[CentOS] Server Replication: CenOS, coda? lustre?

Hi list,
our lab is divided in two sites and we would like to have a clone of the 
mail server on the second site for redundancy only (used in case of a 
lost of connectivity with the primary mail server, with the MX entries 
in the DNS the clone will receive the mails instead). The problem(s) 
comes with the mail partition who must be replicated and synchronized 
over the time (and over the net) to the cloned mail server, so the users 
can still access their mails while we deal with the primary server. My 
search leads me to the coda FS (and maybe the lustre solution, but we 
only use opensource soft so it may only be the one year old version - 
which is open...).  Is there on the list people who are doing such 
things and if you are, can you give me some clues? What I think is 
something like a 'one read - multiple writes' FS who can handle the flow 
of mails and securely replicates it to the clone server. Do we have no 
choice but to tolerate the lost of some emails when the  primary server 
stops responding? btw is it realistic?
Thanks,
Manuel