Your antispam settings can be found by logging in as the System Administrator. Click on the Settings icon and then select Antispam from the navigation pane. On the URIRBL Lists tab, you can click New to create these checks. In the modal window, the settings listed below will be available. I don't know the exact configuration to apply, but I took some guesses based on the URIRBLs showing in my test installation and the configuration details shown at the Spam Eating Monkey link, and here's an example.
Name - SEM-FRESH
Description - Domains registered in the last 5 days
Max Weight -
Hostname - fresh.spameatingmonkey.net
Required Lookup Values - 127.0.0.2
Enable Outgoing SMTP Blocking
For Outgoing SMTP Blocking to work, you need to also enable this functionality on Options tab of your Antispam settings. On the SMTP Blocking card, you'll find Outgoing Weight Threshold. Enable this and adjust the threshold as needed. Messages sent via SMTP will be rejected if the total weight for spam checks that have been enabled for Outgoing SMTP Blocking meet or exceed this threshold. Here's a KB explaining Outgoing SMTP Blocking in more detail: https://portal.smartertools.com/kb/a2661/configure-smtp-blocking-to-prevent-outgoing-spam.aspx
Scarab, care to make any corrections or suggest some weight values?