Real-time Black Lists (also called Real-time Blackhole lists, or DNS-based Blackhole Lists), are lists of IP addresses, published through DNS, often of computers or networks linked to spamming or malware. Many corporate mail servers are configured to reject or flag messages which have been sent from IP addresses listed on one of these RBLs.
Most large-scale Email systems, like Gmail and Hotmail, also use RBLs to filter email for addresses on their system. If IP addresses from your network end up on an RBL, you and your customers can experience problems sending as well as receiving email, which can easily damage your business.
More information about RBLs can be found on Wikipedia.
The RBLTracker system automatically checks over 60 RBLs, multiple times per day, to see if any of your IP addresses are listed, giving you the peace of mind you need to focus on your business.