[Dovecot] What are the options for HA and\or replication and\or storage?

[Eliezer Croitoru]

I am considering couple options of converting a current service into a 
HA using multi servers or any other solutions.
The point is that I am thinking about the options and which I am looking 
for.
NFS can be a storage which has it's own pros and cons.
There is the option of DRBD but as of now it's limited for two hosts.
What options do exist out-there as a solution?

The main issues are that the current service is based on Maildir and is 
on one server with resierfs.
The service is composed of one host with 50k users couple disks in raid6.
On the same machine we have postfix as mailer.

I would like to hear about HA environment that I can use to replicate 
the boxes.

My basic research leaded me to smtp on one host or more and using lmtp 
to send the messages to dovecot o another host which will have a 
replicated host and will be used with storage system based on ISCSI or NFS.

Any other idea to enhance the current one or to replace it is more then 
just welcome.

Thanks,
Eliezer