New anti-spam program worth a try.

Most anti-spam programs take the form of add-ons to your normal e-mail program or service. And most rely on filtering, the effort to guess, usually imperfectly, which e-mails you receive are spam and which are legitimate.

But I’ve been testing an anti-spam system that takes a better approach. It’s a complete e-mail service that has anti-spam intelligence built right in. Instead of filtering, it uses a far more effective method that stops 100 percent of mass-mailed spam.

The program is called Mailblocks, and despite a few downsides, I like it a lot. It’s aimed at consumers and small businesses, and doesn’t work with corporate e-mail. But it has lots of sophisticated features, and can consolidate all of your e-mail accounts – even AOL accounts – into a single in-box.

Mailblocks is a Web-based e-mail service, like Yahoo Mail or Hotmail, that works on both Windows and Macintosh computers. But it is slicker and cleaner than Yahoo or Hotmail, with a very good, uncluttered interface that responds to commands quickly.

The service is inexpensive. A free version, which has ads, gives you 5 megabytes of message storage and a Mailblocks e-mail address. For $9.95 a year – that’s a year, not a month – you get 15 megabytes of storage, an ad-free screen, a Mailblocks e-mail address and the power to consolidate other e-mail accounts. For $24.95 a year, you get all of that and 100 megabytes of storage. On Hotmail or Yahoo, 100 megabytes of storage costs more than twice as much, and you get less effective spam protection and have to look at ads.

Mailblocks also allows you to send e-mail attachments of up to 6 megabytes each, a generous limit that’s enough for multiple high-resolution photos. And if you don’t like having the clumsy word “mailblocks.com” in your e-mail address, you can choose from 19 alternatives.

You can divert e-mail from up to 10 of your current e-mail accounts to Mailblocks, and Mailblocks will apply its antispam system to all, including popular services like EarthLink, AOL, MSN, Yahoo Mail and Hotmail. You can also import address books from other programs.

Mailblocks uses a spam-control system called challenge/response. It’s designed to stop all e-mail from the automated mass-mailing programs spammers use, while letting in e-mail from humans. Here’s how it works.

All e-mail you receive from people in your address book is passed directly to your Mailblocks in-box. If any are sent to you from addresses not in your address book, they are met by an automatically generated “challenge” e-mail that asks the sender to copy a randomly generated number into a box. These e-mails go into a special Pending folder while awaiting a response.

If the sender copies the number correctly, the e-mail is moved to your in-box, and the address is added to your address book. Mailblocks will also add the sender to a master list, so he or she will never be challenged again.

If the sender doesn’t respond correctly to the challenge within 14 days, the e-mail will be deleted from the Pending folder.

All automated spam systems will fail this test, either because they can’t copy the number, or because they use false return addresses. Only human senders with genuine return addresses can pass the challenge.

Walter Mossberg writes about personal technology for The Wall Street Journal. Copies of his columns are available at http://ptech.wsj.com.