Mail server redundancy (kinda offtopic)

[Eugene Crosser <crosser@online.ru>]

Maybe you guys who are running big mail servers may give me some advice.

At this moment, our mail server is essentially a single computer.  It is
the primary MX for a bunch of domains, and also a POP/IMAP server.  We keep
$POSTOFFICE on a fast standalone disk, $LOGDIR on another such disk, and
user mailboxes on a hardware RAID5.

Now, we are planning to improve the reliability of service.  When it
comes to just mail relaying, things are easy.  We actually have "backup"
MXes.  The hard part is local delivery.  What I would like to have is
a backup machine that would take over local delivery to the mailboxes
for the domains that we serve if the "primary" machine fails.  And this
opens the problem of sharing the same disk space between two machines.
I see a few options here: a disk array with two SCSI channels, or
exported over NFS from a third machine, or maybe something else?

Have anybody on this list implemented anything similar?  Whould you
please share your ideas?  I remember Matti was talking about redundancy
infrastructure at Sonera, is any kind of redundant POP/IMAP implemented
there?

Thanks.

Eugene