Log in to your account using a computer;
https://www.outlook.com
Click the gear icon in the upper right corner to open settings.
Click on Rules and see if there are any you don't recognize; if so, delete them.
Also check conditional format and IMAP; if there's anything you didn't create, delete it.
Check if you have any aliases that you don't recognize; if so, delete them.
https://account.live.com/names/manage
Finally, access your account security information:
https://account.live.com/proofs/manage/
Scroll down and check if any app passwords have been created and remove them.
Click "Sign out of all locations."
Change your password and enable 2-step verification.
Then click on Device in the menu above and remove any device that is not yours.