What is an inactive account? If it is IMAP I am guessing the folder contents are deleted as the account probably no longer authenticates.

Hi Matt,correct, all accounts are IMAP. All 3 - even these 2 inactive, when I reopen TB try to connect to their servers and of course, these 2 inactive, fail this attempt. Strange as for these 2 inactive I have unchecked: "Check for new message and startup" in Server Settings. If as you said "folder contents are deleted" why do I see them all once I import them and open TB for the 1st time? That means they must be sitting in Profile, correct? If deleted what to do to not allow TB to delete them?

Personally I would create two folders inside 'Local Folders', like 'Mail1' 'Mail2'. Inside each of them create subfolders as You want, for example: Inbox, Sent etc.

Then manually move mails from each mailbox to those created folders and remove inactive mailboxes, so they won't try to synchronize and display errors.

Hi, thanks for suggestion but what you mean saying "manually move ..." ? Copy & Paste emails from .thunderbird profile folder? If yes which folders and files? If not how would you do it if as I said emails don't show up under inactive accounts ?

You can drag and drop messages from one folder to another, for example select all from inbox folder and then drag them into Local folders -> Mail1 -> Inbox.

You said You can import those messages, start TB and see all mails - don't close TB and try to move them to local folders.

Thanks. It works. Just Copied all emails from inactive Inbox, Sent and subfolders and Pasted them to two new Folders with the same name child folders under the Local Folder. Yes, it is kind of a workaround for this issue, but I still think there must be some Tweaks that make TB on Linux Mint behave the same way as it does on Win10. Thanks a lot for this tip. If you find something else, pls let me know.