[Exim] Exim as a backup MX

I'm interested in how Exim works as a backup MX server. Aside from setting
up the DNS, and telling Exim it can relay the given domain, how does Exim
deal with the following:

1. How does Exim know where the final delivery should be ? Assuming it's a
backup MX that simply relays the mail on to the main MX server. Am I correct
in saying it will try and delivery to any other listed MX server with a
higher priority than itself ?

2. Can Exim be told to relay all mail it receives for the domain it's
running as backup MX for to a specific host ? I believe this is done using a
router, as you would for a smarthost, but just for the specific domain ?

3. Say Exim was the only backup MX, and the main MX server was down for an
extended period of time (say 24 hours). Will Exim queue the mail for that
domain normally and thus generate the usual "mail has not been delivered for
x hours" messages ? Surely from an end-user point of view when an e-mail is
delived to any MX server for a domain it's considered "received" ?

Regards,

Tim Bishop.