ClickZ - March 16, 2005
By Kirill Popov and Loren McDonald of Email Labs

A common e-mail marketing misconception is e-mail is filtered because it contains words such as "free" in the subject line or body. By itself, that won't get your e-mail filtered. Though certain content combinations may get a message filtered, ISPs may be trapping your legitimate e-mail for infractions you rarely pay attention to.

Take HTML code. Using outdated or incorrect code is a major reason why e-mail to domains such as MSN/Hotmail and AOL are blocked or delivered to bulk or junk mail folders.

You may think you don't have to worry about this. Your e-mail may render correctly and look just fine to you. Wrong! Pivotal Veracity, a delivery-monitoring service provider, estimates nearly 100 percent of all HTML e-mail doesn't comply with World Wide Web Consortium (W3C) standards.

Because each ISP handles e-mail differently, messages that get past the filters at one destination may be filtered or entirely blocked at another. Why are some ISPs so concerned about HTML code? You can thank spammers, of course. HTML syntax and format errors are common tricks spammers use to foil standard content filters.

Some W3C infractions are minor and won't cause e-mail to be filtered. An example is not using "alt" tags, which describe the content in an image tag. Many other innocuous-appearing coding errors or tricks may send your e-mail straight to the bulk folder.

Pivotal Veracity recently tested hundreds of HTML e-mail messages to see whether they landed in the inbox or the bulk folder, or were blocked outright. They came up with these surprising results:

• A tracking beacon below the closing HTML tag will get e-mail filtered to the bulk folder at MSN/Hotmail.
  
  
• A poorly constructed boundary between the text and HTML portions of a multipart e-mail message also sends the e-mail to the bulk folder at MSN/Hotmail.
  
  
• Using hex-encoded domains in URLs (substituting the code "%20" for a space in a URL, for example) can get your e-mail blocked or sent to the bulk folder at AOL, CompuServe, and MSN/Hotmail.
  
  
• Using a decoy link that shows one URL in the e-mail but actually redirects to another URL when clicked also gets e-mail directed to the bulk folder on MSN/Hotmail.
  
  This isn't a common technique used by most legitimate e-mail marketers, but if you're thinking about doing it, we advise against it. Using decoy URLs is a technique commonly employed by phishers, scammers who impersonate financial institutions to steal Social Security, bank, or credit account numbers. For example:
  
  <a href="http://yahoo.com/maintainyourprofile.php">http://companyx.com/maintainyourprofile.php</a>
  
  Readers see the second URL in the message, but they'd be sent to the first URL.

AOL's HTML Validator

While conducting tests for our clients several months ago, we discovered a new AOL e-mail filter that scans incoming messages for HTML syntax and format errors. If it detected invalid HTML, it rejected the message. AOL even created a special bounce code it used when rejecting a message for this reason.

Common errors, such as using "<a/>" to close an HTML tag instead of the "</a>," could trigger the filter. Pivotal Veracity's recent testing suggests AOL may no longer be applying this filter. The bottom line is check your code carefully and correct any syntax errors.

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