Two identical email servers can be setup to share a private NFS partion off a raid array. Each server provides the exact same services as the other machine. All clients are told to connect to a third IP for services. If this server ever dies and goes offline, the second server, using vqalive, will detect the failure and assign the IP to itself. As a hot swap over.
Clients can then continue to get services.
And the IP address is switched transparently. We provide commerical support and installation services. Take a look at these other free software packages. QmailAdmin - web admin for vpopmail. SqWebMail - web email client program.