As others have said, with care, you can move a /var/spool/mail store of mbox-format folders to a NetApp; yes, it introduces NFS, which has problems with locking, but NetApps are some of the more skilled and competant NFS talkers:-). Make sure you don't have NFS-related problems with the hosts that mount the /var/spool/mail, make sure they're all locking well and take care with parameters, and it can work.
If you want to go this route, I'd recommend a testing period during which you occasionally yank the network cables into your netapp, and occasionally pull the power, of the netapp, and of your mail server[s]; recovery after such incidents is where NFS locking really has the worst problems, and you want to learn all about it in testing. Make sure you're really loading the system while you're doing these failure tests.
Once you move into prod, of course make sure your netapps and your servers are all provisioned with the most reliable power you can, often that's UPS, so you don't have to worry about exercising this stuff as often.
If you can direct all your mail client accesses through IMAP or POP, leaving no programs but the delivery agent and the imap/pop daemon doing direct mailbox accesses, then you get the ability to relatively painlessly change mailbox formats. That's worthwhile; Maildir+NetApp is one of the sweetest spots in the whole mail server design spectrum. Forget locking, forget recovery, everything Just Works.
When I build a server, even if it's a single box for everything, I go with maildrop delivering to virtual user Maildirs, with Courier-IMAP (which offers pop, and the /ssl varients) providing access for MUAs; this sets me up to grow as big as ever I could want and never worry about mailbox correctness.
-Bennett