[Exim] Redundant exim servers?

I need to come up with a plan for making our email servers fully
redundant...

Theres always the approach of keeping a second machine on standby and
copying the /var/spool and /var/mail across every ten minutes, locked with
exim_lock of course. We use uw-imapd/pop3d and something similar would need
to be done there too to prevent accesses to files.

The other option i was considering was one of the linux network mirroring
filesystems, but this may be too much overhead and Im not sure how well it
will do locking.

Another option is to have a dual port raid system, but this isnt truly
redundant because you have a SPOF in the raid system itself.

Has anyone actually implemented a two or three machine exim cluster, all
machines the same, smtp/pop3/imap load balanced across machines in the
network? This sounds like what I want but what were your experiences?

I checked the mailing list, but couldnt find any substantive discussion of
this subject.

Thanks
Andy