Sending, receiving, and organizing email in Outlook.com
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Check your Junk or Spam folder to see whether the email from the specific sender ended up there.
If the email is not in any of your Outlook folders, go to Outlook Settings > Mail > Junk Email > Blocked senders and domains.
https://outlook.live.com/mail/0/options/mail/junkEmail
If the sender’s email address appears in the blocked senders list, remove it. Then add the sender’s address to the safe senders list so future emails are delivered directly to your inbox.
Also, check your email rules to see whether you’ve set up one that automatically moves emails from that sender to another folder.
Let me know if you still need more help.