From the Windows Central article:
Microsoft is building a universal Outlook client for Windows and Mac that will also replace the default Mail & Calendar apps on Windows 10 when ready. […] The project will deliver Outlook as a single product, with the same user experience and codebase whether that be on Windows or Mac.
This most definitely means they’re going to be using something like Electron.
And who could blame them? For a software product that is expected to run everywhere, having only one desktop client codebase just makes sense.
Shipping features frequently and consistently across platforms is a really complex problem that takes a remarkable level of coordination and engineering discipline. Not everybody can pull it off, and even those who do, still have to deal with big tradeoffs.
For a market as competitive as email, having cross-platform desktop applications that work consistently for every platform can be a huge differentiator. For enterprise customers, this might even be expected of your product (see how many platforms Slack supports).
People often criticize when a big company switches their native clients to Electron, and with good reason. Since the framework bundles apps with its own instance of Chromium, Electron apps are usually bloated, glitchy, and resource intensive.
But even so, Electron’s value proposition is too attractive.
This wouldn’t be the first time Microsoft moves a fully native app to Electron, and it certainly won’t be the last. They did it with Skype, and were quite successful at it.