Version:           4.8 (using KDE 4.8.0) 
OS:                Linux

Having migrated Kmail 1.8 (most recently) mbox style mailbase (directories, mbox files, local static indexes) to a Fedora 16 box with KDE 4.7.4 as standard, and having already encountered similar problems (when the Akonadi helper just ran away with memory until the kernel assassinated it) when upgrading another box from Fedora 14 to 16, I replaced KDE 4.7.4 with KDE 4.8.0 from the fedora-kde48 repo as advised on here.

On first and only run of kmail it advised me that it needed to migrate to the Akonadi storage model as expected from previous experience.  Realising the catastrophe the last attempt had been, and further that I had not yet backed up the .kde directory, I told the process to cancel, only to find that it appears to have already part trashed the existing mailbase by replacing part of the mbox hierarchy with empty maildir directories.

No, I don't have a back trace (there was none).  Yes, I was running the latest release.

Purely as a PS, I find myself asking how did this piece of (expletive) make it into a supposedly "stable" release when it does not (and akonadi and its accompanying helper, and even kmail2 don't) seem to even understand mbox format files any more?  I have said it before, Akonadi and all its overheads, were a wrong choice all along.  KDE used to integrate perfectly well without it.  5 years down the line and KDE4 STILL has not got the simple interworking, and efficiency that it once had working with open standard format flat files and simple APIs.

Reproducible: Didn't try

Steps to Reproduce:
Start kmail2 on an unmigrated kmail1 mailbase.  Whether this particular quirk is reproducible I know not.

Actual Results:  
Mailbase trashed.

Expected Results:  
Kmail2 running with same pre-existing mbox folder hierarchy, or following a cancel instruction - as in this case - no action taken.

I have inferred - looking at wider problems with the migration process because sooner or later I would like to be able to get back into my mail from the last 5 years - that at present kmail/Akonadi are completely ignorant of mbox format files.  When started on a virgin mailbase kmail does not offer the option to create mbox style directories, and Akonadi's memory runaway suggests an attempt to digest 2GB mail files as single messages.  As to the cause of this behaviour problem - overwriting the mailbase despite explicit instructions to take no action - I have no further information.