re :I've tried to copy it back into my imap.gmail.com folder, but nothing populates for the deleted folders when I open TB.

That's because Thunderbird starts up and the first thing it does is access the gmail server to check and update the gmail imap account. As the folders/labels do not exist on server they get removed from the imap account. Imap accounts can only show what is on server.

But it is possible to upload emails back onto the server, but Thunderbird has to be started and connected to server first and you need those emails accessible in 'Local Folders' mail account.

re:I have a copy of the .sbd file that still shows all the .msf files The .msf files are only index files. They do not contain emails. Do you also have mbox files (no extension) - these files contain emails? For a .sbd folder to display it would need an mbox no extension file of same name.

So if you have - this is an example: mbox no extension file called 'Orders' and also an 'Orders.sbd' folder and If the 'Orders.sbd' folder contains mbox no extension files.

• Exit Thunderbird now - this is important.
• Copy the mbox no extension file called 'Orders' and also an 'Orders.sbd' folder
• paste them into the 'Local Folders' mail account located in the 'Mail' folder.

Then when you restart Thunderbird, access 'Local Folders' mail account and you will see a new folder called 'Orders' and it should contain subfolders. Those folders should also contain emails.

• Recreate the required folders in the imap gmail account.
• Subscribe to see those folders.
• Copy batches of highlighted emails from the 'Local Folders' > 'Orders' folder/subfolders
• then Paste them into the newly created imap account folder.

This will then get updates to the server.

If you do not have mbox files then you do not have emails. The only alternative is to create everything on the server first.

• Logon to webmail account via a browser.
• Create the various folders/labels and make sure they are set to 'show in imap'
• Access 'All Mail' and move relevant emails into the various labels you have created.

• Then in Thunderbird, subscribe to see those new folders and subfolders.

• Check Account Settings > Synchronisation & Storage for gmail imap account
• If you have already selected to 'keep messages in all folders for this account on computer.....'
• click on 'Advanced' button and make sure all folders and subfolders are selected so a full downloaded copy can occur.
• Also select 'Synchronise all messages locally regardless of age'

Thanks for writing all that up!

I did have mbox files and was able to use ImportExportTool NG to import them into my local folder. I'm now working to drag them all back to the IMAP account where they started.

The emails were always on the gmail server, just with no label. Copying folders around in TB is tedious, but not nearly as tedious as relabeling hundreds of thousands of messages in gmail. That was never going to happen.