Kmail with a fatal error

Suddenly Kmail just vanishes of the window with the following information: KMail encountered a fatal error and will terminate now. The error was: Message could not be added to the folder, possibly disk space is low. My /home is not really empty but with still 35% to fill cannot imagine a folder of a such a big size . Ideas? Kmail runs on 9.3 with Kde 3.5. No updates of late. Only security patches.