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 that you did not create, delete them. If you do not use rules at all, you can delete all of them.
Conditional formatting
If anything is set up there that you did not create, delete it.
Forwarding and IMAP
If you see any entries or settings you did not create, remove them. Turn off POP and IMAP if you do not use them.
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If nothing is there,
If you use Classic Outlook on any Windows device, do this on those devices
Close Outlook completely.
Press Win + R on your keyboard to open the Run dialog.
Type outlook /cleanrules and press Enter to clear and reset client-side rule stores
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Then try again, send a test email to yourself and see if it is working.