This anti-spam tactic works by checking the IP address of the client attempting to complete the form against a database of IP addresses of known spammers - if found in the list, the form submission will be rejected.
To use this service, you need to register with an RBL checker. Some offer a low-level free service where you can upgrade if you need a faster or more in-depth service. The RBL checker will be kept up to date by users reporting suspected spam from IP addresses anywhere in the world.
Every time your form is completed, your server will pass the client IP address to the RBL checker. If the IP address is found, you know that this form submission is very likely to be spam.
Pros:
Once installed, it works quietly in the background and does not affect the user experience when completing the form.
Cons:
The speed performance may be affected as the connection to the RBL checker may take several seconds to return with a response.