Email clients and their rendering quirks

No two mail clients render HTML the same. Here are the most common gotchas.

Outlook desktop (Windows)

• Uses Word's rendering engine (not a browser engine)
• Strips CSS3 features (flexbox, grid, custom properties)
• Auto-adds whitespace around tables
• Blocks remote images by default

How MailManagers compensates: table-based layout, inline CSS, fallback attributes on every element.

Outlook web / 365

• Much better than desktop — uses a real browser engine
• However: forces its own font stack sometimes
• Image blocking optional (user-controlled)

Apple Mail (macOS / iOS)

• Renders HTML well, closest to browser output
• However: scales images automatically on Retina displays — use 2× resolution images for logos
• On iOS: the "preview" often shows little, open the mail for full layout

Gmail (web / mobile)

• Good HTML support
• However: clips mails after 102 KB ("View entire message" link). Keep your signature compact.
• Removes <style> tags — therefore inline CSS is required

General best practices in MailManagers

• Keep your signature under 20 KB
• Use email-safe web fonts (no Google Fonts or custom fonts)
• Test in at least 2 mail clients before using on your domain

Testing tools

• Litmus or Email on Acid for professional cross-client tests
• Mail-tester.com for spam score
• Or just: send a test mail to yourself via different clients