The designer spent two weeks perfecting the email. Pixel-perfect spacing, beautiful typography, a hero image that loaded instantly. They previewed it in Chrome, nodded approvingly, and sent it to the CEO for approval.
The CEO opened it in Outlook 2016. The two-column layout had collapsed into a single column. The custom font had fallen back to Times New Roman. The hero image was blocked by default, leaving a broken placeholder. The carefully crafted email looked like it had been assembled by someone who'd never seen a computer.
This is the fundamental problem with email: there is no standard rendering engine. Gmail uses its own. Apple Mail uses WebKit. Outlook uses Microsoft Word's rendering engine (yes, the word processor). Each interprets your HTML differently, and each has its own bugs and limitations.
Preview tools solve this by showing you exactly what your email looks like across dozens of clients before you send. Here's what's available.
The major players
Litmus dominates the email preview market for good reason. Upload your HTML or send a test email, and within seconds you see screenshots from 90+ email clients and devices. The interface displays previews in a grid, making it easy to scan for problems. Click any preview to see it full-size, with tools to compare against other clients.
Beyond previews, Litmus analyzes your code for common issues—missing alt text, broken links, accessibility problems, spam trigger words. Their Builder tool lets you edit HTML and see changes reflected across all previews in real-time. The Analytics feature tracks opens and engagement after you send.
The price reflects the comprehensive feature set. Plans start around $99/month for individuals, scaling up for teams. For organizations sending critical emails, the cost is justified by prevented disasters. For occasional senders, it might be overkill.
Email on Acid offers similar preview capabilities at a slightly lower price point. Their preview library covers major clients across desktop, mobile, and webmail. The interface is functional if not quite as polished as Litmus. Where they differentiate is their unlimited testing on higher tiers—if your team iterates rapidly on designs, the unlimited model can be more economical than per-test pricing.
Their Campaign Precheck feature runs through a checklist of deliverability factors, spam testing, and accessibility compliance before you preview. It's a useful forcing function to catch issues early in the process.
Developer-focused options
Mailtrap started as an SMTP testing server—a fake inbox that catches test emails during development. They've expanded into a full email testing platform, including HTML previews across major clients.
The Email Sandbox remains their core strength. Point your development environment's SMTP settings at Mailtrap, and all outgoing emails get captured instead of delivered. You can inspect each email's HTML, plain text, headers, and attachments. The preview feature shows how the email renders across clients.
The free tier is generous—100 test emails per month with basic previews. Paid tiers add more volume and advanced features. For developers who need both a testing sandbox and preview capabilities, the integrated approach is convenient.
Parcel positions itself as a code editor for email, but includes preview capabilities. You write HTML (or MJML) in their editor, and see live previews as you type. The preview panel shows how your email renders across selected clients, updating in real-time as you make changes.
The collaboration features let teams work on emails together, with version history tracking changes. For teams doing serious email development, the integrated editing and preview workflow reduces context switching.
Budget-friendly alternatives
Testi.at offers email previews at a fraction of the cost of Litmus or Email on Acid. The preview library is smaller—around 30 clients compared to 90+—but covers the major ones that matter most. The interface is straightforward: upload HTML, get screenshots.
For teams that need basic preview capabilities without the full feature set of premium tools, Testi.at hits a reasonable price-to-value ratio. Just verify that the clients you care about are in their preview library before committing.
PutsMail from Litmus is a free tool for sending test emails to yourself. It's not a preview tool exactly—you still need to open the email in actual clients—but it's useful for quick tests. Enter your HTML, specify recipient addresses, and it sends the email. You can then check how it renders in your own email clients.
The limitation is obvious: you can only test clients you have access to. But for verifying rendering in Gmail, Outlook, and Apple Mail (the big three), it's a free option that works.
Self-hosted options
For organizations with strict data requirements, sending email HTML to third-party services might not be acceptable. Self-hosted alternatives exist, though they require more setup.
The challenge with self-hosting is maintaining the preview infrastructure. Email clients update constantly, and keeping a farm of virtual machines running different Outlook versions, mobile devices, and webmail accounts is significant operational overhead. Most organizations find that the security review of a reputable SaaS provider is easier than maintaining their own preview infrastructure.
That said, for basic testing, you can set up virtual machines with different Outlook versions and use BrowserStack or similar services for mobile device testing. It's manual and time-consuming, but it works for organizations that can't use external services.
What to actually test
Having preview tools is one thing; knowing what to look for is another.
Start with the big three: Gmail (web and mobile), Outlook (2016, 2019, and 365), and Apple Mail. These cover the majority of business email users. If your email looks good in these, you've addressed most rendering issues.
Pay special attention to Outlook. Its Word-based rendering engine has unique bugs that don't appear anywhere else. Background images often don't work. Certain CSS properties are ignored entirely. If something looks broken only in Outlook, that's normal—you'll need Outlook-specific fixes.
Check mobile rendering carefully. More than half of emails are opened on mobile devices, and a desktop-optimized email that's unreadable on phones is a failed email. Verify that text is large enough to read, buttons are large enough to tap, and the layout adapts sensibly to narrow screens.
Test dark mode. Many email clients now offer dark mode, and they handle it differently. Some invert colors automatically, which can make your carefully chosen palette look terrible. Some respect your specified colors. Preview tools increasingly offer dark mode previews—use them.
Finally, test with images disabled. Many email clients block images by default, especially in corporate environments. Your email should be comprehensible even when images don't load. Alt text should convey meaning, and the layout shouldn't collapse without images.
Frequently asked questions
How many email clients do I really need to test?
At minimum, test Gmail, Outlook, and Apple Mail—they cover the majority of users. Add mobile versions of each. If you have analytics showing which clients your audience uses, prioritize those. Testing 10-15 clients catches most issues without being overwhelming.
Why does my email look different in Outlook than everywhere else?
Outlook uses Microsoft Word's rendering engine instead of a browser engine. Word was never designed to render HTML email, so it handles CSS differently (or not at all). Many CSS properties that work everywhere else simply don't work in Outlook. This is why email HTML uses tables for layout—it's the only reliable method in Outlook.
Are free preview tools good enough?
For occasional emails and basic testing, free tools like PutsMail combined with manual testing in your own clients can work. For regular campaigns or transactional emails that affect revenue, paid tools provide more comprehensive coverage and catch issues that manual testing misses.
How often do email clients change their rendering?
More often than you'd expect. Gmail updates its rendering periodically, sometimes breaking emails that previously worked. Outlook updates with Office releases. Mobile clients update with OS versions. This is why retesting existing templates periodically matters—what worked six months ago might not work today.