Milter-greylist

Teergrube (German for "tarpit") is a system that slows down spammer
machines by taking a long time to respond to SMTP.  If enough machines
are keeping spammer systems (whether their own or zombies) tied up
talking to teergrubes, it slows down the rate at which they can spam
people, increases the chance that they'll be noticed on the machines
they're sending from, and makes it easier to track them down.  <p>

Does it make sense to add a lightweight version of teergrube to
greylisting?  You could do something like sleep(10) the first time a
tuple shows up, and not do the sleep() if the tuple comes back later
or is whitelisted.