you almost got it!

This was likely winmail.dat/TNEF[0], Microsoft's proprietary replacement of HTML.

[0] https://en.wikipedia.org/wiki/Transport_Neutral_Encapsulatio...

TNEF is a container/encapsulation format rather than a markup language. It can contain HTML as well as RTF and plain text.

Granted HTML rendering of email is full of quirks in Outlook

It's using Word to render html. They tried using a browser engine but then it rendered a copy-pasted Word text differently and users hated that.

Now that they are transitioning the desktop version of Outlook into an Electron-like webview, I suppose showing HTML directly in the webview is out of the question, and they must be using the Javascript version of Word to render HTML...

And everyone on Microsoft ecosystem hates the new Outlook version.

We already have the 365 Web version for that.

Yet another example of Microsoft's failure to hire employees with native Windows development skills, now with the old guard that grew up with Win32 are reaching retirement age.

If you ever though nobody had an use-case for WASM...