How Content and Rendering Affect Deliverability

The content of your email is the biggest key to communicating your message effectively to subscribers. Countless hours are spent working on wording, catchy slogans, eye-catching images, and creative new approaches to capture your subscribers interest after they open an email. Subscribers are not the only ones reading your email, though. Spam filters read your email too!

Do you know what your email content is saying about you?
It is impossible to avoid having your emails filtered, so instead you need to understand how modern filters work with your content. These days, content analysis by spam filters goes well beyond a simple search for “spammy” keywords such as “free” or “Viagra”. 

Fingerprints: we all have them, and so do your emails. Fingerprinting is a process used by spam filters to track sections and themes within emails instead of focusing on specific, individual words. A fingerprint is a small (64 bit) hash made from part or all of your content. Each fingerprint is then compared to other similar fingerprints, much the same way law enforcement does. If your email fingerprint matches (or is very similar to) other fingerprints that were determined to be spam, then your email will be marked as spam as well. 

One of the best known users of the fingerprint method is Cloudmark. Fingerprints can be made from any and every part of the email: the header, footer, time stamp, subject line, body text, images, images-to-text ratios, URLs, and even text color can be a factor. Because of all of the factors that go into fingerprints, they tend to be fairly dynamic and change from one email to the next.

Recently we noticed that one of our clients had the exact same Cloudmark fingerprint (P= in the header) for every single email they were sending. We worked with our client to get answers from Cloudmark as to the cause, and discovered that the URL provided by the click tracking company they were using had been fingerprinted as spammy because it was used as part of a spam campaign. Once the client updated the method of click tracking, the fingerprint was no longer applied to their email campaigns and inbox placement increased more than 20%.



Wait, I got blocked because of what!?
As in the client example above, it is sometimes possible for you to be sent to the spam folder through no direct fault of your own. For this reason, it is vitally important to know the current status of all of the images and URLs you utilize in your emails. Blacklists update dynamically, and a link or image that was safe yesterday could prevent you from reaching your subscribers today.

To avoid unpleasant surprises, we recommend checking your URLs and images each time before sending an email campaign using a checker such as Return Path’s Inbox Preview. It is also important to control your content as much as possible by hosting your own images and only linking to URLs you know and trust. This is not always possible, such as when dealing with click tracking and other services, but you should do what you can by limiting your exposure to outside sources.

One of the most common questions we get asked is, “What is the right ratio of imagery to text?” A quick Google search reveals a vast number of articles discussing this question. Let me save you some time… There is no magic ratio. You cannot avoid or fool a filter by doing all text or all images, and filters are not programmed to auto-pass emails with certain ratios. The real key to success is balance.

Keep a “reasonable” balance between text and images and you will do just fine. Many inboxes have images turned off by default, and you need to still get your message across to subscribers if this is the case. Others (such as Apple) have images on by default, and you need to take full advantage of the opportunity to wow your subscribers. Meeting both goals can only be done through careful balance. Leaning too far one way or other other can have unfortunate consequences.

What can I do about all this?

  • Test, test, test! Check your images and URLs regularly before sending your campaigns to make sure they won’t be causing issues.
  • Compare your subject lines to others in your industry for performance. Subject lines are part of the content too, and can play a big part in deliverability and engagement. One easy way to do this is with Subject Line Optimizer, which is now built into Inbox Preview.
  • Check your code, make sure there are no errors or glitches in your HTML code that might be considered spammy by filters. This can be done with a tool like Inbox Preview.
  • Avoid using Base64, which is a common tactic of spammers and is frequently restricted by filters.
  • Balance your ratio of text to images. This should also be tested to see what ratio works best for engaging your audience on each type of mail you send.
  • Test some more! When contemplating making a change to one of your email campaigns or creating a new one, be certain to perform A/B split tests before sending the main campaign. This allows you to confirm that nothing was missed in the steps above.

To learn more about how content and other elements of your email can impact deliverability, check out our latest ebook The Ultimate Guide to Email Deliverability.

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