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.

Greylisting may result in loss of legitimate mail if:

a) The transmitter's retry time is not within the minimum and maximum allowed by the greylisting receiver.

b) The retry comes from a different IP address, causing the receiver to not recognize it as a retry. Retry from a different IP is most likely with large organizations that use multiple transmitters.