[Dovecot] Configuration advices for a 50000 mailboxes server(s)

[Cor Bosman]

>     Here we have approx. 200K users with 4000 concurrent connections  
> (90% POP3 users)  All servers in virtual environment Vmware, supermicro  
> servers and Netapp Metrocluster storage solutions (nfs storage with 10G  
> ethernet network)  POP3 sessions take betwen 40 and 300 milisecons at  
> connect, auth and list.  All accounts lives in LDAP, CentOS 5 and exim  
> like a mta relay.

Similar setup here. Maybe 15-20K concurrent connections, imap only
(pop is not handled by dovecot yet)  about 800K mailboxes. We have all
bare metal servers, linux. Currently 35 servers handling the load
easily. We could run on 1/3rd of the load probably.

In front of the 35 servers are 3 directors, handling imap only, although
im in the process of adding LMTP/sieve to the mix. 

Backend storage is NetApp Metrocluster over 2 datacenters. 

Cor