Difference btw. Dovecot Director and Dovecot Proxy

[Nathan Schultheiss]

Hello,

For a few days I try to understand what program (Dovecot Director or 
Dovecot Proxy) I should use for my email architecture.
We are a hospital, and for security reasons, we must host ourselves our 
emails, and we must leave Google Apps...

I wish to make a Dovecot backend for each department of the hospital.
Each backend Dovecot has a local storage.

Server 01: cardiology
Server 02: emergency
...
Server 05: administration

Dovecot looks in the database which server to use to store email (lmtp) 
and transfer the mail for storage.

I do not understand if I have to work with Dovecot Director or Proxy if 
I would that LMTP or IMAP request are forwarded to the right backend.
Each have self storage and have not access to the other backend storage 
(no nfs, no network storage, ...).

Users indicate imap.hospital.local, this point to Dovecot Director or 
Proxy and Dovecot check the login/password and forward the user to the 
right backend.
In my database I put user at hospital = storage host 172.16.2.10 = folder 
/home/vmail/user/ (Maildir)

I've read that the Director can refer the user to the same backend when 
there is a activ session (POP, IMAP, LMTP)
But can we tell him to always refer the user to the same backend (IMAP 
and LMTP) ?

I've found this picture schema who user have Dovecot Proxy => Dovecot 
Director => Dovecot Backend
And I'm confused why they have proxy + director ?!?

Can anyone tell me if I need to focus on Dovecot Director or Dovecot 
Proxy to create the architecture of this hospital ?

My question is probably stupid for Dovecot sysadmin, but I'm lost in the 
WIKI :)

Thanks in advance,

Nathan