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How HTML Code Affects E-Mail Deliverability

  |  March 16, 2005   |  Comments

Have you hugged your e-mail's HTML code lately?

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

Take HTML code. Using outdated or incorrect code is a major reason why email to domains such as MSN/Hotmail and AOL are blocked or delivered to bulk or junk mail folders.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 email may render correctly and look just fine to you. Wrong! Pivotal Veracity, a delivery-monitoring service provider, estimates nearly 100 percent of all HTML email doesn't comply with World Wide Web Consortium (W3C) standards.

Because each ISP handles email 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 email 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 email straight to the bulk folder.

Pivotal Veracity recently tested hundreds of HTML email 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 email filtered to the bulk folder at MSN/Hotmail.

  • A poorly constructed boundary between the text and HTML portions of a multipart email message also sends the email 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 email blocked or sent to the bulk folder at AOL, CompuServe, and MSN/Hotmail.

  • Using a decoy link that shows one URL in the email but actually redirects to another URL when clicked also gets email directed to the bulk folder on MSN/Hotmail.

    This isn't a common technique used by most legitimate email 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:


    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 email 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 "" to close an HTML tag instead of the "," 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.

Minimizing HTML Filtering Problems

How do you minimize or eliminate HTML filtering problems at ISPs? A few suggestions:

  • Make sure your email designer/programmer is familiar with W3C standards.

  • Before distributing an email, run the message through your design software's or email application's HTML validator, if it has one. Better yet, use a third-party validator such as the ones provided by the leading delivery monitoring firms or the W3C Markup Validation Service.

  • Consider using a delivery-monitoring service to track your messages after pre-tests and full-list distribution. These services use seed lists to monitor whether your email is being delivered into recipients' inboxes or bulk folders or are missing across the first and second-tier ISPs.

  • If your email does get trapped in a bulk folder, check your code and isolate what's causing the filtering. Use a series of tests with modified versions of the email.

In a future column, we'll walk you through an actual diagnosis of code problems and show you how to resolve them. Stay tuned.

Till then, keep on deliverin'.

Want more email marketing information? ClickZ E-Mail Reference is an archive of all our email columns, organized by topic.


Kirill Popov and Loren McDonald

As director of ISP relations and delivery, Kirill Popov creates and enforces strict usage and anti-spam policies, maintains ISP and community relations, and oversees all abuse and policy investigations and inquiries for EmailLabs clients. Kirill works with clients on best practices, content, design, and list hygiene to minimize potential delivery issues. He's a registered member of the SpamCon foundation and representsEmailLabs on AIM's Council for Responsible E-Mail.

Loren McDonald is vice president of marketing at e-mail marketing automation company EmailLabs, overseeing corporate marketing activities and client consulting services. He has 20 years experience in marketing, consulting and strategic planning. Earlier, Loren was founder and president of Intevation, an e-marketing services firm specializing in e-mail and SEM. He's held executive marketing positions at companies including USWeb/CKS (marchFIRST), NetStruxr, and Arthur Andersen.

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