Definition
Render testing involves sending a test email to multiple email clients and devices to verify how it displays before sending to your full list. Email clients — Gmail, Outlook, Apple Mail, Yahoo, and dozens more — each render HTML and CSS differently. An email that looks perfect in one client may break entirely in another.
Render testing tools capture screenshots of your email across hundreds of client-device combinations so you can identify and fix layout issues, broken images, font problems, CTA misalignment, and other display bugs before they reach subscribers.
Common Render Issues
| Issue | Affected Clients | Cause |
|---|---|---|
| Missing background images | Outlook | Outlook strips CSS background images |
| Font fallback problems | Gmail, Outlook | Web fonts not supported or blocked |
| Button rendering | Outlook | Outlook uses Word's rendering engine |
| Image blocking | All clients | Images blocked by default in some clients |
| Mobile layout breaks | Various | Inconsistent media query support |
| Gmail CSS clipping | Gmail | Gmail strips unrecognised CSS |
Best Practices
- Test every email in at least the top 10 email clients by your audience's usage
- Test on both desktop and mobile versions of each client
- Include dark mode rendering in your test suite
- Test with images blocked to verify alt text and layout
- Use a dedicated render testing tool rather than manual testing only
- Create a test matrix of your most important clients and run it before every send
- Save screenshots for regression comparison between campaigns
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Frequently Asked Questions
Start with the clients your audience actually uses. Analytics from your ESP or previous campaigns will show the top clients by open rate. Generally, the top 10 clients cover 90%+ of your audience — Gmail, Apple Mail, Outlook (desktop, web, mobile), Yahoo, and Samsung Mail are almost always included.
At minimum, test Gmail (web and mobile), Apple Mail (mobile), Outlook (desktop), and a generic mobile client. These four cover the majority of email opens for most audiences. More complex or design-heavy emails require broader testing across 10–20 clients.