[Dovecot] HA Mailbox Design

[Nikolaos Milas]

Hi,

We would like to implement a Highly Available Mail Server and would like 
to ask advice on how to architect this.

Some details on our setup:

Currently we have only one internal mail server (Postfix/Dovecot 2.0 - 
planning to move to 2.1), receiving mail from a gateway server 
(filtering spam/viruses) - a Cisco Ironport - which we are considering 
to replace with one (or a set of) cloud-based 
Postfix/Amavis-new/SpamAssassin/ClamAV VMs (currently in testing mode).

Delivery uses Dovecot LDA. User accounts are LDAP-based.

We use Maildir and the load is low (aside spam). Only about 250 
users/mailboxes (4G each). All servers are CentOS 5.8 (planning move to 
6.3) KVM VMs (on a cloud where we don't have control on the host, but on 
highly reliable hardware/networks).

We can have VMs on two different clouds and we also have at least two 
different connections (routes) to the cloud(s) (to support HA).

Any directions will be appreciated. Hoping to design an HA architecture 
but aiming to keep it simple and (as much as possible) easily 
maintainable one.

Thanks for your response and help,
Nick