SmarterMail just uses DNS, querying the DNS Servers you set in SETTINGS > GENERAL SETTINGS. If you were using your own DNS Servers and the hostname of your RBLDSD server is pointed to the Internal IP then you can simply use RB.LOCALRBL for the Hostname in your SmarterMail Antispam checks.
Since you are using Public DNS Servers and you don't have this domain in your DNS then alternately you can add an entry to your HOSTS file (located at %Windir%\System32\drivers\etc on the server running SmarterMail) that points to the local IP. For example, if RB.LOCALRBL is at 192.168.1.100 then you would add the following line:
192.168.1.100 rb.localrbl #local RBL Server
Then, any attempts to connect to RB.LOCALRBL will circumvent DNS entirely and go directly to the internal IP Address you have mapped in your Hosts file!