Why Outlook considers the latest Epiphany release a "Legacy Browser"?

Outlook on the web and Outlook.com only support specific, modern browsers and versions. Anything outside that tested list is treated as unsupported or “legacy,” even if it is technically up to date.

According to the documented requirements, the supported desktop browsers are:

• Windows 11: Microsoft Edge, Mozilla Firefox, or Google Chrome
• macOS: Microsoft Edge, Apple Safari (version 16 or higher), Mozilla Firefox, or Google Chrome
• Linux: Firefox or Chrome (some features may not be available)

If a browser is not one of these (for example, Epiphany/Web on Linux), Outlook.com may:

• Block access entirely and show an error page, or
• Force a degraded or “light” experience designed for older or less capable browsers.

Additionally, starting in late August 2024, Outlook.com explicitly blocks older versions of otherwise supported browsers (for example, Chrome ≤79, Firefox ≤78, Safari ≤13.1, Opera ≤76, Internet Explorer, Microsoft Edge Legacy, and New Microsoft Edge ≤79). These are all categorized as old/legacy for security and compatibility reasons.

So, even if Epiphany uses a modern engine, Outlook.com’s compatibility logic only recognizes the documented set of browsers and versions. Anything else is treated as legacy/unsupported because it is not part of the tested and guaranteed set, and some features may not work correctly.

On Linux specifically, the documentation only commits to Firefox and Chrome for Outlook.com/Outlook on the web, which is why Epiphany is not accepted as a fully supported browser.

References:

• Supported browsers for Outlook on the web and Outlook.com
• Outlook Web App opens in the light version when it's used in Microsoft 365 even though the user has a supported web browser