The use of botnets to send spam would not matter as much if e-mail filters could still make effective use of the second spam-fighting strategy: analyzing the content of an incoming message. Traditional antispam software examines the words in a text message and, using statistical techniques, determines if the words are more likely to make up a legitimate message or a piece of spam.

The explosion of image spam this year has largely thwarted that approach. Spammers have used images in their messages for years, in most cases to offer a peek at a pornographic Web site, or to illustrate the effectiveness of their miracle drugs. But as more of their text-based messages started being blocked, spammers searched for new methods and realized that putting their words inside the image could frustrate text filtering. The use of other people’s computers to send their bandwidth-hogging e-mail made the tactic practical.

“They moved their message into our blind spot,” said Paul Judge, chief technology officer of Secure Computing.

Antispam firms spotted the skyrocketing amount of image spam this summer. A technology arms race ensued. The filtering companies adopted an approach called optical character recognition, which scans the images in an e-mail and tries to recognize any letters or words. Spammers responded in turn by littering their images with speckles, polka dots and background bouquets of color, which mean nothing to human eyes but trip up the computer scanners.

Spammers have also figured out ways to elude another common antispam technique: identifying and blocking multiple copies of the same message. Pioneering antispam companies like the San Francisco-based Brightmail, which was bought two years ago year by the software giant Symantec, achieved early victories against spam by recognizing unwanted e-mail as soon as it hit the Internet, noting its “fingerprint” and stopping every subsequent copy. Spammers have defied that technique by writing software that automatically changes a few pixels in each image.

“Imagine an archvillain who has a new thumbprint every time he puts his thumb down,” said Patrick Peterson, vice president for technology at Ironport. “They have taken away so many of the hooks we can use to look for spam.”

But don’t spammers still have to link to the incriminating Web sites where they sell their disreputable wares? Well, not anymore. Many of the messages in the latest spam wave promote penny stocks — part of a scheme that antispam researchers call the “pump and dump.” Spammers buy the inexpensive stock of an obscure company and send out messages hyping it. They sell their shares when the gullible masses respond and snap up the stock. No links to Web sites are needed in the messages.