We use Outlook Desktop and we save emails into the BOX Drive Folder for all staff to access.  We will then drag these files into our other salesforce program for upload.  This works fine.  

The problem occurs when we try to delete that file in the Box file explorer.  We receive an error message stating that the file cannot be deleted because it is open.  Even though it is not open.  We have to close down Outlook and then delete the file.  This is not a solution as we do this over 200 times a day with many staff.

Here is some troubleshooting I have found:

1.) After you move it to the Box Folder, DO NOT open the file in the Box Folder.
2.) If you do, you can open the email using Outlook, then close the email.  Then try deleting it again and it will delete.
3.) You can also close Outlook and reopen, which you already know.
4.) I also tried the "New Outlook" and this deletes the file form the folder no matter what.

My conclusion:  I think there is something within the Outlook Desktop version that doesn't like Box and it releasing the file if you open if from the Box Drive.