Relay only server

[Dan Egli]

How does one setup Qmail to accept mail for a particular domain, and then
simply hold it and que it till it can be delivered to another server? i.e.

server MX1 == Primary server.
server MX2 == Alternate server

MX1 is down, so MX2 recieves a mail message. The users on MX1 do not exist
on MX2, but I want MX2 to accept the message, then relay it to MX1 when MX1
is back up.

How is this accomplished?

Thanks!
-- Dan