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Why is email rendering so complex?


Editor’s note: Email rendering has grown in complexity since this article was first published. Chad has since shared an updated view of the different potential renderings a single email might have, including that there are more than 300,000 potential renderings for an email.

A print campaign has one rendering. Website rendering is significantly more complex, since sites can look different depending on a device’s operating system, browser, and screen size. But even that is no match for the complexity of email rendering.

In addition to OS, browser, and screen size, the email marketing platform the brand uses, the subscriber’s email client, and whether images are enabled or blocked dramatically affect email rendering. If we bundle webmail clients together with browsers, there are five major factors that determine how an email renders:

  1. Email service providers (ESP)
  2. Operating systems
  3. App and web-based email clients
  4. Screen sizes
  5. Images enabled/blocked

Let’s look at each of these layers.

Email Service Providers

Before you send your email, your ESP can change your email’s code. In addition to adding tracking pixels and redirecting links for tracking purposes, your email marketing platform may strip out certain parts of code.

This is generally for your benefit. For instance, they remove code that may get your emails caught in content filters, resulting in deliverability issues. However, some of these changes may harm the rendering or functionality of your emails—which is why Litmus always recommends previewing emails in Checklist before you send from your ESP.

Be extra mindful of this fact if you’ve recently changed ESPs or plan to. Your emails may have been rendering fine, but your new email platform may cause some of your emails to break.

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Preview emails with images-off and get notified if any of your images are missing ALT text with Litmus Checklist.

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Depending on how you define an email service provider, you could say there are upwards of 400 email marketing tools. But let’s assume there are approximately 150 ESPs of reasonable scale.

That said, we’re talking about how your emails render and you don’t use that many ESPs. On average, brands use 1.8 email marketing platforms to send their marketing emails. For the sake of easier math, let’s round that up to an even 2.


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Operating Systems

The OS of your subscriber’s computer, tablet, smartphone, or other device limits or enhances the capabilities of the email client that sits on top of it. While a handful of major OSs dominate the market, many different versions of those OSs are in use by consumers. For instance, Windows 10, 8, and 7 all have considerable market share.

Across the major desktop and mobile operating systems including Windows, OS X, Android, and iOS, let’s say there are about 15 OS versions of reasonable scale.

App and Web-based Email Clients

The email client is the heart of email rendering. Email clients can run in desktop, mobile, and web environments. Web-based email clients have an extra layer of complexity because they render differently in different browsers. For example, Gmail functions a little differently depending on whether a subscriber is using Chrome, Firefox, Internet Explorer, or Safari.

Litmus has identified about 1,000 webmail clients around the world, plus roughly 250 mobile email apps, along with a much smaller number of desktop email apps. Litmus Email Previews currently supports more than 70 different email apps and webmail-browser combinations, and that number is growing.

Ignoring the email clients with the smallest market share and including the necessary webmail-browser combos, let’s assume that every brand should be concerned about approximately 50 email clients that have reasonable scale in their markets.

Screen Size

The size of the subscriber’s screen or monitor also has a big impact on email rendering. Interacting with an email on a 21-inch monitor and a smartphone with a 4-inch screen are dramatically different experiences.

In the smartphone category alone, there are hundreds of unique screen dimensions. To simplify things, let’s assume there are just 5 screen size categories that we care about: desktop, laptop, tablet, phablet (phone-tablet hybrids), and smartphone.

Images On/Off

Lastly, images can be enabled and blocked in many email clients. This generally creates two very different renderings of an email—sometimes wildly different renderings.

Anthropologie email with images enabled and blocked

So let’s double the number of email rendering possibilities.

The Grand Total

If we put all those factors together, we get…

2 email service providers x 15 operating systems x 50 email clients x 5 screen sizes x 2 image states =

Every email that marketers send has approximately 15,000 potential renderings (and that’s using conservative math).
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Plus, that’s ignoring instances where an email client doesn’t support HTML and displays the plain-text version or watch-html version of the email instead, and instances when a screen reader is used.

Some might view that as an argument for sending more plain-text emails. However, HTML emails have generally proven to be much more effective. So, this exercise in rendering permutations is simply a reminder that rigorous testing should be a part of every brand’s email workflow.

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