I think mail is the wrong application for nfs, because nfs is slow for metadata operations. Would rather use it for vm hosting than mail.
We used to have a small clustered netapp with 10k hdds and three frontend servers with postfix and courier imap/pop3. the setup was stable however the performance was not good.
So we build a imap cluster out of a pair of dell r710 (6 x 15K hdds) with centos 5 drbd and heartbeat. I will scale this setup by adding another pair of r710 servers and randomizing the mailboxes between the imap/pop3 cluster pairs. An imap proxy will direct the users to the right server and the frontend mx servers will also send the mail to the right server by using smtp as tranport and postfix transport maps for routing.
In the future i would like to switch from courier to dovecot and using lmtp as transport to our mailstore.
We currently have 10000 mailboxes only 300 - 400 imap connections but a lot pop access
the load an the active r710 is only 0.10 :)
I think mail is a problem which you can easily partition so why have all eggs in one basket :)
alex