On my servers, I host several domains. One of these domains was fiweb-com.net. Although I didn't used that domain often, besides of receiving spam (something I found out the last few weeks). At the end of this month, this domain will expire and I decided not to extend the registration.
So, I removed the entries out of the DNS-servers and Sendmail-config, and guess what... I reduced the amount of spam with 75%
You can see the reduction in week 01 in the next graph:
On a one-year overivew you will see a drop much clearer: