How to Test Your Email Campaigns Across Devices

You spent hours crafting the perfect email campaign — the subject line is punchy, the copy is compelling, and the call-to-action is impossible to miss. You hit send. Then the replies start rolling in: “Your email looks broken.” “The images aren’t loading.” “I can’t read this on my phone.”

It’s every email marketer’s nightmare — and it’s entirely avoidable.

Testing your email campaigns across devices before you send them is one of the most important (and most overlooked) steps in the email marketing process. With subscribers opening emails on everything from a 27-inch desktop monitor to a five-year-old Android phone, a message that looks great in one environment can fall completely apart in another. This guide walks you through everything you need to know to make sure your emails land well — every time.

Why Cross-Device Testing Matters More Than Ever

The way people read email has changed dramatically. According to industry data, more than 60% of emails are now opened on mobile devices. That means if your campaign looks beautiful on desktop but breaks on a smartphone, the majority of your audience is having a poor experience.

But device type is only part of the puzzle. The email client — the application people use to read their email — plays an equally important role in how your message renders. Gmail strips certain CSS styles. Outlook on Windows uses its own rendering engine that’s decades behind modern web standards. Apple Mail supports dark mode in ways that can completely invert your carefully chosen color scheme.

The result is that a single email can look radically different depending on whether someone is reading it in Gmail on an iPhone, Outlook on a Windows desktop, or Apple Mail on a Mac. Without testing, you’re essentially flying blind.

Step 1: Start With an Email Preview Tool

The most efficient way to catch rendering issues before you send is to use a dedicated email preview tool. Platforms like Litmus, Email on Acid, and Mailtrap let you see how your email renders across 90 or more combinations of email clients and devices — simultaneously, within minutes.

These tools work by sending your email through their testing infrastructure, which captures actual screenshots of how the email renders in each client. What you see is exactly what your subscriber would see. That makes them invaluable for catching layout breaks, font fallback failures, missing images, and button display issues before a single real recipient is affected.

When you run a preview test, pay particular attention to these environments, as they account for the majority of email opens worldwide:

• Gmail (web, iOS, Android)
• Apple Mail (iPhone, iPad, Mac)
• Outlook 2016, 2019, and 2021 on Windows
• Samsung Mail on Android
• Yahoo Mail

If your email looks good across those environments, you’re covering the vast majority of your audience. That said, preview tools aren’t perfect — they simulate renders rather than using live clients — which is why real-device testing is also essential.

Step 2: Send to Real Devices

There’s no substitute for opening your email on actual hardware with actual email apps. Simulated screenshots are a great starting point, but nuances like scroll behavior, touch target size, load timing, and interactive elements can only be truly evaluated on a real device.

Set up a small collection of test accounts and devices that covers the major combinations. Ideally, your testing kit should include:

• An iPhone running the native Mail app
• An iPhone with the Gmail app installed
• An Android phone (Samsung Galaxy or Pixel are good representatives)
• A Windows desktop running Outlook
• A Mac running Apple Mail
• A web browser on desktop with Gmail open

When you send your test, go through each device methodically. Don’t just glance at it — read it the way a subscriber would. Scroll through it. Tap the buttons. Try clicking your links. Check how the preheader text looks in the inbox view before you even open the message.

Step 3: The Outlook Problem

Outlook deserves its own section, because it is uniquely problematic — and it continues to be one of the most widely used email clients in the world, particularly in corporate environments.

Unlike every other major email client, which uses a browser-based rendering engine, Outlook on Windows uses Microsoft Word’s rendering engine. That means it doesn’t support many modern CSS properties, including flexbox, CSS grid, and background images applied via CSS. It also handles fonts, margins, padding, and image rendering in its own idiosyncratic way.

If you don’t test specifically in Outlook, you are almost certainly sending a broken experience to a significant portion of your list. Some things to watch for in Outlook:

Background images will often not render. Always have a solid background color fallback.

Custom fonts will revert to a system font. Make sure your fallback font (typically Arial, Georgia, or Times New Roman) still looks good and doesn’t break your layout.

Padding and margin behavior is inconsistent. Test any content that relies on spacing to look correct.

Buttons created with CSS may render incorrectly. Use bulletproof buttons — table-based button markup that renders reliably in Outlook — rather than styled <a> tags alone.

Step 4: Dark Mode Testing

Dark mode has become a mainstream preference. iOS, Android, macOS, and Windows all support it, and a growing number of email clients — including Apple Mail, Gmail on iOS, and Outlook — offer dark mode rendering for emails.

The challenge is that email clients handle dark mode in different and sometimes unpredictable ways. Some invert all colors automatically. Some partially invert colors. Others pass the email through unchanged and let the device’s OS settings handle it. The result is that an email with a white background and dark text might look fine in light mode and then flip to a nearly unreadable dark background with light text in dark mode — or it might have some elements inverted and others unchanged, creating a patchy, inconsistent look.

To test for dark mode, enable dark mode on your iPhone (Settings → Display & Brightness → Dark) and open your test email in Apple Mail, which has one of the most aggressive dark mode implementations. Check Gmail on iOS in dark mode as well. Then look at Outlook on a Windows machine with dark mode enabled.

The goal isn’t necessarily to design a beautiful dark mode version of every email, though that’s ideal. At minimum, ensure your email is readable and not visually broken when dark mode is active.

Step 5: Check the Specific Elements That Break Most Often

When reviewing your test sends, it helps to have a mental checklist of the elements most likely to cause problems. Here’s what to look at on every test:

Subject line and preheader text. Open the email in the inbox view — don’t jump straight to the message. Check that your subject line isn’t cut off and that your preheader text is adding value rather than showing garbage text like “View this email in your browser…” On most mobile clients, you have roughly 40–50 characters before the subject line is truncated.

Images. First, check whether your images are loading. Then deliberately disable image loading (most desktop clients have this option) and look at your email again. This simulates the experience of subscribers who have image loading turned off — a common setting in corporate Outlook environments. Your email should still communicate its core message even without images, which is why alt text is so important.

Buttons and calls-to-action. On mobile, tap your buttons with your actual thumb — not the tip of your finger. Apple’s human interface guidelines recommend a minimum tap target of 44×44 pixels for a reason. If your button is smaller than that, mobile users are likely to miss it or tap the wrong thing.

Text size and readability. Body copy should be at least 14px, and ideally 16px, on mobile. Font sizes that look fine on a desktop monitor become eye-straining on a phone screen. Check that your text isn’t so small that subscribers need to pinch and zoom to read it.

Multi-column layouts. Two-column or three-column layouts are a common source of mobile rendering failures. On mobile, columns should stack vertically into a single column. If your email uses a multi-column layout, verify that the stacking behavior works correctly and that the resulting single-column version still looks intentional rather than accidental.

Link spacing. On mobile, links that are placed too close together can be difficult to tap accurately. Review any areas of your email where multiple links appear in close proximity.

Step 6: Don’t Forget the Plain-Text Version

Every email you send should have both an HTML version and a plain-text version. The plain-text version is what subscribers see if their email client doesn’t render HTML, and it’s also what many spam filters and email clients reference when evaluating your message.

Most email service providers generate a plain-text version automatically from your HTML, but automated generation often produces a mess of unstructured text. Take a few minutes to manually clean up your plain-text version so that it’s actually readable — with logical structure, clear spacing, and all your key links spelled out in full (since hyperlinks don’t work in plain text).

Test your plain-text version by temporarily enabling it in your email client’s settings or by using a tool like Mail-Tester.

Step 7: Run a Spam Check

Rendering is only half the battle. Your beautifully designed email is useless if it ends up in the spam folder.

Before you send, run your email through a spam score checker. Mail-Tester (mail-tester.com) is a free and widely used tool — it gives you a temporary email address to send your test to, then analyzes your email and returns a spam score along with specific issues to address.

Your email service provider may also have a built-in spam checker. Common issues that hurt your spam score include:

• Missing or misconfigured SPF, DKIM, or DMARC authentication records
• Excessive use of spam-trigger words (“FREE!!!”, “ACT NOW”, “CLICK HERE”)
• A high image-to-text ratio with very little actual text content
• Broken links or links pointing to flagged domains
• A mismatch between your “From” name and email domain

Fixing these issues before you send can make a meaningful difference in deliverability.

Step 8: Test Load Time

A slow-loading email is a frustrating experience, particularly for subscribers on mobile data connections. Large, unoptimized images are the most common culprit.

As a general rule, keep your total email file size under 100KB for the HTML itself, and make sure individual images are compressed appropriately. Tools like TinyPNG or Squoosh can significantly reduce image file sizes without visible quality loss. Litmus also provides estimated load time data as part of its preview testing reports.

Building a Pre-Send Testing Workflow

Ad hoc testing is better than nothing, but the real gains come from building a consistent, repeatable testing process that your whole team follows before every send.

Here’s a workflow that covers all the essential bases without becoming overwhelming:

1. Draft and code review. Before testing, have a second set of eyes review the email’s copy and HTML for obvious errors.

2. Preview tool scan. Run your email through Litmus, Email on Acid, or a similar tool and review the results for the top 10–15 client/device combinations that match your audience’s actual usage.

3. Dark mode check. Specifically review dark mode rendering in Apple Mail and Gmail on iOS.

4. Real-device send. Send to your internal test seed list and open on at least three real devices: an iPhone, an Android phone, and a desktop Outlook client.

5. Plain-text review. Read through the plain-text version and clean it up if needed.

6. Spam check. Run through Mail-Tester or your ESP’s built-in checker and resolve any issues.

7. Final human review. One last read-through on a real device, reading the email exactly as a subscriber would.

Build this checklist into your team’s pre-send process and make it non-negotiable. The time it takes — typically 30 to 60 minutes for a complex campaign — is a small investment compared to the cost of sending a broken email to your entire list.

The Mobile-First Mindset

Perhaps the most important shift you can make in how you approach email design and testing is to treat mobile as your primary format rather than an afterthought.

When you design for mobile first, you build emails that are simple, focused, and easy to read on a small screen — and then scale up gracefully to desktop. You use single-column layouts, large tap targets, short subject lines, and concise copy. The result is an email that works well everywhere, not just in ideal conditions.

Testing is what closes the loop. It turns assumptions into certainty, catches the issues you didn’t anticipate, and gives you the confidence to hit send knowing your message will land the way you intended — regardless of where, when, or how your subscribers choose to open it.