Email greylisting
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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.