Email Optimization

Everything You Know About Email Content Filtering is Wrong

Posted by Tonya Mitchell on

You’ve spent hours developing what you think is the perfect email content.  It looks great.  It tells the story you intended.  And that subject line?  Genius!  You’re sure it’s going to generate the ROI you want and hit the button to deploy the message to your subscribers.  But wait…what happened?  Why is your message being filtered as spam?

Back in the early days of email people thought getting delivered to the inbox was all about content.  Today, we know that isn’t true.  When it comes to your email being filtered, the Return Path Sender Reputation Report identified the following key factors ISPs consider:  IP Reputation, Infrastructure and Content.

Today’s spam content filters take a more holistic view of your message.  Spam filters now consider the type of message being sent and whether or not information in the body of your email as well as the subject could be considered spam to help determine if the message should be rejected.  Filters “learn” this information over time based on the spammy versus legitimate mail it has seen come through its system.  They also consider the feedback from the end users regarding whether or not messages have been marked as spam.  So while content filtering may only come into play 17% of the time, it’s still valuable to understand the various types of spam filters where your message could get caught.

There are major three levels of enterprise spam filtering:  hosted, gateway and desktop.   Hosted spam filters are companies that have developed a proprietary method of using content and reputation metrics to identify spam versus legitimate mail.  Since these spam filters have a large book of clients using their service, they have a broader scope of information to use in their decision making process to determine whether to deliver to the inbox, spam or quarantine folder, or to block it completely.  Hosted spam filters can include technology that is integrated by vendors directly into their own products.   Examples of hosted spam filters include Brightmail, Cloudmark, MessageLabs, and Postini.  According to our latest benchmark report, these hosted spam filters only delivered email to the inbox 86% of the time.  Some of these hosted spam filters are more difficult than others to pass through.  Postini is the most challenging filter with only 47.55% of emails being delivered as you can see in the table below.

Filter Inbox Spam Missing
Mailtrust 82.02 16.76 1.23
MessageLabs 85.02 14.34 0.63
Postini 47.55 52.23 0.22
Brightmail 98.36 1.29 0.35

Gateway spam filters are physical servers with software that are installed at the border of a company’s network.  All mail attempting to come into your company must pass through this “gate” before it can enter your system.  This spam filter is learning what it deems as spam based on all the email coming into your company, thus having less email to learn from than the hosted spam filter.  Examples of a gateway spam filter include IronPort and Barracuda.

The desktop spam filter lives on the end users computer.  It is highly customizable by the individual so is often seen as one of the more difficult filters to pass through.    An example of a desktop spam filter is Outlook.  Outlook uses Microsoft’s anti-spam filter SmartScreen, which also looks at reputation, to help filter email.  SmartScreen uses the feedback from Windows Live Hotmail users to help distinguish legitimate emails from spam.

While the triggers for each content spam filter can vary, the following are some best practices to help reduce your chances of getting caught in the various content spam filters:

Rule Out Reputation. Most of the filters mentioned above also measure IP reputation.  You should first verify that your issues aren’t due to IP reputation first before testing content:

Return Path Sender Score:


Microsoft SmartScreen:




Test, test, test. Testing message content in a pre-deployment tool such as Return Path’s Campaign Preview can identify potential spam filter issues to you prior to deploying to your subscribers.  Once you identify that your content is being flagged by spam filters, you should continue testing to isolate what is causing the issues, such as subject lines, URLs/links, text and images.  Content testing can be a time consuming process as you’ll want to test the various message components separately until you identify what is passing and failing the spam filter.  Also, check to see if any of your URLs are on URL blacklists, like URIBL and SURBL.

Have a balance between text and images in your message. Don’t create a message with one large image as this is a spammer technique used in attempt to bypass spam filters.  Embedding large images in emails or using a lot of graphics can also slow the email servers’ ability to process mail.  As a result, content spam filters will often flag such emails and stop delivery.  Not to mention images are off by default at most ISPs so you won’t be getting your message across anyway.

Don’t use URL shorteners. A URL shortner is a tool that takes your full URL and creates a small version of it to place in content, like and tinyurl for example.  Since URL shorteners are commonly used by phishers and spammers, they are usually blocked by content spam filters.

Get Return Path Certified. Return Path’s Certified program is a 3rd party whitelist used by over 2 billion mailboxes worldwide to help legitimate senders achieve higher inbox placement rates.  As a Certified member you will bypass many content spam filters, have the value of your links and images being enabled and ultimately enjoy higher inbox placement rates.  Only the best of the best are accepted into the program, so having a strong Sender Reputation is key.  Visit the Return Path site for more information on this valuable program.

  • Big Jason

    Seems like you're generalizing pretty heavy here with “Not to mention images are off by default at most isps” so you won't be getting your message across anyways”.  As if there weren't best practices to get users to turn on images or the fact that images can be a strong supplement to your core message and not take away at all even with images turned off. Now if you are directly tying that comment to using one large image, I can see your point.

  • Noel Butler

    HUH?  You need to talk to real ISP's, though, I suspect l ike many of RPs stories they are an attempt to upbeat and sell their services, in the 20 years I've been in this game  “CONTENT” is the number one deciding factor at any ISP I've been with, including those I've been in head honcho of mail servers, my opinion also reflects those other admins of ISP's and corporations and SMB admins who we regularly converse with… IP reputation is not even considered in most cases, even  DNSBL's are becoming less of a need, they are pretty much only good “after the fact” in many cases, our SA rules catch about 95% of spam with rare but occasional false  positives, about the same these reputation things have.

    Reputation and trust can only be earned, not bought!

  • morgaine

    I agree. Content is king. Eve with provider spam filters now flagging phrases within email as potential spam or grey mail. Testing email before sending can catch many issues with code, DKIM, image use, and yes… I've even found a product that tests content phrasing for potential spam filtering in addition. And it's worth noting the company offers the automated testing product for free to anyone, not just those who pay for their other email marketing software and services. 

  • Tonya Mitchell


    Thanks for your feedback Noel.  I completely agree with
    your statement that “reputation and trust can only be earned, not
    bought!”  You can’t “pay to play” in this industry – you have to build a
    positive, solid reputation.  Our goal at Return Path is to promote best
    practices in order for senders to have a positive reputation that translates
    into positive inbox placement.  We have relationships with a large ISP
    network (but no, there is no bat phone!) that have indicated an overall IP
    Reputation is key to achieving inbox placement.  There are many factors
    that contribute to that reputation including complaint rates, data hygiene,
    infrastructure, how subscribers engage with your program (spam versus not spam
    votes) and yes, content. 


    In fact, there are situations where content can be a key
    factor considered.  For example, Enterprise filtering systems do currently
    rely heavily on content (you’ll often see Postini filtering come into play
    here)  where Webmail providers rely more on reputation.  But as
    businesses move to hosted email solutions from providers like Google and
    Microsoft, Reputation will be more important for those senders than it is now.

  • Tonya Mitchell


    You are correct Jason – there are definitely ways to build
    content so it has a balance of both images and text.  Use images in your
    content – just use them correctly versus sending an email as one large image
    that creates a risk of looking spammy.