With iOS and iPadOS 14 available as of today, Microsoft has updated its Outlook app for iOS devices to enable support for the new iOS 14 feature that lets third-party mail and browser apps be set as default apps (via The Verge).
After installing the new version of the app, Outlook can be set to be the default mail app on your iPhone or iPad, so when you click a mail link to compose an email, it will be opened up in Outlook rather than the standard Mail app.
To set Outlook as the default, download Microsoft Outlook, open up the Settings app, scroll down and select Outlook, tap on Default Mail App, and then choose Outlook.
Expect to see other mail and browser apps implementing support for third-party details in the near future. Chrome has already updated its iOS app with support and it can now be set to replace Safari as the default browser.