Try changing your mailfilter script around to filter for "no such user",
i.e. look at:
> I'm using VpopMail 5.2.1 , and i've recently noticed a whole bunch of "no
> such user [EMAIL PROTECTED]" directories in my root partition. i have some
> that are for real users of virtual domains i'm hosting and i have some
> addresses of emails i've sent out to people. inside the folder is
> nothing, if i delete them they come back. any idea what this is and why
> they are being created?
> thanks for the help!