deb5_64 spamfiler blacklist rbl questions

[Turbanator]

On an old SME server I was running, we used to have a setting for Real Time Block Lists for spam (RBL).
Example:

sbl-xbl.spamhaus.org

The system would connect to those servers and block senderd who are on the list before the mail even gets to the scanners.

I know ISP3 has the Blacklist list control but what I haven't been able to find in the forums is
1- what the exact valid format is
2- is I can even use those black list servers or if the isp3 blacklist is NOT for blacklist servers but rather for single domains only.

And I suppose this could be asked as the postfix blacklist also.

fyi, my spam settings are faily high 1,3.2,10 but plenty of spam gets through without being tagged. And many messages come through without ANY SPAM-X tag which I'm still trying to figure out.

thank you.

EDIT:
Maybe this is actually a postfix question instead of spamassassin??