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Why Testing the Rendering of Every Email You Send Increases Your ROI


Email design is a moving target. Whether marketers like it or not, it’s impossible to create a single email design because the rendering of any individual email changes depending on the email client and screen size used to view it, on top of  many other factors.

Recent research shows up to 300,000 different potential renderings for each email you send. The challenge is to manage and control that variability through savvy coding and design. And you can’t do that if you’re not testing the rendering and functionality of every email you create.

While doing this takes extra time and costs, the effort is well rewarded. Email programs that test every email they send report generating an average return on investment (ROI) of 40:1, whereas programs that never test report an average ROI of 34:1, according to our CMO’s Guide to Email ROI.

Chart of the email marketing return on investment (ROI) for brands that QA test every email (40:1) vs. those that never do (34:1)

Even beyond ROI, email preview testing correlates strongly with success. For example, marketers who describe their email marketing programs as very important to company success are 95% more likely than those with programs not important to success to test every email they send (41% vs. 21%), according to our 2020 State of Email research.

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3 reasons for vigilance

Marketers should be constantly on the lookout for changes in email rendering because there’s variability in all three links in the chain between you and your subscriber.

1. Inbox providers, devices, operating systems, and browsers change routinely

New operating systems (OSs) and devices are announced, so we generally have a good heads up about those changes and can be on alert to possible impacts on our email designs.

That said, the market share of new OSs and devices are not always so evident. The adoption of new Apple OSs tends to be fairly rapid, but the adoption of Microsoft Windows and Google Android OSs is much slower. So marketers need to know not only the effect of a new OS or device but also the market share of it among their email subscriber audience. To find yours, use a tool like Litmus Email Analytics.

Industry-level market share is helpful as a general barometer of adoption, but knowing usage rates of all of the various email clients among your unique audience is key if you’re going to make sound email design and coding decisions.

While OS, browser, and device changes are quite public, the changes made by inbox providers often go unannounced. Our research shows there’s an email app update every 1.2 days on average.


Generally, marketers stumble across changes in code support at inbox providers and start talking and asking questions about them on Twitter and various forums before they’re being written about on industry blogs.

2. Your email service provider can make changes that affect the code of your emails

Your email service provider (ESP) will strip out code it doesn’t support, as well as code that it or inbox providers consider vessels for malicious payloads, such as <embed> tags and JavaScript.

This sometimes surprises marketers—and caught Litmus off guard one time quite spectacularly, breaking our first live Twitter feed email.

It’s worth noting that our email’s code was fine until we viewed it in our ESP’s WYSIWYG editor. Doing that is what stripped out the code that was vital to our email functioning.

Consider asking your ESP for a list of the code their systems will remove. Also, pay extra attention to the rendering of your emails in the months following an upgrade.

Needless to say, when switching ESPs, you’ll want to proactively adapt your email templates to your new provider.

3. Your email designs change

Even if inbox providers and ESPs never changed, we sincerely hope your emails do regularly.

For starters, it’s a good idea to refresh your overall email design routinely. Over 86% of brands redesigned their email templates in the past year, according to our 2020 State of Email research. Most of them did it to improve email performance (hint, hint).

You should also be creating one-off email designs for special occasions, whether it’s for a new product launch or an important seasonal event. This helps those emails get the exclusive attention they deserve.

Beyond email redesigns and special one-off emails, we hope you’re experimenting with a variety of new design elements from time to time, and running A/B tests. For instance, the 2020 State of Email report found that 18% of email marketers have created interactive emails—and another 35% plan to.

What’s your brand’s tolerance level?

Looking at it all together, that’s a lot of potential variability in the email ecosystem that can affect the rendering and functionality of your email campaigns:

  • New email clients and devices
  • Changes to existing email clients, devices, operating systems, and browsers
  • How your ESP processes the code of your emails
  • Changes to your email templates
  • New email design elements in your emails, whether for a special one-off email, experiments with new functionality, or incremental improvements

No matter who’s introducing the instability into the email ecosystem, it’s undeniably there. The key question is:

How long can your brand tolerate delivering poor or broken subscriber experiences to a portion of your subscribers?

For those brands that do email template redesigns every couple of years and only preview their new templates, they’ve decided they’re okay delivering subpar subscriber experiences to at least some of their subscribers for up to two years. Brands that only run preview tests every month or so have decided that weeks of potentially broken email designs are fine.

While brands would never accept that level of risk to their brand image in other channels, in email marketing, they sometimes do. Brands just need to accept they’re taking that risk and reducing their email marketing ROI in the process.

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Originally published on January 11, 2019, by Chad S. White. Last updated August 11, 2021.