[Dovecot] mbox configuration

[Nina Pham]

Hi, I change the dovecot.conf
default_mail_env = mbox:/Mail/IMAP/%u/:INBOX=/var/mail/%u
After I did the change, I restart dovecot with the hope that folder that 
I create will be stored in /Mail/IMAP/%u. Then I use Mozzila as my email 
client  and change the IMAP server directory to ~nina/IMAP/ which is the 
link to /Mail/IMAP/nina/.
But then I can not create folder anymore from Mozzila, or view any thing 
other than inbox. I am still able to see message in inbox since I 
haven't change the location of inbox.
 Please help.

Nina