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.

• if you don't get your mail via incoming SMTP, then RBL can't work
• if you get your incoming mail feed via a third party, eg your ISP or a mail filtering company, then RBL can't work
• if you get your incoming mail feed via a proxy server, eg a security appliance, then RBL can't work
• if the VPOP3 PC isn't configured to use a working DNS server, then RBL can't 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.