i do a fresh install of opensuse 13.1
then
i do a fresh install of kde 4.12
then
i configure kmail : i create 2 identities , different folders
then
i import my mails with kmail import tool from local-mail in my mandriva in another partition .

all is ok .
kmail runs well . i send mails , i receive mails .

i can access to my mails in all of the folders , i can view , read them
but when i go to scan the contents of ~/.local/share/local-mail/ folder there is nothing .
it seems that all is stored in akonadi db .

i checked that ~/.local/share/local-mail/ is the path for local folder

my addressbook and my calendar are well duplicated in a local storage

it seems that kmail has created the akonadi version of my folders but no local version .

it's pb , i can't make a backup of my mails .

You are not the only one, see viewtopic.php?f=215&t=117005
You can make a backup of your mail by clicking right on the folder (or local mail folder to get it all) and select archive.
You can also import that archive, for example when you begin KMail from scratch.

this morning after opening a kde session
i have 2 notifications :

Dossiers locaux : Le dossier « Maildir » « » pour la collection « » est non valable.
Dossiers locaux : Le dossier « Maildir » « » pour la collection « » est non valable.

in english : local folders : "Maildir" " " for collection " " is not a valid one .

in "local-mail" there are :
cur
new
tmp