This is known under various names, the most common one being "tagged message queuing".

And it sucks. Hard.

The potential for abuse notwithstanding, it is completely unworkable for people who often get legitimate mail from strangers. These are the people who need of a way to filter spam reliably the worst. These are the people who cannot viably use a traditional whitelist. And these are the people for whom tagged message queuing means a manifold increase in mailtraffic. Because much of their traffic comes from as yet unknown sources, nearly every legit mail they get will require four actual messages to be sent (mail, confirmation request, confirmation, confirmation accept notice).

If you want to kill your mailserver, tagged message queueing is the quickest and most reliable way to do so.

Not to mention it's automated mail sending which means it needs to be configured carefully. I've seen people's message queuers junk mailinglists repeatedly because they were too stupid to set it up right.

Of course it's also a giant pain in the bottocks for the legitimate senders of mail, but who cares, right?