The fact that maildrop attempts a cwd to $HOME before executing the maildroprc or .mailfilter scripts. If $HOME doesn't exist, maildrop will error out before even looking at your script. Sam is opposed to adding auto maildir/home creation to maildrop, arguing that doing so is the responsibility of the admin at account creation time, not the delivery agent and also that things would get ugly real fast should the maildirs be on a remote filesystem that is unmounted... There are a few patches floating around though that add this funcitonality, search the list archives...