One action that IMAP does is syncing between server and client. Probably thats what happened and maybe your client loged in to the server when that account was empty. that way deleting all mails in inbox and other server-oriented folders. BUT deleting in Thunderbird means marking them as to be deleted. So if you have NOT compacting you might still have them. First for security make a copy somewhere of the Inbox (and every other you have in that account. Download and install the add-on Recover Deleted Messages Run this on the inbox.

I think you'll find it helpful to look at a basic guide to IMAP and POP -- something like this: http://whatismyipaddress.com/imap You'll see that with IMAP, your emails live on the server: what you see on your computer is only a copy of the message. That's a fundamental difference from POP mail, which delivers the message to your computer (though it can be set to retain a copy on the server). Consequently, when dealing with an IMAP account, Thunderbird constantly tries to keep 'in sync' with the server: it checks the account repeatedly and adjusts its list of emails accordingly. As you discovered, when you interrupt that checking process by taking your computer offline, the emails in Thunderbird remain unchanged; but as soon as you reconnect to the account Thunderbird updates its file to reflect what is (and is not) on the server. The best way to preserve your emails and stop them from disappearing when they go from the server is to remove them from the email account altogether. Thunderbird offers you the 'Local Folders' account for that purpose. You can create folders there which will not reflect changes on the server. Email folders that are within the account itself are linked to the server, and therefore vulnerable to deletion. I hope that explains somewhat. If you didn't keep a backup copy of your Thunderbird profile then your best hope for recovering the deleted emails is to contact the email provider. Although your account with them has been closed they may keep a backup for a limited period, to cover cases like this.

I fully agree in what amanchesterman say.

But first try that add-on, It works on my "empty" IMAP Inbox until I compact it.

Hi, thanks for your replies.

Yes, I'm well aware of what IMAP is and the difference between IMAP and POP3. Still, that doesn't explain

1. Why Thunderbird "syncs" with an "empty" account although it couldn't even log in ("password expired"),
2. Why Thunderbird connects to the mail server, although I have unchecked every mark in the server settings of the account,
3. Why the mails were deleted, although I have explicitly stated in the settings to keep the downloaded mail locally, and
4. Why Thunderbird re-downloads every single mail from live mail accounts, although I have pointed to the profile folder, which (supposedly?) has them all already. This doesn't usually happen when regularly opening it on the originally installed OS.

Thanks for suggesting "Recover Deleted Messages", but it couldn't recover anything either.

Fortunately, I still have backups of the profile folder. What is really odd, is that the ~400MB INBOX file is simply gone, but the files of the other folders are still there. I could simply copy the INBOX file from my backups back to the folder, but Thunderbird would just delete them again with the current settings.

If the mails are gone for good, then this must be several horrible bugs at once.

Suggest that you copy that 400MB inbox file into Local folders UNDER A NEW NAME. Then run the add-on on that file.

As I said when you or the server deletes a mail then it is just marked /flagged as deleted and will not show any more. It is NOT removed from the file. If you use trash it is copied to that folder. NOT MOVED When compacting you rewrite the folder to a new file, only moving those NOT marked as deleted. That's when the are removed.

Your 400MB folder has most certainly a lot of deleted mail how many depending on when you compacted last time.

You can't turn of sync in settings. It's fundamental for IMAP. I can't tell if your name and password still works but goes to another place with an empty account. Maybe they want you to pay and reconnect.

Or you unchecked "keep messages ..... this computer" then Inbox will only contain headers. Some mails might have be downloaded to cache for simpler access. But if its 400 MB probably only the very latest are downloaded.

1. Why Thunderbird "syncs" with an "empty" account although it couldn't even log in ("password expired"),

Or the answer from server was account doesn't exist any more so Inbox is empty.

1. Why Thunderbird connects to the mail server, although I have unchecked every mark in the server settings of the account,

You cant uncheck that.

1. Why the mails were deleted, although I have explicitly stated in the settings to keep the downloaded mail locally, and

You stated to keep a COPY on this computer, that is the full email would be copied down to your PC-Inbox. As long as a folder, like Inbox, is under an account and NOT under Local Folders, the original folder and mail is on the server. PC and Server will sync.

1. Why Thunderbird re-downloads every single mail from live mail accounts, although I have pointed to the profile folder, which (supposedly?) has them all already. This doesn't usually happen when regularly opening it on the originally installed OS.

Don't understand

Thanks for suggesting "Recover Deleted Messages", but it couldn't recover anything either.

If You haven't compacted or repaired the folder or if you haven't set any disc-saving actions (under Account-settings /sync&storage or Options/advanced/network& disc space or under properties /retention policies for each folder) then the deleted mails should still be there.