Using Classic Outlook on Windows for personal email, calendar, and contact management
Hello
Besides checking what the AI answer already suggested, make sure you also check the Inbox Rules.
In Classic Outlook, go to Files > Manage rules & alert.
For web based, go to www.outlook.com > Settings > Mails > Rules
On your computer, if you use Windows, exit Classic Outlook, hold down Win+R keys, type in the following command to clear any hidden rules:
outlook /cleanrules
Press enter to run it and restart Outlook.
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If you use smartphone for example to check emails as well, make sure you check the phone's very own contacts app, and make sure in its settings you don't have any contacts blocked, that can also cause issue like this.