iPhone XR no longer receiving messages

Most likely this is a simple sync issue between Microsoft's servers and your Mail app, not Apple. Simply logging out and then back into the Mail app on your iPhone should fix the sync issue, though, if the problem persists, I recommend going to the Microsoft Apps & Connected Services Page and pressing edit, then remove these permissions to the Apple Mail app. Then when logging back into the Mail app on your iPhone it will prompt you to "allow the Apple Mail app access to your Microsoft/Outlook account," in which you can simply press accept after reading the terms and conditions again in order to allow your Outlook account to use the app again. You can also try uninstalling and then reinstalling the Mail App itself. If the problem persists though, despite following the above steps, reach out to Microsoft Support directly, since it would be an issue with Microsoft and not Apple most likely.