Definition
Outlook for Windows renders email using Microsoft Word's HTML engine — not a standard browser engine — which means standard CSS often does not work. mso conditionals are special HTML comments that only Outlook processes, allowing you to apply Outlook-specific fixes.
Common mso Properties
mso-line-height-rule: exactly— Prevents Outlook from adding extra line spacingmso-hide: all— Hides elements from Outlook (useful for progressive enhancement)mso-table-lspaceandmso-table-rspace— Removes table cell spacing in Outlookmso-padding-alt— Sets padding specifically for Outlook
Example
<!--[if mso]>
<td style="padding: 20px;">This only renders in Outlook</td>
<![endif]-->
Why It Matters
Outlook for Windows still holds significant market share in B2B environments. Without mso-specific fixes, emails can break or look unprofessional for a substantial portion of your B2B audience.
Best Practices
- Start with the fundamentals of Email mso Conditional CSS and build from a clear baseline, so later improvements are measurable rather than assumed.
- Keep Email mso Conditional CSS consistent with how the rest of your email programme works, so no single initiative works against another.
- Review how Email mso Conditional CSS is handled in your own data and adjust from what you see, rather than copying what another brand does.
- Test one change at a time and measure the effect before rolling it out more widely.
- Revisit your approach to Email mso Conditional CSS regularly, because audience behaviour and inbox technology keep moving.
- Make sure the basics — relevance, timing, and honesty — are solid before chasing more advanced tactics.
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Related Glossary Terms
Above the Fold in Email
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.
AMP for Email
AMP for Email is a technology that allows interactive, dynamic content to live inside an email message, enabling forms, carousels and live updates.
Frequently Asked Questions
Good practice here means handling Email mso Conditional CSS in a way that is relevant, timely, and honest for your audience. mso conditional CSS uses Microsoft Office-specific HTML comments and CSS properties to target styling specifically at Outlook for Windows, which uses Word's rendering engine instead of a standard browser engine. Done well, it improves engagement and builds trust; done poorly, it creates friction that costs you results.
Because it touches the parts of email that drive outcomes: relevance, trust, and delivery. Small improvements compound, while repeated mistakes quietly erode the health of your programme.
The most common problems are treating Email mso Conditional CSS as a one-off task, ignoring what the data says, and copying competitors without testing. All three lead to effort that does not translate into better results.
Compare the metrics it should influence — engagement, conversions, and deliverability — before and after you make changes. Trends over time matter far more than any single send.
It supports the same goal as the rest of your email programme: the right message to the right person at the right time. Aligned with segmentation and automation, it reinforces everything else rather than competing with it.