dovecot - Aug 2014 - Avoid "451 4.4.0 Remote server not answering (timeout while waiting for reply to DATA reply)"

Hi all,

I have a Dovecot setup (2.1, I will switch to 2.2 soon) with one 
Director and 2 backend servers.

When my MX server (Postfix) tries to deliver mail to Dovecot via LMTP, 
it often gets an error "451 4.4.0 Remote server not answering (timeout 
while waiting for reply to DATA reply)" from the Director.

Service LMTP is configured with "client_limit = 1" on Director en 
backends (others limits are default ones).

Postfix is using "lmtp_destination_recipient_limit = 1" and 
"lmtp_destination_concurrency_limit = 1".

I want that incoming messages are stored one after one per recipient 
(have a queue per recipient). I does not seem to be done actually. I 
think Postfix is sending too many messages at a time to Director.

How do you do to handle that ?

I already applied the 3 patches available on the list for 2.1, but it 
does not seem to solve my problem...

Thank you a lot.

Florent