[Exim] local_delivery in /var/spool/mail/user/

hello eximmers,

I'm trying (as an exercise) to do all local delivery in separate files
in /var/spool/mail/user/; I thought I could just add a '/' to the last
line of the local_delivery transport like that:

local_delivery:
driver = appendfile
group = mail
mode = 0660
mode_fail_narrower = false
file = /var/spool/mail/${local_part}/

..but it didn't work.. why? do I need to change the 'driver =' line?

the next obvious question is: can I configure qpopper to understand my
mail spool with each message in a separate file?

TIA!

--
Otavio Exel /<\oo/>\ oexel@???