The Open Relay Behaviour-modification System is a
database for tracking SMTP servers that have been confirmed to permit third-party relay. (The webpage: www.orbs.org has been shut down!!!)
These servers permit spammers to connect to them from anywhere in the world, usually
from a modem connection, and then forward the spam to its intended victims.
Another simlilar idea you can find under: http://maps.vix.com/rbl.
