Mail deletes current message

For about the last week my wife has complained that messages in her Microsoft Mail account were missing. It downloads messages normally, but everytime you enter the inbox it deletes the most recent message. It does not show up in the Deleted folder; it just disappears! You can see the counter in the folder list decrease. THis happens whether you start Mail from scratch or switch to another folder then go back to the Inbox. It does not happen in my Mail account which is in another User account on the same computer. Any ideas? Do I need to reinstall Mail?

 

That's it!

You got it! I don't know how that setting got changed. Thank you!