[exim] Using Exim as Backup Mail server

I am interested in learning how to use an Exim server as a backup for
multiple separate domains. My goal would be for the Exim server to
receive mail for any of the domains when the primary for a domain is
unavailable, and then automatically forward the received mail when the
primary comes back up.

Is there a tutorial on accomplishing this in terms of the necessary
Exim configuration entries? Otherwaise, can someone give me some ideas
on how to proceed?

Thank you for your time,

--
Mark