Spam Filters

Category: Spam

Between 90 and 95 percent of all email is spam, according to security software vendor Symantec's last study in early 2009. So why isn't 90% of the email in your inbox unsolicited commercial email touting men's health products and get-rich-quick schemes? The answer is spam filters. Here's how they work...

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, 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.

Gmail Spam Filter

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 for more on that topic.

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

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Most recent comments on "Spam Filters"

Posted by:

13 Oct 2010

I vote for MailWasher, it deletes the email from your Host before it hits your in box

Posted by:

13 Oct 2010

how come that there is NEVER spam on my Gmail account ?

Posted by:

Digital Artist
14 Oct 2010

Trying to filter spam by certain words in the content or subject line is useless because spammers just modify the spelling, like ci*lis, or p3nis or just put spaces in the words likely to get
fi ltered.

Posted by:

14 Oct 2010

PC newsletters all seem to recommend the same old antispam software all the time, Mailwasher and the like.
I have tested quite a few spam filters over the years but I have never seen the one I use recommended.
I use SpamBrave, truly excellent.

Posted by:

14 Oct 2010

I also use Mailwasher Pro. PC newsletters all seem to recommend the same old antispam software all the time. I wonder why... Oh, it might be because they work. Kinda makes you wonder, doesn't it.

Posted by:

20 Oct 2010

With Mozilla Thunderbird, and some other email clients, you do this two-step solution:

1. Create a SPAM folder.

2. Create a filter that reads: "If From isn't in my address book, send to SPAM folder.

If you check that folder and see an email you actually want to receive, click the white star (Thunderbird only) to add the sender to the Address Book, and drag the email to the Inbox.

Press Ctrl/A to select all others, and delete the lot.

Posted by:

20 Oct 2010

How come that Gmail allows mail that is a-l-m-o-s-t my e-mail address and proclaims they are to ME? Every day, and it does not help with the G-mail filter. The Help-pages give no help and nowhere to protest either. Other "post masters" bump mails with wrong addresses, but not Google mail, why? Help!

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Article information: AskBobRankin -- Spam Filters (Posted: 13 Oct 2010)
Copyright © 2005 - Bob Rankin - All Rights Reserved