In a campaign called Secure Your Server, the FTC and others sent E-mail warnings to operators of computers that might be improperly configured to permit outsiders to route spam E-mail through them.
Spammers pitching prescription drugs, pornography and cheap loans use these misconfigured computers to disguise their identity and origin. Most such computers are inside corporations; few are located in homes.
"Recipients may think the spam comes from your system," the FTC said in its E-mail. "Securing your server will help you protect your system from being misused."
The FTC identified the misconfigured computers with data published by leading anti-spam activists. FTC experts did not scan the Internet themselves for such computers.
The agency did not attempt to verify that each computer targeted by the warnings actually was vulnerable to forwarding unwanted E-mails, said Don Blumenthal, the coordinator for the FTC's Internet lab. It urged operators of these computers to visit the FTC's Web site for more information and advice on properly configuring their software.