high-availability configurations?

Hi Folks,

I'm currently running a pretty basic high-availability configuration for
our mail server (postfix) - it simply runs in a Xen virtual machine,
with mirrored disks across two machines (DRBD), and failover of the VM
if something goes wrong (pacemaker).

I'm thinking about migrating the failover host to a 2nd datacenter -
which makes disk mirroring and VM migration a bit trickier, and I really
don't like how brittle all that infrastructure is, so I'm starting to
think about application layer redundancy - two mailservers, at remote
locations, multiple DNS records, and doing something to replicate ques,
configurations, and local delivery.  The goal is the same:  keep
processing mail if a machine goes down, and don't lose any data to
machine or disk crashes.

Which leads to a question:  Are any of you running such a
configuration?  If so, can you describe what you're doing?  And.. are
there any good references, presentations, etc. that anybody knows about
re. building high-availability, scalable, distributed mail processing
infrastructure?

Thank you very much,

Miles Fidelman

--
In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra