Jan 8, 2005

Spam Filters and Email Content

We’ve noticed some of our users’ campaigns setting off content-based spam filters lately, so thought it would be good to recap on what seems to set those off. 

Content based spam filters generally assign points to certain "violations" in your email. Once your email’s "spam score" exceeds a certain threshold, your email is tagged as spam.

For instance:

  • Avoid words and phrases like, "FREE!", "LIMITED TIME ONLY", or "ACT NOW"
  • Talking about large sums of money ("We met our sales goal of $2,500,000 this year"), might score you some spam points.
  • Avoid using colors in your fonts too much. Red text and red links seem to make the spam filters angry, especially if you also make them large.
  • DON’T USE ALL CAPS—yelling looks like spam
  • Always have a good balance of HTML and Plain-text. Just having one or the other seems to look spammy to some filters
  • Try not to link to IP addresses—instead, point to domain names
  • Be short and sweet with your opt-out instructions. Going on and on about the legality of your email, or how to "be removed from future mailings by clicking reply and typing remove in the subject line" looks spammy. If possible, use a one-click opt-out method, or link to an online form.

Over the last few months, we’ve observed some of our users accidentally getting "false-positives" from the spam filters. Here are some interesting ones…

  • One user sells industrial equipment to B2B clients, with pretty high price tags. All of the "$1,500,000" values in the content set off the filters
  • Similar situation with another user that sells private jets—large money numbers apparently look like financial scams
  • One user sent an HTML only email to his customers, with no plain-text alternative.  It was a holiday e-greeting, and the message had no copy in it—just a big animated graphic.
  • Another user apparently used Microsoft FrontPage (yes, he was penalized for that) to build his HTML email e-vite, which consisted of nothing but gigantic text, with every sentence in a different color.
  • A pharmaceutical corporation sent an internal campaign to its employees that got flagged as spam, because they mentioned pending approval of their new impotence drug (Cialis, Vioxx, Viagra spam, anyone?)
  • A non-profit that helps students in foreign countries come to school in America, put the words "young" and "adult" and "teen" a few too many times in their content.

Just be careful with your content—some of the most innocent words can result in your email getting tagged as spam. Here’s an example of what Spam Assassin, one of the most popular open-source anti-spam systems, reports when it tags something as spam…