Hello, I’m Virginia a fellow Microsoft/Windows user.
I understand you’ve already checked for message rules so you can skip the first part of this suggestion if you wish or you can double check the settings:
Check to see if there are any message rules enabled which are forwarding emails to another address.
Outlook via the web:
Click the cog icon>Email>Rules.
Also check for hidden rules:
Please check the suggestions here to see if there’s any hidden message rules:
As a last resort you’ll need to contact support:
https://learn.microsoft.com/en-us/answers/questions/5569093/my-outlook-account-was-hacked-auto-reappearing-inb Follow the instructions from the marked answer.