[Dovecot] quota doesn't work?

I'm using dovecot 1.0.rc15 with debian etch. In dovecot.conf I have set
mail_plugins = quota imap_quota for imap mail_plugins = quota for pop3 and quota = maildir:storage=10240. I seems that it doesn't work 'cause I send messages over 10M and it doesn't block my maildir box. I have to do something more?