The primary view in the Outlook mobile app Inbox is Focused Inbox. The Focused Inbox is replacing Clutter and is meant to filter important emails into your main Inbox view as further explained here. Regardless of what view you’re in, particularly in the Outlook mobile app, you’ll still receive all emails but the mobile app does default to the Focused Inbox upon setup. One important note is that items sent to Other will not trigger a notification on your mobile device, however, and you’ll need to open the mobile app and choose Other to see those emails.
Below are the steps to turn off Focused Inbox in the mobile app (the steps are from Outlook for Android but iOS is very similar).
Turn Off Focused Inbox
Go to Settings.
Look for Focused Inbox under Mail.
Tap Focused Inbox to turn it off.
Navigate back to your Inbox and you’ll see all mail, there will no longer be a Focused Inbox slider but simply reverse the steps to re-enable Focused Inbox.