Please confirm: When you copied the Win10 'Local Folders' folder confirm you are talking about the one in this location; C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ Mail\Local Folder.

and then you pasted it into the same location on the Win7 machine, overwriting the default one?

Was the Thunderbird program fully closed on both Win7 and Win10 before you did any copy pasting? Thunderbird must be closed when updating/copy pasting etc anything to do with profiles or contents thereof.

Regardign the addon. There is no problem with ImportExportTools: https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Post an image of the Win10 'Local Folders' folder showing me the contents. Tell me which files and/or folders you copied.

Hi Toad-Hall. Apologies for not responding earlier but I came back from a trip, was hit by a series of urgent jobs, went away again, returned to more problems. Now catching up desperately. I've just tried to copy a screen dump of the folder structure but will have to try again as it doesn't want to copy. More apologies, will continue soon.

If you want to share messages between email clients on different computers then IMAP is really far more suitable than POP.

If you need to continue using POP, you could set the travelling computer to leave copies on the server. Then these messages will remain available to the home based computer. But if you do use POP you'll be limited to the Inbox; it won't offer you Sent, Trash, Junk or user-defined folders.

Exporting and importing email messages to transfer them between computers feels perverse to me; email is a network based communication system, so it seems just natural to use the network (meaning an IMAP based account) to share the messages rather than messing about with import/export/memory sticks etc.