In October 2026, Microsoft will end support for the Word rendering engine in Outlook desktop. This is the most significant change to email rendering in two decades. Here's what changes, what breaks, what improves, and exactly what to do about it — from a Mailchimp Pro Partner with 18+ years in email infrastructure.
What changes in October 2026
Since 1997, Outlook for Windows has used the Word rendering engine to display HTML emails. This is the reason email developers have spent two decades fighting with VML, ghost tables, MSO conditional comments, and table-based layouts that wouldn't hold a margin.
The new Outlook for Windows — rolling out now and becoming the default in April 2026 — uses a Chromium-based web rendering engine, the same engine behind Outlook.com and Microsoft 365 web. This means it supports modern HTML and CSS: flexbox, media queries, background images with standard CSS, border-radius that actually rounds corners, and web fonts that render as expected.
Microsoft's stated goal is rendering consistency across Windows, web, and mobile. For email senders, this represents a genuine leap forward — but the transition period is where the real complexity lives.
The key date: April 2026 — new Outlook becomes the default for commercial customers. October 2026 — official end of support for classic Outlook with the Word engine. Users can still opt back to classic temporarily, but no further updates or security patches will be issued.
What breaks or becomes unsupported
The following techniques, which have been standard practice in Outlook email development for years, will stop working — not immediately, but progressively as your recipients migrate to the new Outlook. Each has a specific replacement:
MSO conditional comments
The new Outlook ignores MSO conditional comments. Any code wrapped in