I had same issue, I use Private IP address range and NATing with Firewall.
The solution is pretty simple, assign your server second IP address and use Public IP address.
You private IP address will have gateway while second IP address ( public IP address) will not have gateway option.
The go to BINDINGS - IP address and assign ports to Public IP address , just like you would to private IP address.
Now, when you go to Security and Server Blacklist Check - your public IP address is being checked.
Trick is to assign second IP address to network card and use Public IP address.
This configuration will not interfere with firewall NAT or any other networking since there is no gateway configured for second IP address.
I am not sure what this does to system resources now that SmarterMail has to have open ports on two different IP addresses.
Make sure to setup event notification to sent you email when your Blacklist status changes - see help page: