dimap, where are mails stored locally and how to back up?

Hi and happy new year to everybody :)

I use kmail2 (kde 4.8 rc1) and have several IMAP accounts set up. For all 
the option offline in kmail is activated. To my understanding this should 
be the same as disconnected IMAP (DIMAP) in kmail1 and mails should be 
not only stored on the IMAP server but also on the local hard drive. 
Indeed I can access my mails when the machine is offline. So far so good.

Now http://blogs.kde.org/node/4503 claims that the akonadi database is 
just an interface plus a data cache and the actual mails are still stored 
in files. But where (apart from the IMAP server of course)?

To find mails I got before I changed from kmail1 to kmail2 I did a

grep -r "Happy new year to you too." $HOME > whereismymail.txt

and found last years mails in
~/.kde/share/apps/kmail/dimap/.2107837688.directory/
and last years and this years mails in
/.local/share/akonadi/file_db_data/
plus matches in the binary
~/.kde/share/apps/nepomuk/repository/main/data/virtuosobackend/soprano-
virtuoso.db

It seems that all mails of all DIMAP accounts are in 
/.local/share/akonadi/file_db_data/
What I find disturbing, there is no folder structure whatsoever in this 
folder, just the plain files. How is one supposed to make a sensible 
backup of this?

Cheers,
Dirk

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