Hello,
I am going to share with you the comprehensive list of things to do, please feel free to skip those you already did., but do not skip anything you haven't done.
This process can take 24 hours to see result, so please do not expect immediate results.
Please complete these steps on a computer, not on a smartphone or tablet.
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First, go to https://www.outlook.com and sign in.
Click the gear icon in the top right corner to open Settings.
Under Mail, review the following areas:
Rules
If any rules are listed, delete all of them.
Conditional formatting
If anything is set up there, delete it.
Forwarding and IMAP
If you see any entries or settings you did not create, remove them. Turn off POP and IMAP.
Junk
Review Safe sender and blocked list. If you don't recognize any, remove them.
After that, exit Settings and return to outlook.com.
Open the To Do section by clicking the blue checkmark icon on the left side. Delete anything there that was not created by you.
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Go to https://account.live.com.
Under Your info > Sign-in preferences, review all aliases on the account. If you see any alias you do not recognize, remove it.
Next, go to the Devices section of your Microsoft account and remove any devices you do not recognize.
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Please also do the following in Security section
Change your password
Enable two-step verification
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Go here https://account.microsoft.com/privacy/app-access Click on "Don't allow" for anything you don't recognize.
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Then go to Security > Manage how I sign in and make sure all contact information belongs to you.
On that same page, scroll down to App passwords and remove any existing app passwords.
Also on the same page, click on Sign out everywhere. This is intended to disconnect any active sessions that may still be connected. Please note that this can take up to 24 hours to fully take effect.
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In addition, this problem can sometimes be caused by a malicious script or infection on one of your devices.
Are you using a Windows computer to check email? Do you use an email app such as Outlook Classic or New Outlook?
If so, please open the app and remove the account from the app settings.
Then go to the Microsoft Safety Scanner download page:
https://learn.microsoft.com/defender-endpoint/safety-scanner-download
Download the 64-bit version, run a full scan, and let me know what the final results say.