If you want to "solve" spam (it evolves and is a constant fight), messages need to be vetted and signed by a 3rd party that specializes in fighting spam.
Do they? It seems like you are talking about companies who decide what is spam and what not with "messages need to be vetted".
My definition of spam:
Spam is any kind of unwanted, unsolicited digital communication that gets sent out in bulk.
So adding some amount of "effort" to a message solves spam, since for the spammer, it will not make economically sense since he will lose more than gain with spam (think unsolicited advertisement which only works if sent out in bulk), no?