Using older versions of Outlook for Mac for personal email and calendar
Hello,
To figure out why this is happening I need to know which version of Outlook you are using right now. Are you on the classic desktop program, the New Outlook app, the web version, or the mobile app?
This usually happens if you are looking at emails in the reading pane and your software is set to mark emails as read only after a few seconds. If you read it fast and hit delete before that exact timer runs out, it throws the email into the trash folder while it is still technically flagged as unread by the system.
Let me know what version of Outlook you are on so I can point you to the right settings menu to check and turn that delay off.