[Exim] local delivery and home dirs

Hello.

I am using Exim 4.10 on a FreeBSD 4.6.2 server. Local delivery is
configured for Maildir as follows :

local_delivery:
driver = appendfile
group = mail
mode = 0660
mode_fail_narrower = false
envelope_to_add = true
return_path_add = true
directory = /home/${local_part}/Maildir
maildir_format


My problem is related to the `directory' definition. How can I make
exim search users home dirs using the system definition, instead of
this hard-coded definition /home/${local_part}/ ?

Most of our users have their home dirs located on /home/<login>, but
some others not, and for these, I have the homedir redefined in the
local /etc/passwd database (most others have NIS definitions).

Here is an example for /etc/passwd :


[...]

user1:*:2000:2000:Local User1:/var/local/user1/:/bin/csh
+::::::

[...]


Thanks in advance.

--
Nicolas