Trash folder

My Trash folder seems to automatically empty every day (there is never anything older than yesterday in it).  This includes things erroneously marked spam before I even get to realized i missed an important email.  Its also really annoying since even if I move somehting to trash, if I needed to go back and look at it (missed something the first time) I am completely out of luck.  24hrs seem to be an awfully short window.  Sap can sit in the spam folder for 3 weeks before it moved to trash, why can't I set the trash emptying frequency myself?  It didn't used to be that way.