...snip... 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.
I'd like to echo those sentiments: I build mail systems - the Maildir / NetApp combo means that my largest client currently has a system with 1.6 million Maildirs ( = users) and it's still growing nicely. I wrote to highlight qmail - "The original and best" mailserver to use Maildirs. Qmail is a particularly strong mailserver implementation and is very stable - I have some concerns with Courier mentioned by Bennett in that a new version seems to appear frequently.
The other nice thing about Maildirs is that because of their (as Bennett says) "everything Just Works" design, you can have multiple machines writing entries (messages) to the same Maildir without their needing to be aware of one another's existence; that really helps systems to scale.
More info on qmail at www.qmail.org and cr.yp.to/qmail.html
cheers,
Andrew.