Classic Outlook for Windows still uses Microsoft Word to render HTML email in November 2026, so the limits that annoyed designers in 2007 still apply to a large share of business inboxes. The new Outlook for Windows and Outlook.com use a modern browser-based engine, and Microsoft has said it will support classic Outlook until at least 2029. That leaves you designing for two Outlooks at once. Rendering is no longer the only Microsoft headache either: since May 5, 2025, Outlook.com has required SPF, DKIM and DMARC from senders who send more than 5,000 messages a day, in line with the bulk sender rules Gmail and Yahoo introduced in February 2024.
How Outlook 2007 swapped Internet Explorer for Word
OK, “hate” might be pushing it. But in mid-January 2007, barely two weeks into the new year, email marketers were already unhappy with how the year was shaping up. Campaign Monitor reported that the new release of Outlook would drop Internet Explorer’s HTML rendering engine and use Microsoft Word’s much weaker engine in its place.
What did that mean in practice? The Outlook of the near future would be badly limited. Microsoft’s own developer reference for Outlook 2007 said it “uses the HTML parsing and rendering engine from Microsoft Office Word 2007” and supports only a subset of CSS properties. The Word engine did not support any of these:
- Background images (HTML or CSS)
- Forms
- Flash
- CSS floats
- Replacing list bullets with images in unordered lists
- CSS positioning
- Animated GIFs (Outlook 2007 showed only the first frame)
What a standard HTML email looked like before and after
The same email can look very different depending on which engine renders it. This is a standard HTML email in Outlook 2003, which still used Internet Explorer’s engine:
And this is the same email in Outlook 2007, which used Microsoft Word’s HTML engine:
Why the change mattered so much in 2007
Outlook was the dominant email client at work back then. Figures quoted at the time put it at 79.6% of business users and about 15% of consumers, behind only AOL and Yahoo! Mail. So designers and email marketers had to rethink their coding standards to fit what Outlook 2007 could actually display.
You would expect a company as powerful as Microsoft to raise the bar. Instead it took ten steps backward, dropping to the level of rival clients such as Lotus Notes and Eudora.
For more detail on Outlook 2007’s rendering capabilities, Microsoft’s original Word 2007 HTML and CSS documentation is still available in its archive of previous versions, and there is a CSS support guide for the Word-based Outlook versions here.
Way to go, Microsoft. Way. To. Go.
Where Outlook rendering stands in November 2026
Nearly twenty years later, the Word engine still hasn’t gone away. Every classic desktop version from Outlook 2007 through Outlook 2016, 2019, 2021, 2024 and the classic Outlook in Microsoft 365 renders email with Word. The new Outlook for Windows works differently. It is built on the same web technology as Outlook.com and uses Microsoft Edge WebView2, a Chromium engine, so it renders email much more like a modern browser.
| Feature from the 2007 list | Outlook 2007 (Word engine) | Classic Outlook today | New Outlook and Outlook.com |
|---|---|---|---|
| CSS background images | Not supported | Not supported, VML workaround needed | Supported |
| Forms | Not supported | Not supported | Not dependable, avoid them |
| Flash | Not supported | Not supported | Dead everywhere since Adobe ended Flash in 2020 |
| CSS floats | Not supported | Not supported, use tables | Supported |
| CSS positioning | Not supported | Not supported | Not dependable in webmail |
| Animated GIFs | First frame only | First frame in older versions, plays in current Microsoft 365 builds | Plays |
The support timeline you need to know
Classic Outlook will stay around for years, so Word-engine rendering isn’t going anywhere soon. These are the dates that matter:
- October 14, 2025: Office 2016 and Office 2019 reached end of support.
- October 13, 2026: Office 2021 and Office LTSC 2021, including Outlook 2021, reached end of support with no extended security updates. The app still runs but gets no more fixes.
- March 2027: the enterprise “opt-out” phase is now scheduled to start. Microsoft moved this from its original April 2026 date. In this phase the new Outlook is on by default in Microsoft 365 Apps for enterprise, but users and admins can still switch back. Until then, classic Outlook stays the default for existing enterprise deployments. New installs since Version 2502 include the new Outlook by default.
- At least 2029: Microsoft has committed to supporting classic Outlook for Windows until at least 2029 for existing installs with a perpetual or subscription license. It has promised at least 12 months’ notice before the final cutover removes it.
The new Outlook still lacks features many companies rely on, such as COM add-ins and full PST support. So classic Outlook is likely to remain the default on many corporate desktops through the late 2020s. If you send B2B email, plan for the Word engine for the foreseeable future.
How much Outlook still matters to your list
Outlook’s share depends heavily on who your audience is. Litmus market share data from February 2026, based on more than a billion tracked opens, put Apple Mail at about 45.5%, Gmail at about 23.5%, and Outlook desktop at about 5.7%. That is a long way from Outlook’s lead in 2007.
Treat those numbers with care. Apple Mail Privacy Protection preloads images on Apple’s servers, which inflates Apple’s share of opens and makes open rates less reliable in general. A list of corporate buyers can still open mostly in classic Outlook. Check the client report in your own email service provider (ESP) before you decide how much effort Outlook deserves.
How to code email for both Outlooks today
Put simply, build for the Word engine first, then add improvements for everything else. Designers have used the same core techniques since 2007:
- Use tables for layout. Nested tables with fixed widths survive the Word engine. Floats and CSS grid do not.
- Inline your critical CSS. Classic Outlook ignores many properties in the head, and some ESPs inline styles automatically when you send.
- Use MSO conditional comments. Code wrapped in these comments shows only in Word-based Outlook, so you can serve fixes without affecting other clients.
- Use VML for background images and buttons. Vector Markup Language is the long-standing workaround for backgrounds and rounded “bulletproof” buttons in Word-based Outlook.
- Test in both engines and in dark mode. Outlook’s dark mode can invert colors, so check your logo, text contrast and button colors with dark mode on and off.
A conditional comment that shows a fixed-width wrapper table only to classic Outlook looks like this:
<!, [if mso]>
<table role="presentation" width="600" cellpadding="0" cellspacing="0" border="0" align="center">
<tr><td>
<![endif], >
<div style="max-width:600px; margin:0 auto;">
<!, email content, >
</div>
<!, [if mso]>
</td></tr></table>
<![endif], >
Modern clients read the flexible div and skip the commented table. Classic Outlook ignores max-width but honors the 600-pixel table.
Microsoft’s sender rules matter as much as rendering
A perfectly coded email doesn’t help if it never reaches the inbox. On May 5, 2025, Outlook.com, Hotmail.com and Live.com started enforcing requirements for domains that send more than 5,000 messages a day. You need passing SPF, passing DKIM, and a DMARC record of at least p=none that aligns with SPF or DKIM. Mail that fails can be junked or rejected outright.
A minimal DMARC record that meets that floor looks like this:
_dmarc.example.com. TXT "v=DMARC1; p=none; rua=mailto:[email protected]"
Gmail and Yahoo have similar rules, in place since February 2024: authentication, one-click unsubscribe for marketing mail, and a spam complaint rate kept below 0.3%. Google said enforcement would ramp up from November 2025, including rejecting mail that doesn’t comply. If you meet Google’s and Yahoo’s rules, you already meet most of what Microsoft asks.
FAQ
Does Outlook still use the Word rendering engine?
Yes, classic Outlook for Windows does. That covers Outlook 2007 through 2024 and the classic Outlook in Microsoft 365. The new Outlook for Windows, Outlook.com and Outlook for Mac, iOS and Android use more modern engines, so the same email can look different across Microsoft’s own apps.
When will classic Outlook go away?
Not soon. Microsoft has committed to supporting classic Outlook until at least 2029 and has promised at least 12 months’ notice before it is removed from Microsoft 365 Apps for enterprise. The new Outlook becomes the enterprise default in March 2027, but users can still switch back during that phase.
Do background images work in Outlook?
They work in the new Outlook and Outlook.com but not in Word-based classic Outlook. For classic Outlook, use a VML fallback inside an MSO conditional comment, and always set a background color so the email still reads well if the image doesn’t load.
Why do animated GIFs freeze in Outlook?
Outlook 2007, 2010, 2013 and 2016 show only the first frame of an animated GIF. Current Microsoft 365 builds of classic Outlook play them. Make the first frame carry your key message or offer, so readers on older versions still get the point.
Is it still worth designing for Outlook if my list mostly uses Gmail and Apple Mail?
Check your ESP’s client report first. For a consumer list where Outlook desktop shows only a few percent of opens, a table-based layout that degrades cleanly is usually enough. For B2B lists, where classic Outlook can be the main client, careful testing in the Word engine pays off.
Does Outlook rendering affect deliverability?
Not directly. Microsoft filters mail on authentication, reputation and complaints, not on how well your HTML renders. Broken layouts do hurt engagement and can lead to more spam complaints, though, and complaint rates count toward Gmail’s and Yahoo’s 0.3% limit.
Sources
- Google Workspace Admin Help: Email sender guidelines and the bulk sender guidelines FAQ
- Yahoo Sender Hub: Sender best practices and bulk sender requirements
- Microsoft Tech Community, Outlook blog: Outlook’s new requirements for high-volume senders (April 2025)
- Microsoft Support: End of support for Office 2021
- Microsoft Learn: Word 2007 HTML and CSS rendering capabilities in Outlook 2007
- Federal Trade Commission: CAN-SPAM Act, a compliance guide for business
- Litmus: Email client market share
- Mailjet documentation: Rendering in Microsoft Outlook