Comment on A ‘frighteningly ambitious’ way to improve emailparentComments−emillon13yGraylisting is more or less this, but instead of a human receiving a challenge, the outbound server receives a "please try again later" (which is enough to stop a lot of spam already).
Comments
Graylisting is more or less this, but instead of a human receiving a challenge, the outbound server receives a "please try again later" (which is enough to stop a lot of spam already).