If you check Automatically compact folders & kmail; removes the messages you deleted from the server as soon as you leave a folder. Otherwise the messages are only marked as deleted and it is up to you to compact the folders manually by using the menu item File Compact All Folders.
Se selecciona Compactar os cartafoles automaticamente, o & kmail; retira do servidor as mensaxes que vostede elimina logo de que saia do cartafol. Se non, as mensaxes só se marcan como eliminadas e depende de vostede o compactar os cartafoles manualmente mediante o elemento do menú Ficheiro Compactar Todos os Cartafoles.