RBL stand for Realtime Blackhole List, a list of IP addresses which spread or refuse to stop the proliferation of spam.

Multiple RBLCheck answers you about whether an IP is listed in specific RBL or not. There are total over 70+ RBL lists compared. The list is still increasing.

The backend of this function is a C program, rblcheck v1.5, by Edward S. Marshall at 2001. The frontend is a simple PHP program which you can download it from here.

Check whether an IP is listed in RBL or not: