The beauty of business cards and webpages.
I have neither. But let's say that I did have a web page -
what's to prevent the key from being reaped by the spam
service industry?
Given the immense volume of legitimate email that crosses
the net every day, any solution to the spam problem has to
scale. Yours doesn't. What you're proposing is basically a
semi-P-K-no-I, where you've got a great big bootstrapping
problem with the credentials. I really don't think the spam
problem is one that can be solved cryptographically, anyway.
Melinda