[jdfreefly]

I've worked on a couple of antispam solutions that did something akin to a more intelligent grey listing. Think quality of service meets IP reputation.

The problem with braindead simple things like postgrey is that they are incredibly easy to circumvent and the more attractive a target you are, the quicker the spammers will get around the solution. For instance, if one of the major providers (Yahoo, Hotmail, Verizon, Comcast) were to implement this, spammers would work around it within the hour. If you're low enough on their radar they won't care that they can't get email through to you but as soon as it starts to bother them enough to care, it stops being effective.