I am in need of saving 20,000 plus emails saved over past 6 years.

Please provide more detail so that you get useful information. Is this a POP account? I presume so, since you state that the account has been shut down. My inference is that you want to save the messages elsewhere from account, such as in Local Folders. Thank you.

It is an Imap acct.

It is an Imap acct.

Then technically if you have already shut down the account you have lost the emails involved.

It might be possible to recover some but probably not all of the relevant email by copying the base files before Thunderbird gets done deleting everything.

1. GO to the troubleshooting information on the help menu.
2. Click on the show profile button.
3. Close Thunderbird
4. In the Windows file manager window that opened copy the folder "IMAP mail" and all of it's sub folders to another location (desktop USB dive. The where is not important.
5. Open Thunderbird.
6. Install this addon https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/?src=ss
7. from a folder in local folder, right click and select ImportExportToolNG >import MBOX files > All MBOX fiules from a directory with structure.
8. When asked point to the IMAP mail folder you copied to the backup location

Eventually the import will import all IMAP mail you copied not necessarily from just the account you closed but being with structure will allow you to determine which accounts belong with which folders.

Now you have a local backup and can work with it however you like.