[Exim] local delivery to hashed directories

Hi there

I would like exim to deliver local mail in this format...
mail sent to say tom@localdomain should be delivered to /var/mail/t/tom

my currennt transport is..

local_delivery:
driver = appendfile
group = mail
mode = 0660
envelope_to_add = true
directory = "/var/mail/$local_part"

which delivers to say /var/mail/tom

Please help

TDN