An RBL or Real-time Blacklist is an antispam mechanism consisting of a list of IP addresses or hostnames that are known spam sources. There are many RBL sources available across the internet, both free and commercial.
CanIt users have access to several different lists that are populated by the activity of our users and curated by our staff. You can find a description of these different lists here.
Users can also add their own external RBLs if they choose as described here.
If you are are trying to find out whether or not a certain IP address or hostname is currently listed on any of Roaring Penguin's RBLs we have a tool that will allow you to search our current lists. Note that these lists are constantly changing and so an address's status at the time of your check may not reflect it's status at the time a message comes in. For CanIt admins, the Administration->Search Logs tool will allow you to find out more information an why a message was blocked.
For more information on RBLs, including how to request removal from our lists, see this page.