Exim (v3) - How to configure a backup MX?

[Dominik Schulz <dominik.schulz@gauner.org>]

Hi,
I'm wondering how it's possible to configure Exim as a Backup-MX for my
Primary-MX. The Backup-MX should recieve all Mails adressed to him and
try every x Minutes (let x be 5) to forward them to the primary MX. In case
this fails it should keep on trying for an unlimited time.

The background:
My Primary-MX is going to have a scheduled downtime of about 12 to 36
hours and since there is relatively high mail traffic it'd be very  inconvenient
to lose some of them. Since I've got another spare server in the same
data center I'd like to use this as a primary MX as described above.
I've got no problems setting up the DNS-stuff or installing Exim -  since
it runs very well on the primary mx - but I just don't know how I can
get it working like described above.

Any help would be greatly appreciated.

Mit freundlichen Gruessen / Best regards
Dominik Schulz