How do I setup a secondary incoming mail server?

I currently have a server running Debian 6, with postfix and dovecot handling email. This server hosts email for a number of domains and users, so I use MySQL as my backing store for users and forwardings and everything related.

Currently, this server is the only server listed in an MX record for all of the domains it serves. I would like to create a secondary server that would be listed in the DNS with a lower priority (e.g. current primary server is priority 5, secondary would be priority 10), so that in the event that I need to reboot the primary server, or otherwise make it unavailable, the secondary server would receive email, and hold it until the primary server came back up, at which point it would deliver any held email to the primary server.

I do not need the secondary server to function as a backup sending server. Users would never need to see the secondary server, they would simply not lose incoming emails if the primary server is down, and they would be unable to send or receive until the primary came back up.

How would I go about doing this? I would like to use the same software if they can handle this task, because I’m already familiar with managing them.

There is a complete documentation on how to setup Postfix as primary and secondary MX. Just follow the guide: http://www.postfix.org/STANDARD_CONFIGURATION_README.html#backup

When your primary isn't available, the foreign mailserver queues mails und does a delivery retry up to five days. I haven`t setup an additional MX, because most spammers deliver to the secondary first. Often this server isn't maintained as good as the primary.

summa summarum: much work, less benefit. Let the world queue your mails!