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Now, an anti-spam company is drawing fire for using the same tactic.

Ads, called "spamlets" by one privacy expert, have begun appearing in the signature files typically used to place personal information, such as a name, telephone number and custom greeting, at the bottom of e-mail messages. Some software downloads now include code that inserts a marketing message in this signature file. Once triggered, all e-mail from that address will carry the promotional text.

Recent targets of the practice include Web surfers who installed a test version of an anti-spam product from MailFrontier, a Palo Alto, Calif.-based software developer. When Web surfers install its Matador product, the download automatically alters their signature line in Microsoft Outlook to read: "This mailbox protected from junk email by Matador from MailFrontier Inc."

Users can't prevent the alteration unless they erase the note each time or go to the company's FAQ to read about how to remove the tag line, which is not a simple process. An early version of the software is buggy, too, so that in the event the program is uninstalled, the tag line remains.

MailFrontier said it plans to release a bug fix in the next week, and promised that an upcoming paid version of the software would give consumers the ability to disable the message completely.

Critics say the strategy is ironic for a company readying to sell an anti-spam product.

"It's a form of deceptive spam," said Larry Ponemon, a privacy expert who is starting an institute for privacy activism. "If you're going to change someone's e-mail process in any way, you have to give them a right to make that choice."

MailFrontier CEO Pavni Diwanji countered by saying that information in the FAQ and license agreement is "upfront and clear."

"Currently the product is in beta. When the product is in full release, people choose to pay for it and to not have the tag," she said, adding that not many users have complained.

New wave in ad invasion
The aggressive tactic is only the latest ploy from advertisers aiming to invade every last square inch of open space, virtual or real, and get in front of consumers in places otherwise considered off-limits. And the Internet has proved novel new ground for experimentation.

Some Web operators have been criticized for taking liberties with consumer desktops and Web browser settings in numerous ways, actions that have raised questions about consumer privacy and companies' responsibility to properly disclose their actions. On the darker side of the spectrum, sites have preyed on browser vulnerabilities to change homepage settings without permission, or to automatically install programs that can capture personal information such as credit card numbers and passwords.

Other companies have used downloads of popular file-sharing software to disseminate their own advertising, or adware, programs. The Gator online advertising network, for example, piggybacks its technology onto other top downloads, then uses it to deliver targeted advertisements to people while surfing the Web. Gator's pop-up ads landed the company in hot water with several publishers, which sued it for copyright infringement and were granted a preliminary injunction against the company last month.

"There's a constant struggle here," said Richard Smith, a security expert and purveyor of Computerbytesman.org, who dubbed Matador's commercials spamlets. "Advertisers and marketers are always looking for new places to stick their messages."

MailFrontier, which raised $5 million in funding from investors, including Draper Fisher Jurvetson, launched a free test version of its anti-spam software in recent weeks. At the same time, the company is attempting to build up its customer base by updating a viral marketing tactic pioneered by Hotmail--seeding a subscriber's signature line with a commercial for itself.

The company got the idea from Tim Draper, one of its board members and an early investor in free Web-based e-mail service Hotmail, which is now owned by Microsoft. Hotmail rose to fame and enormous popularity among the Web community through the use of a standard tag line in each subscriber's e-mail that sold the service--one of the first uses of viral marketing. Yahoo has since used the same space in subscribers' outgoing e-mail to promote news and various features. On Sept. 11, the company changed the outgoing message as a memorial to the attacks on the World Trade Center, for example.

MailFrontier's strategy differs in that it tinkers with corporate or personal mailboxes outside of a free Web-based e-mail service, privacy experts said.

For her part, Diwanji called the signature modification "mild viral marketing" and a relatively well-known technique, but the company does not want to cause customer ire. In the coming paid version of the software, which will not have a free counterpart, customers will have a choice, she said.

Matador uses another unique marketing tactic in the process of fighting spam, too. When the software detects e-mail sent from an unknown person that may be spam, it will send a "challenge" or automatic verification notice to the recipient. That recipient must prove that he or she is human through a quick questionnaire before the e-mail can get through.

Still, privacy experts say that the company is offering poor notice and choice when it comes to altering the tag line. Though MailFrontier includes information about the mailbox change in its terms of service, which consumers must agree to before installing the software, privacy experts dismiss the terms as material nobody reads. People can also find out information about the change in the company's FAQ.

"It's pretty tacky," Smith said. "They're messing with outgoing e-mail they have nothing to do with. This is a bad trend. It's the same as sticking an advertisement in the few seconds before you answer a phone call--the phone companies just don't do it."