I need to setup redundant mail servers, using qmail, freebsd, and sqwebmail. The servers each need to have the same data, so that if the primary fails, the secondary can take over seamlessly. I have 2 problems:

1) how do i mirror the data between the 2 constantly?

2) how do I make the secondary take over as soon as the primary fails?

The servers will be on a lan, the pipes to the internet are already redundant.

A couple of solutions to bring to discussion.

The matter of failover should be pretty well taken care of using linux-ha (heartbeat). If you're not familiar with it, you configure the daemon on two boxes and set up a virtual IP to be the active one. Designate one of the hosts to be the primary or master host. The two hosts are connected via a null-modem cable through which hello packets are sent from the secondary to the master to determine if he is still alive. As soon as the secondary host detects no response from the master, he siezes control of the virtual IP and continues to serve where the other guy left off. We're using this to make our Squid proxies redundant; coupled with round robin DNS, you can also enable load balancing between two hosts (two virtual IPs).

As for the matter of data replication, you may consider something like rsync, but this I fear will he harder with mail servers than some other types of hosts, what with syncing mailboxes and queues, etc.

Hopefully that gives you something to look at.