In suse the kmail folder is situated in /home/user/.kde/share/apps/kmail.
If you have reinstalled your system and opening kmail and clikkin on mails they magically disappears … well I think it is a kmail bug!!!
Anyway just stop clikkin on mails, close kmail, go in emails folder (the folder above) and delete all indexes (index)
hope to have been clear and helpfull as usual.
happy suse, happy people.