8 years ago when I built and put on the net my first web server. I learned the hard way what could happen if I did not have a secure system. One of the lessons I learned was when my ISP handed me a bill for excessive bandwidth utilization that did not match the type of traffic we were generating. Turns out my SMTP server was mis-configured and was acting as a open relay. It had become so popular that after we locked it down traffic hit it for several months trying to bounce mail through.
Well the feds are sending letters to system admins and domain owners who have open relays. In an attempt to close some of the doors that spammers are using. [Cnet]