If you deliver more than a few emails to the outside world, especially if a good portion of those go to Yahoo, you may want to read this message:
http://marc.info/?l=postfix-users&m=127689518629249&w=2
Actually, read the whole thread, it's interesting and the discussion still continues:
http://marc.info/?t=127619611600001&r=1&w=2
TLDR: The 2.3.3 Postfix version that comes with RH / CentOS 5 doesn't do delivery rate control very well and has a habit of annoying some email providers. A simple upgrade from 2.3.3 to 2.7.0, no config changes, can improve delivery rate by 50% in some cases (e.g. if you deliver thousands of emails to Yahoo). With config changes, the improvement might be even bigger.
Fun fact: Postfix-2.3.3 has been released in August 2006. Think about that.