Todays Debian Jessie's update brings a fix in Akonadi that you certainly want in your system.

There was a bug in Akonadi that made it leak files. And if you use Kmail you will certainly want to keep reading: most of us who tested it before pushing it to testing (and now to stable) removed more than 4 GiB of useless data from our homes.

The bug that makes Akonadi leak files gets solved with the latest stable update (and has been in testing for a couple of months already). But you need to purge the leaked files. It's pretty easy: with your normal user account just run:

akonadictl fsck

That's all. After a while you will get back a lot of disk space. Note that you don't need the Akonadi fix in order to run this tool and recover your space. The fix just makes sure this won't happen again.