I started moving folders from the old email account to the new one on Windows 10. The old account is POP3, and the new one. I used Move Folder. When I came to a folder with multiple subfolders I found there was a synchronisation problem with error code 0x8A404005. I left it running and went to the rest of the folders, all of which transferred OK. I deleted the problem folder from the new account and started searching for a solution to the problem. I identified the subfolders with data, and went through deleting emails up to the last few years. The syncing was still running..
whatever it was that I did I managed to leave all the subfolders empty. I think what I need is help in stopping the syncing before trying to move the folder again.
I had an inspiration, and changed the default Outlook email account to the new one. Synchronization started up immediately, and I was able to move the folder that caused the problem over, and watched that one through the syncing process. Not bad for a 93 year old (in the UK)