Email greylisting

Greylisting involves returning a temporary reject on the theory that only legitimate transmitters will retry after a temporary failure.

Greylisting is controversial as to its long-term effectiveness. If enough spam transmitters add retry capability, greylisting will be like a partially-effective anti-biotic. The pathogen population will mutate to a more resistant form.

On the other hand, a delay will always give more time for new sources to be blacklisted.