GuideMar 1, 202611 min read

How to Test Email Rendering in Outlook

Arjun Desai — Email engineer from Mumbai, India. Arjun specializes in Outlook Word rendering engine quirks and has debugged 500+ emails for compatibility issues across Outlook versions.

Outlook desktop uses Microsoft Word as its HTML rendering engine — which means most modern CSS doesn't work. If you're sending HTML emails, Outlook is the client most likely to break your design. This topic is covered in depth in our ultimate guide to HTML email testing.

Why Does Outlook Break My Email?

Unlike Gmail or Apple Mail which use WebKit-based renderers, Outlook desktop uses the Word HTML engine. This means it doesn't support many standard CSS properties that work everywhere else. For comparisons with other clients, see our guide on Gmail vs Outlook rendering differences.

Not Supported in Outlook

background-image (use VML instead)
border-radius
box-shadow
flexbox and grid
max-height
max-width (partial, unreliable)
CSS animations

Safe for Outlook

Table-based layouts
Inline CSS
width / height in px
MSO conditional comments
VML for background images
Basic font properties

How Can I Preview My HTML Email in Outlook Before Sending?

You don't need an Outlook account to test your emails. MailViewr's email preview tool lets you paste your HTML code and instantly preview how Outlook will render it — across desktop, iOS, and Android variants. The compatibility checker flags Outlook-specific issues automatically.

Pro tip: MailViewr's compatibility score highlights which CSS properties will fail in Outlook, so you can fix them before sending.

MSO Conditional Comments

Outlook supports Microsoft Office (MSO) conditional comments, which let you include code that only Outlook will read. This is your best tool for Outlook-specific fixes.

<!--[if mso]>
  <table role="presentation" width="600">
    <tr><td>
      Outlook-only content here
    </td></tr>
  </table>
<![endif]-->

<!--[if !mso]><!-->
  <div style="max-width: 600px;">
    Modern email client content
  </div>
<!--<![endif]-->

VML for Background Images in Outlook

Since Outlook doesn't support background-image in CSS, you need to use Vector Markup Language (VML) to achieve background images. MailViewr's builder generates Outlook-compatible code automatically.

Outlook Desktop vs Outlook Mobile

Outlook on iOS and Android uses a different rendering engine than desktop Outlook. Mobile Outlook is much more modern and supports properties like border-radius,background-image, and media queries. Always test both.

Test Your Email in Outlook Now

Free Outlook Preview

Paste your HTML, select Outlook, and see the rendering instantly. Get a compatibility score and fix issues before they reach inboxes.

Test in Outlook →