Here is how to set a whitelist in Postfix so that you can whitelist specific IP's against being checked in your RBL's:
http://www.howtoforge.com/how-to-whitel ... in-postfix
I had to use it to whitelist a Xerox scanner at a school that sends you what you scanned in your email. It somehow ended up on zen.spamhaus.org. It works beautifully.
Other than that, I will reiterate what others have said, SpamAssassin and RBL's are not going to be perfect. The only false positive I have ever had using SpamAssassin + SpamHaus is that Xerox scanner, which probably wasn't a "false" positive to begin with.
It sounds like you are also still learning quite a bit about how to properly configure a mail server and its spam scanners and RBL's. I would recommend perhaps gutting your server and installing Virtualmin from scratch (
http://www.virtualmin.com/). It will properly configure Postfix + SpamAssassin + ClamD for you, and give you a nice GUI interface to be able to whitelist different addresses and network ranges if you experience false positives.
In short, it will make your life easier.