Check your Junk Email folder
In your Outlook.com folder list, choose the Junk Email folder and look for the missing email.
If there are messages that shouldn't have been marked as junk, right-click each email and then choose Mark as not junk. The message will automatically be moved to your inbox.
Check the Other tab
Focused Inbox separates your inbox into two tabs, Focused and Other, to help you focus on the messages that matter most to you. If Focused Inbox is enabled for you, some messages sent to you might be in the Other tab. You can right-click any messages you want to appear in Focused and select Move to Focused inbox or Always move to Focused inbox. Focused Inbox will learn which messages should go always go to the Focused tab.
Check email forwarding
Check if email forwarding is enabled.
Go to Forwarding settings.
To enable forwarding, select Enable forwarding. To disable forwarding, clear the check box.
If you want email to go to the forwarding address and stay in your Outlook.com mailbox, select Keep a copy of forwarded messages.
If you've set up another email account to forward to your Outlook.com inbox, make sure forwarding hasn't expired and is properly set up. Check with your other email services support for more information.