Yes: keep Outlook email fallbacks. Outlook 2021 end of support did not remove the Word engine from classic Outlook. The new Outlook for Windows is a different app, built on Outlook on the web. Build on tables, Test in classic and new Outlook if your list uses them, then take export-ready HTML or MJML.
Short answer: keep the fallbacks
By "fallbacks", we mean the unglamorous patterns classic Outlook still needs: table widths, buttons built as table cells, and the offer in live text for when images are off. We're not talking about a full VML cookbook.
If someone tells you end of support means you can ditch every MSO conditional and nested table tomorrow, they're mixing up two things. Support for the Office 2021 family really is ending. The Word engine disappearing from every copy of classic Outlook on the same day isn't happening.
For the practical checks (width, images off, buttons, fonts, Gmail's clip), see the HTML email checklist. This guide is about those Outlook email fallbacks: why they still matter in classic and new Outlook, including after Outlook 2021 end of support.
What ends on 13 October 2026 (and what does not)
Office / Outlook 2021 end of support
Office Watch gives 13 October 2026 as the end of support for the Office 2021 family, including Outlook 2021 (retail and LTSC). The apps generally keep working. What stops is technical support, bug fixes and security updates.
End of support doesn't mean "Outlook vanishes tomorrow". Plenty of computers carry on running older versions of Office long after the updates stop.
The Word engine is not instantly gone from enterprise Outlook
email-dev.com's Outlook glossary still lists classic Outlook for Windows (including the classic app on Microsoft 365) as using the Microsoft Word engine to render HTML email. That's the one that ignores flexbox, drops CSS max-width without width attributes and swallows the padding on ordinary links.
New Outlook for Windows is built on Outlook on the web, so it renders with a browser engine rather than Word. That helps, but it still cleans your CSS the way Outlook on the web does and doesn't load web fonts. It ignores MSO conditional comments, which is usually exactly what you want in a hybrid build.
The same glossary says classic Outlook stays supported on Microsoft 365 until at least 2029, and that the enterprise opt-out from classic to new Outlook has been pushed back to March 2027. Those are second-hand summaries, so check Microsoft's own lifecycle pages before planning any IT migration. For email, the takeaway is simpler: classic Outlook is fading out slowly, not switching off on 13 October.
New Outlook vs classic Outlook: what changes for your email
Less than you'd hope, and nothing that lets you drop your tables. Emailens compared the two across 298 HTML and CSS features (data synced 17 October 2026): 143 behave the same, 149 work better in the new Outlook, 5 get worse and 1 is newly confirmed as unsupported. The new app reads table layouts fine. Classic Outlook can't read the newer CSS. So the email that works in both is still built on tables, with anything fancier layered on top.
| Topic | Classic Outlook (Windows) | New Outlook for Windows |
|---|---|---|
| Engine | Microsoft Word | Outlook on the web (browser engine) |
| Tables / width attributes | Still the safe floor | Read fine; still the layout that works in both |
| MSO conditionals and VML | Used by the Word path | Ignored, so fixes there reach classic only |
| Web fonts | Fall back to the next font | Not loaded; plan on your fallback font |
| Newer layout CSS (flexbox, padded-only buttons) | Poor / break | Better, but treat it as a bonus on top of tables |
Outlook is a family of apps, not just one: classic Windows, new Windows, Mac, web, iOS and Android. The Mac, web and mobile versions are much easier to design for. So when someone says "it broke in Outlook", ask which one.
Because a business audience will be split between the two apps for some time, treat Outlook as two inboxes and test in both before you delete any fallback. For more on width attributes, table-cell buttons and images-off checks, see the HTML email checklist.
What still belongs in export-ready HTML
If any of your readers use classic Outlook, keep these basics in place:
- One column near 600px, with width set as an HTML attribute on the table, the cells and the images
- Buttons as table cells with bgcolor and padding on the cell
- The offer in live text; assume images may be off
- MSO or VML only when the design really needs them. New Outlook ignores MSO, and that's fine
MJML compiles to table HTML that Outlook understands (buttons, columns, widths), so there's less scaffolding to maintain by hand. The inbox still gets HTML, though, and compiling doesn't replace testing in a real inbox.
Test in classic Outlook, then export-ready
The best place to catch an Outlook problem is your own inbox, before your list sees it. In Milo Email Studio, a Test goes to an address on your account, with a subject that starts with Test. If your list includes classic Outlook, open the proof there and in the new Outlook too, and on a phone with images off, as the checklist suggests. Milo isn't a Litmus-style testing lab for every Outlook version, so open the client yourself when it matters.
When the proof looks right, you leave with export-ready HTML (downloaded or copied), the MJML or an assets zip, or you can connect Zoho Campaigns and push a draft. Either way, you send from your ESP as usual. For team sign-off without editing, see email proofing with preview links.
How this differs from GIF, dark mode and clip notes
This guide is about the Outlook email fallbacks that still matter in classic and new Outlook. Animated GIFs that freeze on the first frame in classic Outlook, dark mode contrast and logo swaps, Gmail's clip and getting ChatGPT's HTML to work in Outlook are separate reads. Outlook 2021 end of support is one reason people ask about dropping tables; the Word engine in classic Outlook is the reason to keep them.
FAQ
Does Outlook 2021 end of support mean I can drop Outlook fallbacks?
No. Keep Outlook email fallbacks. End of support for Office and Outlook 2021 isn't the same as the Word engine disappearing from classic Outlook. Keep your Outlook-safe tables until your own readers show you otherwise.
When does Outlook 2021 support end?
13 October 2026, with the Office 2021 family (Office Watch).
Classic vs new Outlook: which one breaks HTML?
Classic Outlook on Windows uses the Word engine and is usually to blame for broken buttons, widths and modern CSS. New Outlook for Windows is built on Outlook on the web, so it copes better, but it still strips some CSS and ignores MSO fixes.
Do I still need MSO conditionals?
If some of your readers use classic Outlook, a hybrid build with MSO where needed still makes sense. New Outlook ignores them, which is usually fine.
Can I design only for the new Outlook now?
Not yet. Plenty of business readers are still on classic Outlook, and the new app reads table layouts fine, so keep the tables and test in both.
Will classic Outlook disappear in 2026?
Not all at once. Summaries such as email-dev.com say classic Outlook will be around on Microsoft 365 for years yet (until at least 2029, though check Microsoft's own wording for IT planning). The move to make new Outlook the default for businesses has already slipped too, with the opt-out now given as March 2027.
Does MJML remove the need to Test in Outlook?
No. MJML compiles to HTML that Outlook understands, so you write less scaffolding, but the inbox still gets HTML. Test it, then export. See why MJML beats hand-written HTML.
Does Milo preview every Outlook variant?
No. Milo sends a Test to an address on your account, so open it in classic Outlook yourself when that matters.
Keep the table floor, then export
Keep your Outlook-safe tables while classic Outlook is still on your list. Send a Test to classic Outlook if you need to, then take export-ready HTML or MJML from Milo, or push a Zoho Campaigns draft. Describe the email to Milo's AI designer if you want a first draft of the blocks, then edit and Test as usual. The HTML email checklist and the MJML export guide cover the practical side.