The folks at Vivaldi, which you might know for their web browser, have officially launched Vivaldi Mail 1.0 after a period of beta testing. The new and free cross-platform mail client sports an integrated calendar and feed reader functionality, with that feed reader being the one thing that Microsoft's new One Outlook experience lacks.
Vivaldi Mail 1.0 is built right into the Vivaldi browser and isn't a separate client, unlike Thunderbird, Outlook, or Apple's mail app. It basically lets you surf the web, while also being able to check your mail and to-do lists easily. Like Outlook, you also can combine e-mail accounts and manage them from within this client, but without the need for a subscription or one-time purchase. Vivaldi mentions that the experience has been optimized for speed, elegance, and customization. Check it out in the video below.
You can access this Mail experience from Vivaldi by going to Settings > General > Productivity features > Enable Mail, Calendar, and Feeds. All types of accounts work with Vivaldi, including POP, IMAP, and Gmail, and Outlook, and all these will be under a single inbox. The big factor here, though, is that all emails are indexed and searchable offline, plus the addition of different views for email like Unseen or Unread. Shortcuts also help with navigation, just like Outlook. Themes, and the ability to schedule emails, are other features, wrap up emailing features.
The other thing that sets Vivaldi apart from One Outlook is the Feed reader. This lets you bring your favorite RSS feeds right within the client, cutting out the need for Feedly or other services in a browser. As for the Calendar, it's a bit more fully featured than One Outlook, with features including various views (minimal, full, or compact,) viewing the calendar in the sidebar, inline event editing, full keyboard navigation, and a quick calendar event option from the right-click menu.
If you're interested in Vivaldi, check it out now via this link. It is available on Windows, macOS, and Linux as well as Android.