LinuxQuestions.org - Postfix "humancheck" or captcha-like thingies. Are they existing?

Hi!

A few years ago i saw a great idea for anti-spam solutions.
It was like:
if a@foo.com was sending mail to b@bar.com then
a immediately got an answer email from the server like:

"Dear a@foo.com!

You are not on b@bar.com's recipient list.
If you want to be on it, then CLICK ON THIS LINK, and your mail will be delivered instantly.
"

After you click on it, then a simple webpage appears (may should improved by captcha) which tells you, that you verified your address, and your recipient got your mail now.

I think with this human-interacting method, white-listing is a really easy and powerful, efficient way. Until you do not pass under a human test, your mail will be delayed or (without verification and an amount of timeout) abandoned.

My first questions is: do you agree? :)

My second: do you know such a solution for Postfix already existing?
If not, then i think, it would only consume a couple dozen lines to make it. Is that right?

I do not want to reinvent the wheel, maybe not needed also :P
Maybe there is a common name for this method, but i don't know even its name.

(When i see sender verification, I see that is all about really automatic stuff, domain check, rbl, etc.
Maybe i'm wrong, and such an easy "hey, if you are my buddy, click on this" solution may already exist)

Any ideas would be appriciated

thx