VPOP3 Enterprise has an RBL checking feature which is configured in the VPOP3 Settings at Services → SMTP Server → SPAM Reduction → Realtime Blacklist Rules
If it does not seem to be working properly, there are a few reasons why the RBL feature may not work.
In the first three cases above, the server/device which is first receiving the incoming SMTP mail should do the RBL testing, because the RBL testing has to be done by the server/device which receives the mail from the last 'untrusted' server.
If you go to Settings → Diagnostics, and set 'Log Level' to 'Full Logging', then look in the VPOP3.LOG file after you have had an incoming SMTP message, you should see lines containing with “Test RBL Server <server> <address being looked up> FOUND|NOT FOUND”. Those will show what RBL checks VPOP3 is doing, and what the results were.