You could create a rule that moves messages arriving from your trainee that have an attachment and a word or phrase in then subject and move it to a different (existing) folder. If you use Outlook Today as your start screen
https://www.technipages.com/outlook-today-startup and customize it to add your inbox and your trainee's folder, you can see at a glance which folders have unread messages. You should add a shortcut to Outlook today to the QAT (Quick Access Toolbar) for ease of return there.