you run the smtp-proxy... so mails are processed by the smtp-daemon and will be processed and did not come to the firewall-process..
You can try host/network blacklist under SMTP/Relaying but i think this will not work as expected.
Create a DNAT-Rule which DNATs the "Hackers" List to an IP-Adress which is not existent in your LAN.. then create an firewall-rule which drops that traffic.