Definition
MSO is the Word-based rendering engine Outlook desktop uses for email. It ignores much modern CSS, so email must be coded with tables, inline styles, and conditionals.
Knowledge of MSO quirks is core to reliable Outlook rendering.
Outlook's Word engine requires special handling, and MSO knowledge is the key to it. Conditional comments and VML provide the fallbacks Outlook needs. Testing the versions your audience uses completes the picture.
Coding for MSO
Test in Outlook 2013-2019 and 365, the most common versions.
Apply MSO conditionals for backgrounds and layout.
Why It Matters
Outlook's Word engine requires special handling, and MSO is the key to it. Understanding it is essential for professional email development.
Best Practices
- Test in Outlook 2013-2019 and 365, the most common versions.
- Apply MSO conditionals for backgrounds and layout that Word ignores.
- Expect limited support for flexbox, grid, and padding on tables.
- Use tables and inline styles.
- Keep Outlook and modern versions aligned.
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Related Glossary Terms
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Above the fold in email refers to the content visible to a recipient before they scroll, a critical area for first impressions and primary calls to action.
Accessible CTA (Email)
An accessible CTA is a button or link coded with sufficient colour contrast, visible focus states, descriptive screen-reader text, and a minimum touch target of 44×44 pixels.
Adaptive Design
Adaptive design in email uses predefined layouts that adjust to specific device or client widths, rather than fluidly resizing to every possible screen.
AI-Generated Content in Email
AI-generated content in email is copy, images, code or subject lines produced by artificial intelligence tools to speed up campaign production and testing.
Alt Text
Alt text is the written alternative to an image in an email, displayed when images are blocked, slow to load, or consumed by screen readers.
Autoplay Video in Email
Autoplay video in email refers to the limited ability to play video content automatically when an email is opened, using specific client-supported techniques.
Frequently Asked Questions
It uses the Word rendering engine rather than a browser engine. Outlook's Word engine requires special handling, and MSO knowledge is the key to it.
Flexbox, grid, background sizing, border-radius, and margins on many elements. Conditional comments and VML provide the fallbacks Outlook needs.
Use conditional comments and VML for background and layout fallbacks. Testing the versions your audience uses completes the picture.
No, test the versions your audience uses. Outlook's Word engine requires special handling, and MSO knowledge is the key to it.
Slowly, but Outlook desktop still matters. Conditional comments and VML provide the fallbacks Outlook needs.