dovecot - Jun 2013 - use Alternative Storage as a cloud mail service

Hi all

I'm trying to create a mail service provider

I like to know is it possible to have some servers as storage servers
And automatically put email datas on the servers
Something like having 3 servers , If server two get full , Automatically
use server three

If it's possible give me some solutions to use

I like to use CentOS OS on my servers

Thanks in advance