When my account was closed by an ISP, Thunderbird (unexpectedly for me) mirrored the lack of access to the IMAP folders and deleted all my local mail. Fortunately this was a temporary issue that I was able to recover from.
In short, don't rely on TB to retain IMAP data, (like Dropbox) it is not a backup and IME will sync data losses, deleting local data if it is no longer accessible on the server.
>When my account was closed by an ISP, Thunderbird (unexpectedly for me) mirrored the lack of access to the IMAP folders and deleted all my local mail. Fortunately this was a temporary issue that I was able to recover from.
This is absolutely ridiculous of Thunderbird.
That said:
1. As you said, like Dropbox, it's a local mirror, not a backup. Someone gaining access to your email and mass-deleting everything would delete your local copy too. A an actual backup of Thunderbird mail files would accomplish a backup.
2. Did your ISP somehow allow Thunderbird to log into the mailbox and gave it empty content? Or did they delete content before killing IMAP sessions? If so, it's outright malicious. Otherwise, Thunderbird shouldn't have deleted anything.
3. Having an always-on desktop box with a mail client that gets all email once a week is a lazy way to decrease the chances of being affected by something like that. Having a script that copies mailbox data into another folder (named according to date) is, well, an actual backup.
Apps like imap-backup (https://github.com/joeyates/imap-backup) allow retaining IMAP data.
In short, don't rely on TB to retain IMAP data, (like Dropbox) it is not a backup and IME will sync data losses, deleting local data if it is no longer accessible on the server.
Take care.