Mapped drives not showing in file browser when attaching file in Outlook

I have a user who, when he tries to attach a file to a new email in Outlook (Office 365 fully patched/Windows 10 fully patched), only sees the C: drive in the Insert File browse window, none of our other drives are showing. These drives show in everyone elses Outlook doing this operation. What could be the issue with his? This is a new computer for this user.