a result the challenge response/whitelist passthrough is probably the way it will end up in the medium term. Then of course the spammers will implement respond bots and the cycle will continue.

But the beauty of that is that they can no longer hide their mail address. It has to be valid. Then you can blacklist it. Setting up numerous real respondbots is much more onerous than just formulating fake return addresses.

The thing that gets me is: what are they thinking? If someone is trying to filter out their offers, how likely is it that that person will decide to become a customer when their efforts are thwarted?

Earthlink has a rather ingenious system: they set up some fake accounts expressly to attract spam. When those accounts receive it, they analyze it and filter it out of clients' mailboxes. It works very well. In addition to that, there is whitelisting.