1. Check the deleted items folder in case the emails were mistakenly put there
2. Check the junk folder in case Outlook mistakenly marked the emails as junk
3. Check if the emails are on another device, for example a mobile device, if the same email account is used
4. Manually check if the emails have been blocked at the server or ISP level
5. Check if a security app is blocking the emails
6. Contact the email service support team, in case any above steps have not found the missing emails