[Exim] Quota checking

Hi!

I'm currently developing a mail system which should allow non-system
users server-side filtering, like sorting their mail into subfolders for
retrieval via IMAP.

To cut a long story short, mail routers rewrite the recipient address to
something like 'user+folder@mailhost' and forward the mail to 'mailhost'.
'mailhost' then has a redirect router, which includes a filter file. This
basically reads:

<snip>
if $local_part_suffix matches .+ then
        save "/var/mail/${lookup ldap ...}/$local_part/.${extract {2} {+} {$local_part_suffix}}/"
else
        save "/var/mail/${lookup ldap ...}/$local_part/"
endif
</snip>

directory_transport is set to a transport which sets a quota based on a
ldap query.

Well, it all works fine - if the message is delivered into the users
normal INBOX. Quota checking then includes all subfolders. But, if the
mail gets delivered into a subfolder, only the size of that directory is
calculated - and therefore renders my whole quota concept pretty
useless.

Is there any possibility to separate the directory used for the quota
check and the one for delivery? I'm afraid not, if I interpret the
following piece from the documentation for 'quota' correctly:

<snip>
This option imposes a limit on the size of the file to which Exim
is appending, or to the total space used in the directory tree when the
directory option is set. [...]
</snip>

I can't see a way out of here, but perhaps I am stuck in my views. Does
anybody of you have any ideas, suggestions or similar problems?

Thanks in advance,

Heinz Ekker