Spam Filter Review

Specialized software can reduce spam levels

Internet security experts say most email -- estimates range from 70 to 95 percent -- is actually spam. A typical inbox doesn't contain anywhere near that, though, because most spam is blocked further upstream. ISPs and large companies use expensive hardware spam filters, while large web-based email services like Gmail have had good results with collaborative filtering.

As anti-spam technology evolves, spammers continue to develop new distribution tactics and profit angles. Scott Petry of email security firm Postini says spam continues to evolve into new forms. It is hidden in email attachments such as PDFs, MP3s, Microsoft Office documents and JPEGs.

The simplest spam promotes illegal or gray-market products (drugs marketed as sexual performance enhancers are popular with spammers). More complex schemes are designed to trick users into revealing sensitive personal or financial information. (This technique is known as phishing.) Tax time sees a surge of IRS-themed spam designed to mislead users through phony rebate or refund offers. Spam can also be used to distribute viruses and other types of malware.

Spam filters, also called spam blockers or anti-spam software, attempt to separate legitimate email from spam. Spam filters vary in their effectiveness and may misidentify legitimate email as spam (called a false positive). The best reviewers evaluate the accuracy and effectiveness of spam filters, along with features, ease of use and potential drain on your system resources.

We found lots of worthwhile and helpful reviews of spam filters. In the last year, Virus Bulletin has begun regularly testing anti-spam software in addition to antivirus products. Choice, Australia's answer to ConsumerReports.org, includes seven standalone products in its recent spam-filtering report and looks at the spam-fighting abilities built into some email clients. But while testing is comprehensive and balanced, discussion could be deeper. The recent spam-filter coverage at ConsumerReports.org itself has the same positives and negatives, but fewer spam filters are tested. PCMag.com provides the most comprehensive comparison of standalone spam blockers, and the site has up-to-date coverage of the spam-filtering capabilities of major Internet security suites. Gizmo's Freeware has recently updated reviews naming the best free spam filters for experienced and average users. While authoritative, we'd like those better if testing was explained. About.com's coverage is similarly helpful, but similarly flawed. The write-ups at TopTenReviews.com seem balanced for this category, but the marketing-driven nature of the site gives us pause. Download.com is a good destination for user reviews.

Differences between types of spam blockers

Each program or service uses one or more technologies (sometimes all) to filter spam. You can also choose between a software filter, which is installed on your computer, and web-based filters that filter your mail, then forward legitimate messages.

Rules-based filters require users to train the software. Most programs start with a set of default rules that label incoming mail as definitely spam, not spam or undecided. You sort through initial batches of undecided mail and determine which are unwanted. The anti-spam software then learns to create new rules so that you don't get similar spam in the future. This method can work very well, but users do need to spend time training the software. Experts add that the best rules-based anti-spam programs use Bayesian spam filtering, a rules-based statistical method that classifies messages into categories based on probability.

Content-based filtering, which blocks keywords or phrases, is a method that has been outsmarted by spammers. Spammers use misspelling and similar tricks to evade content filters.

Whitelist filters use an approved-senders list (usually the contacts in your address book) to sort the good from bad. You can add more contacts to your approved list anytime. This method's inflexibility is both its strength and its weakness. New correspondents, business proposals or responses to classified ads will be classified as junk, making whitelist filters best for those who don't get email from people they don't know. Many anti-spam programs offer the option of a whitelist in addition to other types of spam filtering. That assures you will always receive mail from the email addresses in your contacts folder.

Challenge/response filters respond to messages from unknown senders. The sender must enter a code, go to the software vendor's website or jump through some other hoop. In a study by Brockmann and Company, a specialty market research firm, challenge/response filters produced the highest level of satisfaction by far for business customers. However, they are not without some drawbacks. For example, spammers often use forged addresses, which means that the challenge might be returned to the inbox of someone not responsible for sending the original message -- essentially becoming spam itself. There's also the nuisance for legitimate senders, who could decide going through additional steps just to send an email isn't worth the bother. Challenge/response filters are usually used in conjunction with whitelist filters.

Blacklist filters block specific email addresses. Most blacklist filters rely on web-based databases of known spammers. You can add addresses to the blacklist.

Community-based filters depend on user contributions to assemble a list of known spammers. Once a handful of established users submit spam as junk to the software company, it's added to the database of known offenders, and no users will receive messages from that email address (and sometimes that computer's IP address). This technique requires little user intervention, and legitimate mail is unlikely to be tagged as spam.