It might be due to the Outlook features Clutter or its successor Focused Inbox.

Outlook attempt to help you manage your emails, but sometimes some emails are categorized in a way that you don’t want them to, and therefore are misplaced.

Where can I find my emails?

In Outlook Online and in the Outlook client, your emails can be found in the folder Clutter. If you are using the Outlook app, your inbox is separated into two tabs - Focused and Other. Move any messages that shouldn't be in the Clutter folder back to the inbox. Clutter will learn from this and do better next time.

Focused Inbox will replace the Clutter feature on all platforms this autumn. It is already in use in the Outlook app and soon in Outlook Online and in the Outlook client.

Do you want to turn the function off? Follow the Microsoft guide here.

If you would like to know more about Clutter or Focused Inbox more information can be found here.