I find it hard to believe that there is no way to achieve this. I found some articles on the web that seem to suggest that the folderpath is structured to include the mailbox - e.g. see How to find folder (full folder path) by folder name in Outlook?. But I have not had success in using this kind of structure with the folderpath search operator - e.g. by searching folderpath:\\myaccount@mydomain.com\inbox. I can't figure out why this wouldn't work!