Is this possible to setup backup server
like if my 1 server is down but i want my clients to switch on another server
and check emails and all
is that can possible ?
if yes then how to ?
regards

It is possible, but you would really need to be able to control your own network. It isn't really economical to try this with a dedicated server rented from someone. The problem lies in bandwith. You would have to transfer all files every day or so from the main server to the backup server.

If you have your own network, I believe there are failover methods available, but I am not really familiar with them myself. I know they involve failover switches and such.