Zombies, or hijacked computers, are all over the net now. These are compromised machines that have been taken over by Trojans and other malicious code and are used in botnets to do denial of service attacks and to fire out spam in huge quantities. The spam that's hitting your mailbox right now probably did not come from the sendmail server of some spammer - it probably came from another user's PC, out there on the net, its owners completely unaware that they are being used in this way.
The Federal Trade Commission, Department of Commerce, Department of Homeland Security and 33 other agencies from over 20 countries want this stopped. They have joined together to form "Operation Spam Zombies", a new initiative to help ISPs and users prevent computers from being compromised by spammers. Several tactics are to be employed, such as blocking port 25 that is used for outgoing e-mail, applying rate-limiting controls for e-mail relays, identifying potential zombie machines, and providing customers with trojan-removal tools.
"Computers around the globe have been hijacked to send unwanted e-mail," said Lydia Parnes, Director of the FTC's Bureau of Consumer Protection. "With our international partners, we're urging Internet Service Providers worldwide to step up their efforts to protect computer users from costly, annoying, and intrusive spam zombies."