Yeah a lot of greylisting is naive and uses "sender", "recipient", and "IP" to discover repeats.
If you're going to implement greylisting yourself it makes more sent to hash the sender+recipient pair, allowing IP addresses of the senders to change - which is what will happen with many of the bigger mail-hosts, gmail, hotmail, etc.
If you're going to implement greylisting yourself it makes more sent to hash the sender+recipient pair, allowing IP addresses of the senders to change - which is what will happen with many of the bigger mail-hosts, gmail, hotmail, etc.