I already have something like this with my ISP, they called it the 'Friends' system.

Basically, you setup rules that state messages with a suspected spam level are held on the mail server, while an automated email is sent to the sender asking them to click a link which proves their a human being (Bots don't click them, so that eliminates almost all spam). Once the recipient clicks the link, they are added to the safe senders list and any held messages from them are automatically added to the queue to be downloaded.

The system works great, and in the 4 years I've had it, I've never had a single spam email, plus it's easy to check held messages to manually allow if the sender is too lazy, but often they aren't. It also sends a log every month that states any held messages, so you don't have to check for held messages, it tells you when it happens, which is also great.