The following document was written after countless hours of research and experimentation, in the hope that you will not have to waste as much time in setting up DKIM on Postfix as we had to.
First of I didn't ask postfix to do NIS lookups.
I used to receive around 5,000-7,000 spams daily on angsuman [at] taragana [dot] com email which is publicly available on the internet.
I finally grew tired of email spams, having to delete several thousand spams a day while fishing out for valid emails is no fun.
We are tweaking our postfix servers to offer maximum spam protection upfront with helo checks, header checks, blacklist checks, RBL's and more.