America Online Inc. said yesterday it has bought a small Silicon Valley e-mail company in a move that could boost the use of an anti-spam technology that many experts regard as the most effective in blocking unwanted messages.

Privately held Mailblocks Inc. was a pioneer of a system known as "challenge-response," which can help block messages from spammers, who use computers to generate millions of e-mail messages a day.

When e-mail is sent to a challenge-response user, the sender gets an automated message requiring that someone type in a code to verify that a person, not just a computer, is sending the message. When the sender has done so, the e-mail reaches the recipient's inbox and the sender is verified for all future e-mail.

As a result, users do not need to rely solely on spam filters, which seek to identify spam by either their content or origin, but which spammers often are adept at evading.

"We view challenge-response technology as coming as close as anything we've ever seen to being the coveted magic silver bullet that we and our members have been looking for" to combat spam, said AOL spokesman Nicholas J. Graham.

He said the service would be free and integrated into the e-mail system used by roughly 30 million AOL users worldwide. Members will have the option of turning on the system, which AOL intends to market heavily.

Graham declined to say how much AOL paid for Mailblocks, whose founder, Phillip Y. Goldman, died in December at age 39. Graham said nearly all of Mailblocks' 15 employees have been hired by AOL.

Despite its effectiveness in thwarting mass spam mailings, the technology has taken hold more slowly than some experts expected.

EarthLink, with roughly 5 million e-mail account holders, offered the service to its members beginning last summer. Spokesman Jerry Grasso said about 673,000 customers are now using it.

"We still think today that it's the best solution for the consumer," Grasso said. "It gives the user complete control."

Mailblocks, which charged a small fee for most of its services, never disclosed how many customers it had, but a source familiar with the company said it was about 350,000.

A handful of small e-mail companies also offer versions of the system.

But challenge-response requires a bit more work of both senders and recipients. Users must take care to pre-verify legitimate, auto-generated mail, such as airline-ticket confirmations or information from message groups to which they subscribe.

Grasso said it might not be appropriate for some small businesses that rely on e-mail sales inquiries from unknown customers, who might be put off by the challenge.

Still, boosters of the technology say that once people get accustomed to challenge-response, it has a chance to become predominant.