This is on MacBook Air with Sierra 10.12.6
The user filed saved mail "On My Mac" instead of in the Exchange Account. There are several layers of folders.
Copying folders from "On My Mac" to the Exchange account is not reliable. A lot of folders never get copied.
I know that I can re-create the folder structure in Exchange, and then manually move all the email one folder at a time, but that is a lot of work.
Is there a 3rd party program or some other method for moving all this mail?