Clients6 min read•Audience: All, Developers
Email Client Testing Matrix
How to test across the major rendering engines: WebKit (Apple Mail), Blink (Gmail), and Word (Outlook Windows).
Key Takeaways
- The Big Three Rendering Engines: Apple Mail (WebKit), Gmail (Blink/Gecko), and Windows Outlook (Microsoft Word).
- Outlook desktop is the most restrictive: does not support border-radius, web fonts, or standard CSS background-image.
- PostBrix abstracts these quirks automatically through MJML and VML tags.
- Use third-party preview tools (Litmus, Email on Acid) or test inboxes for mission-critical campaigns.
The 3 Major Email Rendering Engines#
- WebKit (Apple Mail on iOS and macOS): The gold standard. Supports HTML5, modern CSS, web fonts, media queries, and dark mode controls.
- Blink / Gecko (Gmail Web, Yahoo, Outlook.com): Excellent general support, but Gmail clips messages >102 KB and strips non-inline styles in certain forwarding scenarios.
- Microsoft Word Engine (Outlook 2016–2021 Desktop for Windows): The most challenging engine. Treats email like a Word document; requires nested
<table>structures, VML graphics, and explicit pixel dimensions.
Core 5-Client Testing Matrix#
For every high-stakes campaign, verify in these five environments:
- Apple Mail (iOS 17+): Checks mobile typography, Retina asset sharpness, and dark mode behavior.
- Gmail App (Android): Checks mobile column stacking and hybrid fluid table behavior.
- Gmail Web (Chrome / Safari): Checks 102 KB clipping thresholds and webmail styles.
- Outlook 2019 / 2021 for Windows: Checks MSO conditional comments, VML background graphics, and button clickability.
- Outlook.com / Web: Checks partial dark mode inversion and inline CSS compatibility.
Frequently Asked Questions
Do I need a paid Litmus or Email on Acid subscription to test PostBrix emails?
While automated virtual preview tools are valuable for enterprise teams, sending real test emails to personal inboxes (Gmail, Outlook, iOS Mail) covers over 90% of user environments for most businesses.
Does Outlook for Mac use the Microsoft Word rendering engine?
No. Outlook for Mac uses Apple WebKit, meaning it supports modern CSS, web fonts, and border-radius just like Apple Mail. Only Outlook for Windows uses the Word rendering engine.