You explicitly chose not to store copies on your computer, so you can stop looking. They are not there.

What is in this 2 paths mentioned above? These folders are over 200 MB!

Your actual emails are gone. From http://kb.mozillazine.org/Thunderbird_3.0_-_New_Features_and_Changes

Disk Cache

   Thunderbird 3.0 now comes with a disk cache, which can cache any remote content [45].
   These are usually images referred to in HTML messages, or something trivial as the startup page, but also certificate data and even update files are cached.
   Messages and their attachments are now cached on the disk for IMAP accounts as well [46].
   This setting is in the Advanced preference pane, the default is 50MB. If you have privacy concerns, you should disable the disk cache (don't forget the IMAP offline copies as well). To do so, set its size to 0MB or the browser.cache.disk.enable pref to false.
   While "about:cache" has been implemented, there is no easy access to the contents of the disk cache [47]. As a workaround, define the start page in Advanced → General as about:cache?device=disk, which allows you to examine the contents whenever the start page would be shown. 

Start and work offline.

There is no way I am aware of of even looking at the content of the cache. and you have told Thunderbird not to store mail on this computer. So there will be no mbox files.

Gnospen said

Start and work offline.

I had already tried this. It did not display any mail.

Gnospen said

...... As a workaround, define the start page in Advanced → General as about:cache?device=disk, which allows you to examine the contents whenever the start page would be shown. Start and work offline.

This is a workaround, and there isn't a way as I know to retrieve your mails But this is a way to see whats in your cache. Mostly it's pics and .ocsp (google it) The key is a link that opens the contents in Firefox /IE