Today I started using Spamhaus to help prevent spam, I use it now in addition to Spamcop.
It was triggered by many spam emails coming from the same range of IPs, so I tested it with Spamhaus and it blocked it.

Here is the code to add in Postfix main.cf file:
smtpd_client_restrictions =
permit_mynetworks
check_client_access hash:/usr/local/etc/postfix/client_access
reject_rbl_client sbl.spamhaus.org
reject_rbl_client bl.spamcop.net
permit