I think I twigged it. Neither I, or my wife, had "Deleted" set as folders we wanted to keep locally in Tools->Account Settings->Synchronisation & Storage->Advanced. As soon as I set it on for Deleted & restarted TB I got the compact? prompt & compacting the folder shrank the file to zero bytes. So it looks like a bug. If you don't specify you want a copy of the "Deleted" folder locally you get it anyway but TB doesn't police it, so the file grows & grows.

All Imap accounts will subscribe automatically to all the basic system folders eg: Inbox, Sent, Drafts, Trash etc. Emails are stored on the server and Thunderbird is a vehicle to showing you what is on the server and it does this by synchronising with server folders.

All Imap accounts should have all deleted emails going to the server imap Trash folder for same account. Note: some servers will call 'Trash' other names eg: 'Bin' or 'Deleted Items'

Some servers will auto empty the server 'Trash' folder periodically. eg: Gmail does it every 30 days approx. So it happens without any intervention from you.

You can manually empty the Trash - right click on Trash and select 'Empty Trash'. You can set up Thunderbird to auto empty the Trash when you exit Thunderbird. This is done on a per account basis via 'Account Settings' > 'Server Settings' under the section 'Message Storage'

When you delete or move an email, it appears to move to the designated folder. In reality, there is a residual original copy in the original folder which is now 'marked as deleted' and 'hidden', but it still in the file. This facilitates easy reinstating if you move or delete in error. But ultimately, the file needs to be cleaned up to regain space and also to help prevent getting corrupted files. This requires you to 'compact' the files. This can be done manually: Right click on folder and select 'Compact'. You can also set up an Imap account to compact/purge the Inbox upon exit from Thunderbird via 'Account Settings' > 'Server Settings' under the section 'Message Storage'. You can also set up Thunderbird to perform an auto compact when it will save eg: 20MB.

• Menu app icon > Options
• Select 'General'
• Scroll down to 'Network & Disc Space' section
• In 'Disk Space' sub section
• Select checkbox 'Compact all folders when it will save over 20MB in total'

So emails are not truely deleted until compaction has occured.

Thanks for your input, but respectfully I don't think what you're describing explains the situation I had. In the original post I said I'd done a compact on the "Deleted" folder to no effect, and that the deleted emails were not on the server. I don't know why the "Deleted" folder was not on the list of folders to be duplicated in TB, but when I changed the config to include it, deleted emails then went away after a compact, which they didn't before. With the "Deleted" folder listed for duplication the behaviour was as you describe, but not before.