[Dovecot] SMTP configuration for director mail cluster

[Murray Trainer]

HI All,

I have setup my Dovecot director proxies and Dovecot backend mail
stores.  I intend setting up several incoming and outgoing Exim
MTA's.  I am a bit confused about what I need to run on the backend
Dovecot MTA's to get the incoming mail delivered to them.  Do I need
to install Exim on them or use LMTP to receive mail from the MTA's? 
Also, can I directly deliver mail to the mailstores or do I have to go
through the director proxies using lmtp?  Note that I am using
Director with the mailboxes on NFS backed storage.

Thanks

Murray