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How to leverage email inbox preview testing for seamless rendering and increased engagement

Looks aren’t everything, or are they? Learn how email inbox preview tools ensure responsive rendering across device types, operating systems, and email clients.



Email rendering across email clients and devices goes beyond “does it look good”. How your emails load and function affects user experience and engagement, which impacts your deliverability. The trouble is, not all device types and email clients render in the same way.

So, how can you make sure what looks great on your email builder will look just as good on Outlook? How can you ensure that great layout will render properly on mobile?

In this post, we’ll fill you in on how email preview can help overcome rendering roadblocks and show you how you can use email preview tools to test before you send.

What does email preview mean?

Not to be confused with email preview text, the copy that comes up next to the subject in your inbox, an email preview is a view of how an email message will look in the inbox before it is sent - regardless of what device, email client, or operating system your recipients use to view it. The question is, do you need to preview your messages?

The answer is easy when you consider the statistics. When it comes to email, human attention span is shrinking, from about 13 seconds in 2018 to 9 seconds in 2022 – soon it’ll be shorter than the five-second food rule. We'd wager that the average email recipient spends less time reading an email with images that don’t load, or formatting that doesn’t fit their mobile screen than they do thinking about eating that tasty morsel that just fell off their plate.

The role of email inbox preview

Looks matter. Even for email. And you can think about the email previewing as that handy mirror you check yourself on before leaving the house. It’s that one final check before the email goes out.  Email previewing has two main benefits:

  • It ensures consistent appearance of your messaging across systems and devices.

  • It protects the relationship between responsive emails and improved engagement.

Since we’re talking about the five-second rule, we’ll keep the etiquette lesson going and take a page from the southern mother’s handbook. Email previewing tests if your messages will sit pretty and act nice in the inbox. It’s not all about looks, your messages have to behave and arrive at the inbox in working order.

Before we talk about why email previewing is important for rendering, and what some of the industry challenges are, we need to understand a bit more about email responsiveness. 

Email responsiveness is whether or not a message is compatible visually across devices, email clients, and operating systems, and if it is compatible functionally. Do all the buttons work? Do the links register? If you’ve built an interactive email, is it universally accessible?

Learn more: Creating responsive emails can be challenging. Here’s our top 12 tips for creating responsive emails that render.

Why is email preview important for email rendering?

Now that we’ve brushed up on email rendering, here’s how email preview fits in. Not all devices and email clients render emails with the same standards. It’s your job as the sender to develop an email that is compatible with them all. (We were tempted to put a Lord of the Rings reference here, but we’ll hold it in.) 

Since well-rendered emails speak to consistency, email previewing has to be a multi-perspective practice.

In other words, it is important to preview your emails from multiple angles, or from the view of multiple screen experiences and email clients. When we talk about email clients, big names probably jump into your head, including Apple, Gmail, or Microsoft Outlook. But within these three names lie hundreds of email clients.

Now things are starting to get a bit more complex.

“Email rendering across multiple email clients is a constant challenge for businesses. With a multitude of email clients and devices in use today, senders must navigate the intricacies of HTML and CSS compatibility, responsive design, and rendering inconsistencies to ensure emails are received and displayed correctly across all platforms. Email preview tools can help overcome these challenges by testing emails before they are sent to avoid broken elements that can harm brand image, downgrade user experience, and impact engagement and conversions.”

Kate Nowrouzi, VP of Deliverability at Sinch Mailgun

The challenges of email rendering

The biggest challenge of email rendering is that there is no standardization among email clients. None. Zero. Zilch. In fact, each client has its own special email rendering engine that controls how your sent messages are displayed. This means that if you test your emails for Apple, they may not look the same in Gmail. But it actually goes deeper. 

Each primary email client, like Gmail, has a series of email clients under it for web, mobile, and desktop views. And within those nests are further clients for each Android or iPhone OS. Here’s an example:

Mobi­le Emai­l Clie­nts

Mobi­le Emai­l Clie­nts

Gmai­l App Pixe­l 2

Andr­oid 8

Gmai­l App Pixe­l 3

Andr­oid 9

Gmai­l App Pixe­l 4

Andr­oid 10

Gmai­l App Pixe­l 4

Andr­oid 10 (Dar­k Mode­)

Gmai­l App Pixe­l 5

Andr­oid 11

Gmai­l App Pixe­l 5

Andr­oid 11 (Dar­k Mode­)

Gmai­l App Pixe­l 6

Andr­oid 12

Gmai­l App Pixe­l 6

Andr­oid 12 (Dar­k Mode­)

Gmai­l App Pixe­l 6

Andr­oid 13

Gmai­l App Pixe­l 5

Andr­oid 13 (Dar­k Mode­)

iPad­ Pro (11 in)

iOS 15

iPad­ Pro (12.­9 in)

iOS 15

iPho­ne XR

iOS 14

iPho­ne XR

iOS 14 (Dar­k Mode­)

iPho­ne SE 2nd Gen

iOS 14

iPho­ne SE 2nd Gen

iOS 15

iPho­ne 11

iOS 14

Since email clients aren’t standardized, it’s best to build messages with responsive designs that are capable of adapting to different rendering engines and standards. We linked to our best practices earlier in this post but here are a few key tips:

  • Adopting mobile-first design principles that work within the boundaries of smaller screens

  • Using media queries and fluid layouts that are adaptable

  • Optimizing images for different screen sizes

Sounds too complicated? Check out our responsive email templates and use them to build your own!

The email rendering rabbit hole is a long, long tunnel. The best way to navigate it is to simplify your sending with best practices and email previewing before you send.

Learn more: Dive deeper into email rendering across different clients and devices with our top 12 tips.

Email preview and deliverability

Now we’ll cut into the meat of email inbox preview testing and the impact it has on your deliverability.

The number one benefit to an email that renders perfectly across clients and devices is in an increased engagement rate. Correctly rendered emails support user interaction and click-through because all of the elements are functioning and visible to the user.

Testing responsiveness and ensuring rendering of images, fonts, layouts, hyperlinks, etc., means you are sending compatible messages that can adapt to different rendering engines. Correctly rendered emails are less likely to be flagged as spam, and also more likely to drive positive engagement – a great sign for mailbox providers that your email is legitimate and wanted.

Utilizing online testing platforms to preview emails on different devices and across a large volume of email clients allows you to troubleshoot and resolve any rendering issues prior to sending your campaigns, protecting your deliverability from the negative impact of broken emails.

Email preview tools are great for email marketers and for your email campaigns, but don’t forget about your transactional messaging.  Use our templates API to build and optimize your notification emails like abandoned cart and password reset.

How do I preview emails across device types and email clients?

Ok, so now that we’ve convinced you that previewing your emails is in your best interest, how do you do it?

There are lots of solutions on the market to choose from, and before we get into why ours is the best, we need to say that there is no easy way to manually preview your messages across all devices and email clients. To try it, you’d need an arsenal of devices, with varying operating systems, and enough people to check the sea of test messages you’d have to send. That’s hundreds of phones and computers.

Trust us, not only do you need the support of a platform, you want it.

Email Previews with Mailgun Optimize

There are a lot of features associated with email previewing.  Here are some key elements you’ll need in your email previewing tool – and that Mailgun Optimize offers – to ensure a high level of responsiveness and engagement.

  • Email clients. Lots of them: In general, you want your email preview tool to allow you to test across the largest number of devices and email clients possible. With Mailgun Optimize, that’s over 100 email clients for web, mobile, and desktop.

  • Eye-catching. Get the attention of your recipients: You want to ensure your emails stand out once they reach the inbox. Previews apply to more than just the message body, so ensure you find a tool with which to test your subject lines, sender name, and how your pre-header text appears.

  • Optimize elements: Prepare for user preference: Alongside your unique CSS and HTML designs, your recipients also have control over additional visuals. The most common is dark mode, with over 82% of people using dark mode in some way on their devices. If your messages aren’t ready to go dark, you could be in trouble. Make sure you can test it.

Mailgun Optimize’s Email Previews offers this and much more. If you want to learn more about the tool and its features, check out our Email Previews product page.

Learn more: Build an email preview checklist to help you keep up with best practices.

Final thoughts

“Well, I mean you wouldn't buy a skirt without asking your friends first if it looks good on you.”

Gretchen Wieners

Don’t risk souring your looks and your reputation with broken emails. You only have one chance to make a first impression, and when sending, email preview is your second opinion. There is zero chance that you can create the perfect email and send it off into the world – error free – without testing. There are just too many differences between email clients and device restrictions for that to work.

That’s why responsive emails are the key to rendering and email inbox preview testing is the secret to guaranteeing responsive emails. Just because it looks good in your environment doesn’t mean it looks good on the mobile device in the palm of your biggest client’s hand.

Email preview is effective but also simple. Try out our solution and experience how previewing your emails fits into your larger deliverability strategy.

Unmatched email deliverability

Mailgun Optimize Deliverability Suite

Never send another broken email. Mailgun Optimize deliverability apps and services give you the data and the resources you need to build a winning email program.

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