namesppace trouble

[fddi]

Hello,
the cyrus namespace base is located in my default installation in

/var/spool/imap/user/

I expected afterrunning the mkimap script to find the alphabetical 
letters of the usernames in that directory:

a
b
c
d
e

and so on

and have every user mailbox created in the dorresponding alphabetical 
subdirectory

for example user foo would be in

/var/spool/imap/user/f/foo

but it is not like that

when I Create user foo or user bar their mailbox are set up as

/var/spool/imap/user/foo
/var/spool/imap/user/bar

How can I change this behaviour  and have the classical schema ?

/var/spool/imap/user/a/
/var/spool/imap/user/b/
/var/spool/imap/user/c/
/var/spool/imap/user/d/
.
.
.
/var/spool/imap/user/z/

I am using cyrus-imapd-2.4.17 on FreeBSD 9.1


thank you

Rick