I think the even bigger problem is the "Open Relay" issue. That's basically where you don't have any authentication set up on your SMTP server, so just anyone can spam through your own SMTP server.
I made this mistake a few months ago, not knowing enought about how e-mail servers work.
Learn more...
Over 2 millions e-mails were sent out over the course of 4 1/2 days. Obviously, I worked for a day to rid of us on blacklists. If you've found yourself subject to this,
go here for some blacklist tools.