The email looked perfect in Gmail. Clean layout, responsive design, carefully crafted copy. Then someone opened it in Outlook 2019 and saw a mangled mess of broken tables and missing images. The CEO was not amused.
This is the reality of email development: what works in one client breaks spectacularly in another. Unlike web browsers, which have largely converged on standards, email clients remain a fragmented landscape of quirks, limitations, and outright bugs. Outlook still uses Word's rendering engine. Gmail strips most CSS. Apple Mail does its own thing entirely.
Testing isn't optional—it's survival. Here are the tools that make it possible.
Inbox preview tools
Before you send anything, you need to see how it renders across clients. These tools show you exactly what recipients will see.
Litmus remains the industry standard for email previews. Upload your HTML and see instant screenshots across 90+ email clients and devices. The interface shows desktop, mobile, and webmail views side by side, making it easy to spot rendering issues. Their code analysis catches common problems before you even preview—missing alt text, broken links, accessibility issues. The price tag is significant (starting around $99/month), but for teams sending critical emails, it pays for itself in prevented disasters.
Email on Acid offers similar preview capabilities at a slightly lower price point. Their checklist feature walks through deliverability factors, spam testing, and accessibility compliance. The previews are comprehensive, though the interface feels slightly dated compared to Litmus. Where they shine is their unlimited testing on higher tiers—useful for teams iterating rapidly on designs.
Mailtrap has evolved from a simple SMTP testing server into a full email testing platform. Their HTML Check feature previews emails across major clients, and their Email Sandbox captures test emails before they reach real inboxes. The free tier is generous enough for individual developers, and the pricing scales reasonably for teams.
Local development tools
When you're building emails, you need fast feedback loops. These tools let you develop and test without sending anything.
Mailhog is a local SMTP server that captures all outgoing email. Point your application's SMTP settings at Mailhog, and every email gets caught in a web interface instead of being delivered. You can inspect headers, view HTML and plain text versions, and download raw messages. It's perfect for development environments where you don't want test emails escaping into the wild. Installation is trivial—a single binary with no dependencies.
MailCatcher does essentially the same thing with a Ruby-based implementation. The web interface is clean and functional, showing all captured emails with their full content. Some developers prefer it for its simplicity; others find Mailhog's Go implementation faster. Both are free and open source.
Papercut is the Windows equivalent—a simple SMTP server that displays received emails in a desktop application. If your development environment is Windows-based, it's the path of least resistance.
Spam and deliverability testing
Getting past spam filters is half the battle. These tools predict whether your email will land in the inbox.
Mail Tester is beautifully simple. Send your email to a unique address, and it returns a score from 1 to 10 along with detailed feedback. It checks SPF, DKIM, and DMARC authentication, scans content for spam triggers, and verifies your sending IP isn't blacklisted. The free tier gives you three tests per day—enough for most development workflows. The feedback is actionable and specific.
GlockApps goes deeper into deliverability testing. Send your email to their seed list, and they'll show you exactly where it landed across major providers—inbox, spam, promotions tab, or missing entirely. Their spam filter tests simulate how different providers will treat your message. The insights are invaluable for diagnosing deliverability problems, though the pricing reflects the complexity of maintaining seed accounts across providers.
Sender Score from Validity shows your IP reputation on a scale of 0 to 100. While not a testing tool per se, checking your score before major sends helps predict deliverability. A score below 70 suggests problems that need addressing before you'll see consistent inbox placement.
HTML and rendering tools
Email HTML is its own special dialect. These tools help you write it correctly.
MJML is a markup language that compiles to email-compatible HTML. Instead of wrestling with nested tables and inline styles, you write clean, semantic code that MJML transforms into the ugly-but-necessary HTML that email clients require. The VS Code extension provides live preview as you type. It's become the de facto standard for email development, and for good reason—it eliminates entire categories of rendering bugs.
Cerberus provides battle-tested HTML email templates that work everywhere. Rather than starting from scratch, you begin with patterns that have been tested across hundreds of client combinations. The templates are well-documented, explaining why each hack exists and which clients require it. Even if you don't use the templates directly, studying them teaches you email HTML's peculiarities.
Parcel is a code editor built specifically for email. It understands email HTML's constraints, provides intelligent autocomplete, and shows live previews as you type. The collaboration features let teams work on emails together, and the version history tracks changes over time. For teams doing serious email development, it's worth evaluating.
Accessibility testing
Email accessibility is increasingly important—and increasingly required by law in some jurisdictions.
Accessible Email uses automated checks to identify accessibility issues in your HTML. Missing alt text, insufficient color contrast, improper heading structure—it catches the common problems. The tool also provides guidance on fixing issues, not just identifying them.
The WAVE browser extension, while designed for web pages, works reasonably well for email HTML previewed in a browser. It visualizes accessibility issues directly on the page, making problems obvious. It's free and useful for quick checks during development.
Link and content validation
Broken links and typos are embarrassing. These tools catch them.
Litmus and Email on Acid both include link checking in their preview tools, validating that every URL in your email resolves correctly. They'll catch typos, expired links, and redirect chains that might trigger spam filters.
For standalone link checking, Dead Link Checker can crawl your email's HTML and verify every link. It's not email-specific, but it works. The free tier handles reasonable volumes.
Grammarly's browser extension catches spelling and grammar issues as you write email copy. It's not a testing tool exactly, but running your copy through it before sending prevents embarrassing mistakes.
Load and performance testing
For high-volume senders, testing at scale matters.
Postal is an open-source mail server you can self-host for testing. Unlike cloud services with rate limits, you control the infrastructure. It's useful for load testing your email pipeline—can your system handle 10,000 emails per hour? 100,000? Postal lets you find out without paying for cloud sends.
For testing email processing (inbound webhooks, parsing, etc.), tools like Postman can simulate webhook payloads at volume. Combined with load testing frameworks like k6 or Artillery, you can stress-test your email infrastructure before it faces real traffic.
Putting it together
No single tool covers everything. A practical testing workflow combines several:
During development, use MJML for authoring and Mailhog for capturing test sends. Before any significant send, run your email through Mail Tester for spam scoring and Litmus or Email on Acid for rendering previews. Check your sender score periodically to catch reputation issues early.
The investment in testing tools pays off in emails that actually reach inboxes and render correctly when they get there. In email, the best surprise is no surprise.
Frequently asked questions
Do I really need paid preview tools?
For occasional emails, free tools and manual testing across a few clients might suffice. For regular email campaigns or transactional emails that affect revenue, paid preview tools prevent costly mistakes. Calculate the cost of one broken email campaign versus the annual tool subscription.
How do I test dark mode rendering?
Litmus and Email on Acid both offer dark mode previews for clients that support it. You can also test manually by enabling dark mode on your devices. The key is designing with dark mode in mind from the start—using transparent images and testing color inversions.
What's the minimum testing I should do?
At minimum: check rendering in Gmail (web and mobile), Outlook (desktop), and Apple Mail. Run your email through Mail Tester for spam scoring. Verify all links work. This catches the majority of issues without requiring paid tools.
How often should I test my email templates?
Test whenever you make changes, obviously. But also retest existing templates quarterly—email clients update their rendering engines, and what worked six months ago might break today. Litmus and Email on Acid can monitor templates automatically.