Proper secondary MX configuration

[Michael T. Babcock]

What is the correct way to configure a secondary MX machine (qmail using
qmqp) so that the messages are sent with the standard exponential backoff
until '24 hours', and then every day for two weeks?

I've been asked to secondary a machine that is down sometimes for extended
periods of time (they won't upgrade, but they'll pay us to host secondary
...) and they don't want to lose their mail.