KMail with mBox format

I'm running Fedora-18 with KDE. I'm accessing mail from the mBox file /var/mail/tim and this is working fine. (I was previously trying to access mail from /var/spool/mail/tim which is linked to /var/mail/tim but that did not work. Evidently KMail has some problem with linked files?) The only problem I have now is that mail that I delete with KMail, eg mail deleted by my spam filter, is not deleted from the mBox file /var/mail/tim . Is there any KMail setting that will allow this? If there is none, is there any way outside KMail of doing this? It seems incidentally that I cannot include the mBox email in "Local Folders" on KMail. This seems to be set to an maildir setup at /home/tim/.local/share/local-mail , which I am not using and as far as I can see cannot delete, at least within KMail=>Settings . Maybe I have to do something within System Settings? Any advice or suggestions gratefully received.