Many companies don’t realize it but they have compromised computers that are being used by spammers.
http://www.networkworld.com/news/2007/062007-companies-send-spam.html?fsrc=rss-spam
[Editor's Note (Valle): This is a huge liability issue for companies and they need to get their arms around it before some enterprising lawyers do. A good defense against this kind of threat is the IronPort S series that can scan outgoing emails for spam.]