Greylisting relies upon a specific IP address retrying deliveries but with large providers, the retries may come from different IP addresses. 

I therefore propose a configurable option that allows the server administrator to set the greylisting to either apply to the original IP (/32) or other lengths, perhaps /24 and /16. This will not cover scenarios where retries come from different blocks but it is likely to speed up delivery in many scenarios. Otherwise software we use applies similar logic successfully.