Since recent update to kde 4.12.2 akonadi fills my Local folder outbox with thousands of copies of the previously sent mail whenever I restart. In fact it does not seem to stop anywhere. CPU usage and memory uses is going up (due to kmail and akonadi_*). I have a configuration with multiple different akonadi resources. Local mbox and new Kmail format local folders (this outbox is the one which gets populated) and a few imap mail boxes. Akonadi created about 20'000 visible messages in the outbox before the system started to get slow.
The only way I found to escape a system crash was deleting with (Ctrl+A) and (Shift+Del) all the messages akonadi produces. I did this repeatedly and faster than new messages were produced until the outbox was eventually empty. The third time I encountered the bug I also did 'find . -name "*" -print0 | xargs -0 rm' in  '~/.local/share/local-mail/outbox/new' (rm * did not work anymore because there were too many items). I do not know if this would be sufficient however.
I am aware that the amount of information I can provide is probably not sufficient to debug this.
However the bug, in my opinion, is that akonadi is capable in principle of creating so many messages at a time at all. Just consider the situation in which I sent a mail to my boss, then I restart and I am connected to the internet when the system comes up, I don't watch it and the messages actually get send (before my system crashes do to missing memory and disk space). It will be hard to explain that I did not send the messages intentionally.

Reproducible: Always