I'm doing this, albeit I choose iRedMail as a base install on a test server to give us a leg up on not having to create our own ldap schema in-house.
iRedMail in a nutshell is just Dovecot (IMAP/POP3) server + Postfix, and a few other services. If you can load balance those services, then you can load balance iRedMail.
I'm not going to go in-depth how to do this, as I don't have much time. There's a few things to keep in mind, you need a proper load balancer or you need to imitate one to do things "correctly". I say "correctly" because you can likely get away with poor-mans roundrobin DNS or something, but at the end of the day having a proper load balancer is the "right way" to do things as it's important that Dovecot does not write to the same file on your share storage, which in my case our hardware balancer solves in that it will ensure for the life of an active IMAP session the user only goes to Server XYZ.
Once you solve that issue, everything else is a peice of cake. Just do some research into load balancing Postfix and Dovecot and some shared storage (I've got a SAN, but I see no reason NFS couldn't work - I believe Dovecot is NFS aware out of the box with some config options to be mindful of) then just ensure no single service is writing to the same file.
If you're using LDAP backend, then you essentially have your config in a very easy format to replicate between multiple servers. Do some research into setting up OpenLDAP to perform some provider/consumer replication. So your ldap master (provider) sits on Server ABC, and Servers XYZ and 123 replicate (consume) via the master. In the case that your master goes down, things will continue along fine and you can even promote a replica to be a master.
Without putting too much thought into it, if you had a NFS (with file locking) share mounted on a few servers. Installed iRedMail on each server and configured it to use the NFS mount for mail storage, then on your "consumer" servers just re-configure OpenLDAP to be a consumer of the master and I would imagine it would "work".
That should give you a start!