For incoming mails that won't be a problem. The nature of MX records means you can give each server their own MX record - one primary, one secondary - if the primary one fails, other mail servers will send to the other one. However, you're talking about clustering, so to keep both servers synchronised, you'd need some form of shared storage - whether an NFS share mounted on a machine common between the 2, or a NAS device, or whatever.
Of course then your shared storage becomes a single point-of-failure, so you'd then need 2 of those!
The other option is to keep one server sitting as a "hot-spare" and continually rsync the user's home directories off the primary. Use a low TTL value on your mail server's DNS records. If the primary server fails, your hot-spare will be more-or-less up-to-date, so just switch the DNS over, and within a few seconds your clients will switch over. When you're primary one is backup and running, keep that one as the hot-spare instead, or rsync the directories back again when you get a lull in client traffic.
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