Dovecot Director and Dovecot proxy

Hi,

Dovecot Director is used to keep a temporary user -> Dovecot backend server mapping. So, Director decides which backend handles each user and the user is always redirected to the same server. All user data is stored in shared storage (NFS). The Dovecot presentation featured during this webinar http://knowledgebase.open-xchange.com/fileadmin/user_upload/open-xchange/mis... mentions (slide 7) a Dovecot proxy cluster (doing credentials and user info lookup) behind the LB device (F5 Big-IP)

Why Dovecot Director server isn't used to perform this without Dovecot proxy ? Thus, the load balancer (F5 Big-IP) can distribute requests based on IMAP protocol to Dovecot Director cluster.

Thx so much.

gdrub