In order to use RBL lists on your postfix setup to block spam replace this line in main.cf:

smtpd_sender_restrictions = check_sender_access mysql:/etc/postfix/mysql-virtual_sender.cf

with:

smtpd_sender_restrictions = reject_rbl_client sbl-xbl.spamhaus.org, reject_rbl_client cbl.abuseat.org, reject_non_fqdn_sender, reject_unknown_sender_domain, check_sender_access mysql:/etc/postfix/mysql-virtual_sender.cf

You can copy and paste the line with any ISPConfig setup. If you don’t use mysql to authenticate your users then you might need to adapt parts of this setting.