This value can only be available if IsIPAddressInRbl is true (the IP address is blacklisted). If the IP is blacklisted, this means the corresponding A record does exist for the given IP address. This property returns the contents of this record (in the form of an IPv4 address, usually something like "127.0.0.3", etc).
The exact meaning of these records depends on each particular RBL. Refer to your RBL documentation to find out more. For instance, return codes of the popular Spamhaus Zen RBL can be found at https://www.spamhaus.org/zen/.
|The IP address string returned by this property does not denote any real IP address. It's just the requirement that A records must represent IP addresses. So treat them just like error codes.|