And, it can be done on Fedora / CentOS in an easy manner.
The best tutorial is this, and a good ones are these and these.
Greylisting is very effective in reducing spam – almost 90% of servers contacting our mail system are illegitimate, and greylisting reduces the noise from them. Some of the best products is milter-greylist (it is still not aware that postfix is milter compatible). For postfix, there is a similar solution called postgrey.
Postgrey – this one I choose. Best tutorial I found for installing on my CentOS was here.
Milter Greylisting solutions: milter-greylist, milter-gris, Scam-grey milter.