Spam Filters

How Do Spam Filters Work?

Spam filters are silent sentinels that you may not even know are guarding your inbox for you. Spam filters consist of rule and sophisticated software packages that do a pretty good job of differentiating spam from legitimate email. Spam filters use a combination of techniques to evaluate each incoming email, and direct the message to a junk folder or your inbox.

The source of an email is one indicator. Thousands of Internet domains are dedicated to hosting spammers and their spam-spewing software. As these rogue domains are identified they are added to blacklists used by spam filters to block all mail from the rogue domains.

Specific email accounts on rogue or legitimate domains may be spammers, and such individual senders may be blocked by spam filters too. But this is a losing game because spammers can create new email accounts almost instantly on many free email servers. Adding a specific email address to a blacklist is typically left to the end user, as a last resort should a piece of spam slip through the filter.

Keywords are used by spam filters to block objectionable emails from scammers, online pharmacies and other objectionable sources. But keyword filtering is problematic for many legitimate newsletters and RSS feeds delivered via email.

Some spam filters even look at "guilt by association" to label an email-generating Web site as a spammer. Recently, Australia's nationwide government-operated spam filter blocked virtually every online news medium including News.com.au, the nation's largest source of online news. The reason, officials discovered to their chagrin, was that news sites were hosting ads for online matchmaking and dating services.

Spam filters are installed on the email servers of every sensible email service provider including Google's GMail, Yahoo Mail, Hotmail, AOL, and others. They trap spam before it is sent out to users' inboxes, so most users aren't even aware of how much spam is sent to them but never arrives. Besides keeping customers happy, ISPs have a vested interest in avoiding the bandwidth costs of sending spam to users' desktops, so every ISP uses spam filters.

Creating Your Own Spam Filters

You can also add your own spam filters to most desktop or web-based email accounts. Spam control functions are usually under the "Settings", "Options" or "Account Management" tabs. There are several handy things you can do with spam filters:

• Filter by keyword. If you're getting emails related to adult services, pharmaceuticals, or you want to block obscene or vulgar words, you can stop them cold by filtering on the Subject line or body text.
• Add a specific email address or an entire domain to your spam blacklist, meaning mail from that source will always be blocked.
• Add an address or domain to a whitelist, meaning mail from that source will NEVER be blocked. That's useful when a spam filter incorrectly tags an email or newsletter you really want to get.

Similar controls are built into many popular desktop email clients such as Outlook Express, Mozilla Thunderbird, and others. After specifying your filtering criteria, you can specify what to do with messages that match. Options include simply deleting the message, saving it in a specified folder, forwarding the message, and even specifying that a message should NOT be marked as spam.

Desktop email clients usually have a folder for "junk mail" or spam. Whatever makes it past your ISP's spam filter but looks suspicious to your desktop email client goes in there. You can inspect suspected junk emails before deleting them. If you find one that's been incorrectly filtered, you can flag it as "Not Spam" and move it to your inbox. You can also do the opposite and "Mark as Spam" to label a spam sender who makes it past all of your defenses into your inbox.

If you find the ISP-level spam filters combined with do-it-yourself filtering isn't working, or involves too much constant tweaking, you can use a variety of software addons to help protect your inbox. See my related article Antispam Software http://askbobrankin.com/antispam_software.html for more on that topic.

Do you use spam filters? Tell us about it! Post your comment or question below...