Dovecot Director and Dovecot proxy

[rub zorghy]

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/misc/webinar/2013_21_08/Dovecot_Webinar_21.08.2013.pdf>
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