To enable RBL blocking, as root, run the following:
cd /etc/virtual rm -f use_rbl_domains ln -s domains use_rbl_domains
This will setup the list of domains to be filtered to match the active domains list, so new domains added to the server are also included for the spammer IP blocking.
The RBLs will connect out to remote servers for each email, check to see if the sending IP is a spammer, and prevent that email from arriving. These lists are regularly updated, so they are highly effective.
Note that some lists also block entire ISP IP ranges, so if your clients are not able to send email, then can switch to sending email through port 587 instead of port 25.
or to disable RBLs, you can do the followig:
cd /etc/virtual rm -f use_rbl_domains touch use_rbl_domains