I had some difficulty importing saved emails from my old computer when I set up Live Mail on the new computer.  Some folders imported OK, but some gave me the error message "Folder contains no emails, or another application is using the folder." Clearly, there were .eml files there, and I could open them in Live Mail by clicking on them, but Import wouldn't work.  I managed to import them using drag and drop, but why wouldn't Import work?  The files were there, and no other application was using the folder or the files - I even closed File Explorer to eliminate that possibility - so it appears that the error message was incorrect.  Some of the folders that wouldn't Import were quite large, but others were not, so it did not seem to be related to file size - in fact, one folder that did work with the Import function had 30,000 emails in it.