Outlook's iOS app has been updated with support for the new iPhones.
Up until this new version, the email app, made by Microsoft, would appear in a usual size on the iPhone XS Max and not take advantage of the larger 6.5" display. IT will now show more content on the larger display.
Outlook for iOS has also been updated to support the iPhone XS and XR too.
Finally, as with OneDrive too, beta testing is back open with the Outlook app on iPhone, and users who sign up to it can test new features and updates to the main Outlook app before they come out to the public.