How Do Spam Filters Work?

You may be surprised but most spam filters are much more sophisticated than just the words in your email. We’ve all received emails which contain offers with enhancing supplements or get rich quick schemes.  But it’s not just these phrases which trigger a content filters.

In fact, they actually work by assigning a point to an email based on elements found throughout the email.  Each fax-pas will earn a negative point value and once those points are totted up, if the email exceeds the threshold the content filter kicks in.

Thankfully you wont end up being considered a spam sender for one wrong phrase, but multiple offenses.

The free/open source website Spam Assassin list out all the content-matching rules that are used in most spam-detection techniques.  Remember however, this list is constantly being updated as the game between Spammers and Spam Filters ensues, each time spammers adapting to bypass the filters with new techniques.

Common sense usually prevails when it comes to phrases considered by most as spam. But there are a few surprising ones:

  • Subject Line is in ALL CAPS (.763 points)
  • Exclamation points ! and question marks ? in a subject line (.201 points)
  • unsubscribe requests will be honored in 7 days (2.063 points)

Here’s a compilation of the Top 100 Spam Trigger Phrases to Avoid.

Beyond making sure your HTML message wont get caught in the Spam Filter, you should also make sure your email lists are clean and don’t include bad emails.  Using an email verification service like will weed out the Spam Traps so your distribution list is squeaky clean for every send.

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