Earlier today (okay, technically yesterday) I came across the concept of bogons [1], or IP (Internet Protocol) address not officially allocated for use. They even provide a current list of non-routed IP blocks [2]. Curious about the effect of using said list to block potential spam, I setup a test, consisting of 565,012 tuples (we've stepped up testing of the greylist daemon [3] over the past week) previously processed (I'm keeping some extensive logs here), added the 6,803 IP blocks not allocated, and let it rip.
An hour and a half later, I had my answer.
Of 565,012 tuples processed, only 6,117 came from non-allocated IP space.
It's a little over 1%.
I don't think it's worth adding the non-allocated IP space to the greylist daemon. Not that it makes the daemon run slower, it's just that an IP list of that size takes up quite a bit of memory due to the trie structure [4] used to store the table, and for such a small gain, I don't feel it's really worth it.
[1] http://www.completewhois.com/bogons/
[2] http://www.completewhois.com/bogons/data/bogons-cidr-all.txt