Quick answer: Optimise for Apple Mail and Gmail first, because together they account for almost 90% of all email opens (Apple 62.26% and Gmail 27.03% in Litmus’s July 2026 email client market share data). Then test in classic Outlook for Windows, which only has around 6% of opens globally but still dominates many business inboxes and renders email with Microsoft Word’s engine. Design mobile-first, make your emails accessible, check every email in dark mode, and use your own campaign reporting to confirm which clients your audience actually uses.
There’s a message every email builder dreads. It usually lands a few minutes after a campaign goes out, and it comes with a screenshot. The client’s beautifully designed newsletter, now sporting square buttons, a missing background image and a headline that has switched itself to Times New Roman. Nine times out of ten, the culprit is Outlook.
It’s a useful reminder that nobody reads your email in your design tool. They read it in an email client, and every client has its own opinion about how HTML should look. There are hundreds of them out there, which sounds daunting until you look at the data.
A handful of clients account for nearly every open you’ll ever get. Once you know which ones, you can focus your design and testing effort where it counts. So let’s look at who owns the inbox in 2026, which clients deserve your attention, and how to test without losing an afternoon to it.
Email client market share in 2026
Apple and Gmail dominate email opens by a wide margin, with Outlook a distant third. These are the top clients in Litmus’s July 2026 report, calculated from over a billion email opens.
| Rank | Email client | Share of opens |
|---|---|---|
| 1 | Apple (Mail, iPhone, iPad and Mail Privacy Protection) | 62.26% |
| 2 | Gmail (web and mobile apps) | 27.03% |
| 3 | Outlook (desktop) | 5.83% |
| 4 | Yahoo Mail | 2.59% |
| 5 | Google Android (native mail app) | 1.45% |
| 6 | Outlook.com | 0.42% |
| 7 | Thunderbird | 0.23% |
Source: Litmus Email Client Market Share, July 2026. Apple’s figure includes privacy-protected opens, which inflate it. More on that below.
Put Apple and Gmail together, and you’re looking at nearly nine in every ten opens, with the rest spread across a long tail of smaller clients. That’s why trying to perfect your email for every one of them is rarely a good use of time.
Which email clients should you optimise for?
Start with Apple Mail and Gmail, then add classic Outlook if you email businesses. Most other clients are covered by sound, standards-based code and a quick test pass.
Apple Mail
Apple Mail is the default app on every iPhone, iPad and Mac, which explains its huge share. The good news is that it’s also the most forgiving client you’ll design for. It supports modern HTML and CSS well, so your email will usually look close to how you designed it. Your main jobs here are checking the layout on smaller iPhone screens and making sure everything still works in dark mode.
Why your Apple Mail open rates can’t be trusted
Apple’s Mail Privacy Protection loads email content in the background, which can register an open whether or not a person actually read your email. That inflates both Apple’s market share and your open rates.
Mail Privacy Protection launched in September 2021, and Apple Mail users who switch it on effectively disable open tracking. Litmus estimates that it now affects roughly 55-60% of all email opens. That’s a big chunk of your reporting built on opens you can’t verify.
The fix is to judge your campaigns on signals a machine can’t fake. Clicks, replies, form submissions and conversions tell you far more about what’s working than open rate does. Genuine engagement also feeds your sender reputation, which plays a big part in improving your email deliverability.
Gmail
Gmail is the largest email provider by account count, with around 1.8 billion active accounts. It supports less CSS than Apple Mail and has a few habits worth knowing about. The big one is clipping: Gmail cuts off any message larger than 102KB and hides the rest behind a “View entire message” link, which can take your unsubscribe link and tracking pixel with it. Lean code and light images keep you under the limit, and our guide to email image size best practices covers the file sizes to aim for.
Outlook
Outlook looks small on the global chart, with 5.83% for the desktop app and 0.42% for Outlook.com. Don’t write it off, though. Classic Outlook for Windows renders email with Microsoft Word’s engine, which makes it the hardest client to design for, and if you email businesses it matters far more than its global share suggests. We cover both points in detail below.
Yahoo Mail and everything else
Yahoo Mail (2.59%) and native Android mail apps (1.45%) round out the rest of the meaningful share. Both handle standard email code reasonably well. If your email is built on a solid, table-based foundation and looks good in Apple Mail, Gmail and Outlook, a single test pass is usually all these clients need.
Email marketing to businesses: what’s different
If you send B2B email, expect far more of your audience to use Outlook than the global chart suggests, because Microsoft 365 is one of the two biggest business email platforms.
Microsoft 365 and Google Workspace are the two biggest business email platforms, and they’re roughly neck and neck. BuiltWith detects Google Workspace on around 19 million live websites and Exchange Online, the email service behind Microsoft 365, on around 16 million. Microsoft also reported more than 450 million paid commercial seats in January 2026. In other words, if you email businesses, a large share of your audience is likely to have an Outlook mailbox.
So why does Outlook look so small in open data? Not everyone on Microsoft 365 reads their email in desktop Outlook, since plenty of people check it on their phones. Open tracking also relies on images loading, and desktop Outlook is often set to block images by default. Litmus notes that image blocking can leave some clients under-represented in its data, so Outlook’s real share of business inboxes is likely higher than the chart suggests.
Classic Outlook vs new Outlook: what designers need to know
There are now two different desktop apps for Windows that are both called Outlook, and they display your emails very differently. Classic Outlook needs special design care, while new Outlook behaves much more like a modern webmail client.
Classic Outlook is the long-standing desktop app: Outlook 2016, 2019, 2021 and 2024, plus the classic Outlook app that comes with Microsoft 365. Since 2007, it has used Microsoft Word’s engine to display HTML emails. Word was built for documents rather than web pages, so it only understands a limited set of HTML and CSS, and it quietly drops anything it doesn’t recognise. New Outlook for Windows is a rebuilt app that uses a browser-based engine, much like Outlook.com. Outlook for Mac, iPhone and Android don’t use the Word engine at all.
Design considerations for classic Outlook
If a meaningful share of your audience uses classic Outlook, build these into your templates:
- Layout: use HTML tables for structure. Flexbox, grid, floats and max-width are ignored or unreliable, so multi-column layouts need nested tables, often with Outlook-only code to hold column widths in place.
- Spacing: padding and margins on div and image elements are ignored. Apply padding to table cells instead.
- Buttons: rounded corners and padding on links don’t render. Use “bulletproof” buttons built from table cells, or VML (Microsoft’s own vector graphics code), so they stay clickable and on-brand.
- Background images: CSS background images don’t display. Add a VML fallback, or design the section so it still works on a solid background colour.
- Fonts: web fonts aren’t supported and can fall back to Times New Roman. Always include web-safe fallbacks such as Arial or Georgia in every font stack.
- Images: set the width and height as HTML attributes. If you only size images with CSS, classic Outlook can display high-resolution (2x) images at their full size. On Windows screens set above 100% scaling, it can also stretch images and tables unevenly, which a standard Outlook scaling fix in your email’s code prevents.
- Responsive design: media queries aren’t supported, so classic Outlook always shows your desktop layout. A fixed-width layout, usually around 600px, keeps things predictable.
Well-built templates from established email platforms handle most of this for you. Custom-coded templates and ambitious designs are where classic Outlook tends to bite, so test those carefully.
Designing for new Outlook
New Outlook supports modern HTML and CSS, so rounded corners, background images, web fonts and flexible layouts generally work as intended. It renders much like Outlook.com, which still has quirks of its own, so it’s worth testing separately rather than assuming it behaves exactly like a web browser.
Microsoft is replacing classic Outlook gradually and will support it until at least 2029, so both versions will share business inboxes for years. Design emails that work cleanly in classic Outlook, add polish for modern clients, and keep your Outlook-specific code until your own data shows classic Outlook has faded from your list.
Email design considerations for 2026
How people read email has changed a lot in the last few years. Four trends deserve a place in every design brief: mobile reading, dark mode, accessibility and AI-generated inbox summaries.
Design for mobile first
More than 60% of emails are now opened on mobile devices. A single-column layout, readable body text, and large buttons with plenty of space around them will serve you well across every major client.
Plan for dark mode
Litmus reports that over 25% of users now read in dark mode, and each client handles colour inversion slightly differently. Watch out for dark logos that vanish on dark backgrounds, text baked into images that can’t adapt, and brand colours that shift in ways you didn’t intend.
Build accessible emails
Accessibility is the most neglected area of email design. The Email Markup Consortium’s 2026 accessibility report found serious or critical issues in 99.88% of the 376,348 emails it analysed, and only eight emails passed. The most common problems are easy to fix: images without alt text, missing language attributes, and layout tables that screen readers try to read as data tables. Accessible emails are easier for everyone to read, and many of the same fixes help your emails make sense when images are blocked.
Write for AI inbox summaries
Apple Mail and Gmail now use AI to summarise emails. On devices with Apple Intelligence, Apple Mail can show an AI-written summary in place of your preview text, while Gmail’s Gemini summaries mostly appear once an email is open. There’s no reliable data yet on how many subscribers see these summaries, and Apple Intelligence only runs on newer devices, so treat this as a trend to prepare for rather than a crisis. Litmus’s guide to AI summaries covers the detail, but the practical response is simple. Put your key message and call to action in the first lines of live text, keep each email focused on one main idea, and don’t bury important content inside images, which AI can’t read.
How to find out which email clients your audience uses
Global email client market share is a starting point, but your own data is the real answer. A B2B list full of corporate addresses will look very different from a consumer list, and Litmus itself notes that your own top ten may not match the global chart.
Most email marketing platforms break down opens by device or email client in their campaign reporting, so check there first. For a more detailed view, including Outlook versions and dark mode usage, a dedicated analytics tool like Litmus Email Analytics tracks client data across your sends. Just remember that Mail Privacy Protection will make Apple look bigger in your reports than it really is.
A simple testing workflow before you send
Follow these six steps before every important campaign.
- Check your own client data to confirm which clients matter most to your audience.
- Build with email-safe code: table-based layouts, inline CSS, and Outlook fallbacks where you need them.
- Preview your email in Apple Mail on iPhone, Gmail on web and mobile, and classic Outlook for Windows.
- Switch to dark mode and check your logo, text and buttons.
- Run an accessibility check and make sure every image has meaningful alt text.
- Send a real test to yourself and read it on your phone before you schedule it.
Useful testing tools
- Litmus: previews your email across dozens of clients and devices, including dark mode.
- Email on Acid: email previews plus accessibility and code checks.
- Can I email: free lookup of which HTML and CSS features each email client supports.
Final thoughts
You don’t need to design for hundreds of email clients. You need to get three right. Nail Apple Mail and Gmail, respect Outlook’s quirks if you email businesses, and check every send on mobile, in dark mode and for accessibility. Do that, and the dreaded screenshot should become a rare visitor to your inbox.
Need a hand with your email templates?
If your emails keep breaking in Outlook or you’d like your templates rebuilt to render reliably everywhere, get in touch with Cybersolve for an email marketing consultation.
FAQs on email client optimisation
What is the most popular email client in 2026?
Apple is the most popular email client by share of opens, with 62.26% in Litmus’s July 2026 data, followed by Gmail at 27.03% and Outlook desktop at 5.83%. Apple’s figure is inflated by Mail Privacy Protection, which can register opens that no person made, but Apple and Gmail together clearly account for almost 90% of all email opens.
Do I still need to test my emails in Outlook?
Yes, especially if you email businesses. Outlook only has around 6% of global opens, but Microsoft 365 is one of the two biggest business email platforms, and desktop Outlook is often undercounted because it blocks images by default. Classic Outlook for Windows also uses Microsoft Word’s rendering engine, which ignores many modern HTML and CSS features. It’s the client most likely to break your email, so it’s worth testing every campaign in it.
Why do my emails look broken in Outlook?
Usually because the recipient is using classic Outlook for Windows, which displays HTML emails with Microsoft Word’s rendering engine. Word supports only a limited set of HTML and CSS, so rounded buttons, background images, web fonts and modern layouts can break. Table-based layouts, padding on table cells, bulletproof buttons, VML background images, web-safe fallback fonts and image width attributes fix most of these problems.
Do most businesses use Microsoft 365 or Google Workspace for email?
They’re roughly level. Microsoft 365 and Google Workspace are the two biggest business email platforms: technology tracker BuiltWith detects Google Workspace on around 19 million live websites and Exchange Online, the email service behind Microsoft 365, on around 16 million. Microsoft also reports more than 450 million paid commercial Microsoft 365 seats, so if you email businesses, a large share of your audience is likely to have an Outlook mailbox.
When will classic Outlook be discontinued?
Eventually, but not soon. Microsoft is gradually replacing classic Outlook, which uses Microsoft Word’s rendering engine, with the new Outlook for Windows, which uses a modern browser-based engine. The opt-out stage for enterprise customers, when new Outlook becomes the default, is scheduled for March 2027. No final cutover date has been announced, and Microsoft will support existing classic Outlook installations until at least 2029, so both versions will be in use for several years.
Why are my Apple Mail open rates so high?
Apple’s Mail Privacy Protection loads email content in the background for users who switch it on, which can register an open even if the person never reads the email. It affects roughly 55-60% of all email opens, so open rates are no longer a reliable measure of engagement. Clicks, replies and conversions give you a much more accurate picture.
How do I find out which email clients my subscribers use?
Check the campaign reports in your email marketing platform, which usually break down opens by device or email client. For more detail, such as Outlook versions and dark mode usage, a tool like Litmus Email Analytics can track client data across your sends. Keep in mind that Mail Privacy Protection will overstate Apple’s share.
Should I design my emails for dark mode?
Yes. Over 25% of users read their email in dark mode, and each client handles colour inversion differently. Check that your logo stays visible on dark backgrounds, avoid putting important text inside images, and preview every email in dark mode before you send it.
How can I make my marketing emails more accessible?
Start with the basics: add descriptive alt text to every image, set a language attribute, mark layout tables as presentational, use real text rather than text inside images, and keep colour contrast high. The Email Markup Consortium’s 2026 report found serious or critical accessibility issues in 99.88% of the emails it tested, so even simple fixes put you ahead of most senders.
Do AI inbox summaries change how I should write emails?
Yes. On devices with Apple Intelligence, Apple Mail can replace your preview text with an AI-written summary, and Gmail offers Gemini summaries once an email is open. Put your main message and call to action near the top in live text, keep each email focused on one idea, and avoid placing key content inside images, which AI summaries can’t read.
