A real-time block list (RBL) is a method of stopping spammers from being able to send out large quantities of distasteful spam. A real-time block list is managed and maintained by an organization (company, non-profit, or volunteers) who track spam activity and create a list of known violators. Violations can include SMTP configurations to being caught sending spam. Once you are on their list, you can typically request removal. Some site will publish email addresses and all messages received to that email address is spam.
When a computer connects to your Exchange server, Exchange will query the specified real-time block list. If the address is on that list, Exchange will generate an error and refuse the message. The server that was trying to send the spam is then responsible to generate a non-delivery report and send it the sender. This will eventually lock up the sending server until their open relay is detected and resolved.
For a list of real-time block lists, please refer to the Wikipedia article:
The process is the same for Exchange 2008 and Exchange 2010.
To setup a RBL, open Exchange Management Console – the GUI, and under Organization Configuration select Hub Transport. Select the Anti-Spam tab and right click IP Block List Providers selecting Properties. You can add lists and set them up here.