The One Outlook version of Outlook that is eventually expected to unify the various versions of Outlook has leaked and, well, it's basically the Outlook web app.
If you'd like, you can pursue the leak yourself, but be warned: the installer file is technically unverified, and could pose a hidden risk to your PC. It also only works with an educational or workplace email account right now. That may well violate your workplace's educational policies, and it may violate those of our employer, Foundry. (While we were able to verify that the download installs itself correctly via Windows Sandbox, we didn't go so far as to install it on a work account yet.)
In any event, screenshots of the new Outlook client have leaked, including from developer FireCube Studios.
In any case, the screenshots from the leaked version look similar to the Web version of Outlook, with a dash of the Fluent Design language that Microsoft has used. It's unclear whether the new client will offer all of the email management capabilities we've outlined in our previous story about how to manage Outlook. And, of course, it looks like the awesome Windows Mail client is not long for this world either if this replaces it.
Again, unless you understand the risks, don't download an unknown client installer. Given that the installer has leaked, however, it's a good indication that you'll see an official client release soon enough: Microsoft's Build developer conference takes place the last week in May.