[Exim] Backup MX server

I want to setup another exim as a forwarding mail server. ie if the main box
goes down, the mail will be received by second mx server. which in turn will
accumulate the mail and try indefinitely to deliver it to main server. so
the mail will stay in queue for whatever time necessary.

is there howto on it? or has someone done it already and can share the
knowledge?

thx