interesting idea, I know it would work for some people like my Grandparents that get sick of loads of mail when they only want it from a few people in their social groups in person and family.
I have found that folks in my own family, and even some hosting customers, who are not particularly tech savy, are the most likely to be taken advantage of, through spam. They will click on it, become infected with a malware spammer, and the work-load and headaches increase for all of us. Security is compromised. More spam is generated. TOS is violated. And the problem gets worse.
The current work around I am using requires the end user to submit the address they wish to whitelist. I then take it and add it to their smtp layer whitelist. It translates into added labor for me, so I have to charge more for it...... If this step could be eliminated, so the end user could add to their smtp layer whitelist themselves, we might have a very marketable, and desirable configuration availble, that increases security, reduces spam and the associated traffic, and is not offered elsewhere.
Obviously the submittal form would have to be secured... Perhaps something along the lines of the password changing form.... I don't know...
Thanks for taking me seriously on this, Stephen.