This is the idea behind hashcash[1], but instead of currency it's just a small proof of work. Some nominal PoW (a few seconds?) to bypass the spam filter smells like it would be remarkably effective at cutting down spam.
Captcha requires manual intervention, and potentially has the problem of being easy for bots but difficult for humans. A captcha for each email on even a small mailing list would be pretty horrible.
This is also what we do with bitmaelum[1]. Each message needs a proof of work for deliverance. Large (authentic) mailinglists can use an opt-in by a reader which allows the sender to bypass the pow restriction.
