From SecuryTeam:

For those not familiar with RBL, the term means Real-time Blackhole List, it is mainly used for SPAM fighting. I have recently started playing around with Google as an RBL engine, the idea is that if the search term I use hits too many hits it is likely to be SPAM

This is actually interesting, i've been googling urls found in spams for a little while now but those kind of search never return tons of pages, but it does return a few of them, and most of the time they are security related site. This "google rbl" could be pretty usefull coupled with a list of trusted security sites. If the search returns a few urls from those sites, then the mail is likely to be a spam or malware related. What about a mail with "" in the body? uhuh...definitely not bulletproof! So better stick to the IP of the sender and just count the number of hits returned by google.

from securiteam