You can use free and paid subscription blackhole lists with your server.
To switch on spam protection based on DNSBL:
Now, email messages from known spammers should be rejected with an error code 550 (connection refused).
Important: If you use the Qmail mail server and switch on DNSBL, senders with IP addresses from the blackhole list will not be able to send email even if they pass SMTP authentication. To avoid this problem, switch on the message submission as described in the section Configuring Server-Wide Mail Settings.