[Sussex] Backup MX

[Alan F]

I've been running my own mail/dns server from home for nearly a year now, 
and have been very happy with the way it's turned out, spam/virus 
filtering especially. Availability has been good, losing very few mails 
(as far as I know, anyway).

Some redundancy would of course be nice :-)

Setting up another two nameservers was fairly easy; these 2 backups being
at different providers and different locations in the US. This of course
beats the ns2 that pointed to ns1 at home previously =)

Now the new challenge... A backup MX. I have a box in the US that could 
serve as this, but I'm unsure as to how go about it, and thought an 
experienced bunch like you would have an idea.

Using postfix on both (no plans to change), ideally it would receive mail
when the main MX was down, then place it on there when it comes back up.  
Is this what normally happens? From google all I gather is that the second
MX should just receive mail for the domains, there isn't much mention of
how it actually gets back to the primary MX... Am I supposed to have 
scripts in cron to forward any mail or something similar?

Any feedback would be appreciated :)
Alan